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1.
  • Aamodt, K., et al. (author)
  • Alignment of the ALICE Inner Tracking System with cosmic-ray tracks
  • 2010
  • In: Journal of Instrumentation. - 1748-0221. ; 5
  • Conference paper (peer-reviewed)abstract
    • ALICE (A Large Ion Collider Experiment) is the LHC (Large Hadron Collider) experiment devoted to investigating the strongly interacting matter created in nucleus-nucleus collisions at the LHC energies. The ALICE ITS, Inner Tracking System, consists of six cylindrical layers of silicon detectors with three different technologies; in the outward direction: two layers of pixel detectors, two layers each of drift, and strip detectors. The number of parameters to be determined in the spatial alignment of the 2198 sensor modules of the ITS is about 13,000. The target alignment precision is well below 10 mu m in some cases (pixels). The sources of alignment information include survey measurements, and the reconstructed tracks from cosmic rays and from proton-proton collisions. The main track-based alignment method uses the Millepede global approach. An iterative local method was developed and used as well. We present the results obtained for the ITS alignment using about 10(5) charged tracks from cosmic rays that have been collected during summer 2008, with the ALICE solenoidal magnet switched off.
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2.
  • Aamodt, K., et al. (author)
  • First proton-proton collisions at the LHC as observed with the ALICE detector: measurement of the charged-particle pseudorapidity density at root s=900 GeV
  • 2010
  • In: European Physical Journal C. Particles and Fields. - : Springer Science and Business Media LLC. - 1434-6044. ; 65:1-2, s. 111-125
  • Journal article (peer-reviewed)abstract
    • On 23rd November 2009, during the early commissioning of the CERN Large Hadron Collider (LHC), two counter-rotating proton bunches were circulated for the first time concurrently in the machine, at the LHC injection energy of 450 GeV per beam. Although the proton intensity was very low, with only one pilot bunch per beam, and no systematic attempt was made to optimize the collision optics, all LHC experiments reported a number of collision candidates. In the ALICE experiment, the collision region was centred very well in both the longitudinal and transverse directions and 284 events were recorded in coincidence with the two passing proton bunches. The events were immediately reconstructed and analyzed both online and offline. We have used these events to measure the pseudorapidity density of charged primary particles in the central region. In the range vertical bar eta vertical bar < 0.5, we obtain dN(ch)/d eta = 3.10 +/- 0.13(stat.) +/- 0.22(syst.) for all inelastic interactions, and dN(ch)/d eta = 3.51 +/- 0.15(stat.) +/- 0.25(syst.) for nonsingle diffractive interactions. These results are consistent with previous measurements in proton-antiproton interactions at the same centre-of-mass energy at the CERN Sp<(p)over bar>S collider. They also illustrate the excellent functioning and rapid progress of the LHC accelerator, and of both the hardware and software of the ALICE experiment, in this early start-up phase.
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3.
  • Aamodt, K., et al. (author)
  • Midrapidity Antiproton-to-Proton Ratio in pp Collisons root s=0.9 and 7 TeV Measured by the ALICE Experiment
  • 2010
  • In: Physical Review Letters. - 1079-7114. ; 105:7
  • Journal article (peer-reviewed)abstract
    • The ratio of the yields of antiprotons to protons in pp collisions has been measured by the ALICE experiment at root s = 0.9 and 7 TeV during the initial running periods of the Large Hadron Collider. The measurement covers the transverse momentum interval 0.45 < p(t) < 1.05 GeV/c and rapidity vertical bar y vertical bar < 0.5. The ratio is measured to be R-vertical bar y vertical bar<0.5 = 0.957 +/- 0.006(stat) +/- 0.0014(syst) at 0.9 Tev and R-vertical bar y vertical bar<0.5 = 0.991 +/- 0.005 +/- 0.014(syst) at 7 TeV and it is independent of both rapidity and transverse momentum. The results are consistent with the conventional model of baryon-number transport and set stringent limits on any additional contributions to baryon-number transfer over very large rapidity intervals in pp collisions.
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4.
  • Aamodt, K., et al. (author)
  • Production of pions, kaons and protons in pp collisions at root s=900 GeV with ALICE at the LHC
  • 2011
  • In: European Physical Journal C. Particles and Fields. - : Springer Science and Business Media LLC. - 1434-6044. ; 71:6
  • Journal article (peer-reviewed)abstract
    • The production of pi(+), pi(-), K+, K-, p, and (p) over bar at mid-rapidity has been measured in proton-proton collisions at root s = 900 GeV with the ALICE detector. Particle identification is performed using the specific energy loss in the inner tracking silicon detector and the time projection chamber. In addition, time-of-flight information is used to identify hadrons at higher momenta. Finally, the distinctive kink topology of the weak decay of charged kaons is used for an alternative measurement of the kaon transverse momentum (p(t)) spectra. Since these various particle identification tools give the best separation capabilities over different momentum ranges, the results are combined to extract spectra from p(t) = 100 MeV/c to 2.5 GeV/c. The measured spectra are further compared with QCD-inspired models which yield a poor description. The total yields and the mean pt are compared with previous measurements, and the trends as a function of collision energy are discussed.
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5.
  • Aamodt, K., et al. (author)
  • Transverse momentum spectra of charged particles in proton-proton collisions at root s=900 GeV with ALICE at the LHC
  • 2010
  • In: Physics Letters. Section B: Nuclear, Elementary Particle and High-Energy Physics. - : Elsevier BV. - 0370-2693. ; 693:2, s. 53-68
  • Journal article (peer-reviewed)abstract
    • The inclusive charged particle transverse momentum distribution is measured in proton-proton collisions at root s = 900 GeV at the LHC using the ALICE detector. The measurement is performed in the central pseudorapidity region (vertical bar eta vertical bar < 0.8) over the transverse momentum range 0.15 < p(T) < 10 GeV/c. The correlation between transverse momentum and particle multiplicity is also studied. Results are presented for inelastic (INEL) and non-single-diffractive (NSD) events. The average transverse momentum for vertical bar eta vertical bar < 0.8 is < p(T)>(INEL) = 0.483 +/- 0.001 (stat.) +/- 0.007 (syst.) GeV/c and < p(T)>(NSD) = 0.489 +/- 0.001 (stat.) +/- 0.007 (syst.) GeV/c, respectively. The data exhibit a slightly larger < p(T)> than measurements in wider pseudorapidity intervals. The results are compared to simulations with the Monte Carlo event generators PYTHIA and PHOJET. (C) 2010 Published by Elsevier B.V.
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6.
  • Aamodt, K., et al. (author)
  • Two-pion Bose-Einstein correlations in pp collisions at root s=900 GeV
  • 2010
  • In: Physical Review D (Particles, Fields, Gravitation and Cosmology). - 1550-2368. ; 82:5
  • Journal article (peer-reviewed)abstract
    • We report on the measurement of two-pion correlation functions from pp collisions at root s = 900 GeV performed by the ALICE experiment at the Large Hadron Collider. Our analysis shows an increase of the Hanbury Brown-Twiss radius with increasing event multiplicity, in line with other measurements done in particle- and nuclear collisions. Conversely, the strong decrease of the radius with increasing transverse momentum, as observed at the Relativistic Heavy Ion Collider and at Tevatron, is not manifest in our data.
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7.
  • Aamodt, K., et al. (author)
  • Charged-particle multiplicity measurement in proton-proton collisions at root s=0.9 and 2.36 TeV with ALICE at LHC
  • 2010
  • In: European Physical Journal C. Particles and Fields. - : Springer Science and Business Media LLC. - 1434-6044. ; 68:1-2, s. 89-108
  • Journal article (peer-reviewed)abstract
    • Charged-particle production was studied in proton-proton collisions collected at the LHC with the ALICE detector at centre-of-mass energies 0.9 TeV and 2.36 TeV in the pseudorapidity range vertical bar eta vertical bar < 1.4. In the central region (vertical bar eta vertical bar < 0.5), at 0.9 TeV, we measure charged-particle pseudo-rapidity density dN(ch)/d eta = 3.02 +/- 0.01(stat.)(-0.05)(+0.08)(syst.) for inelastic interactions, and dN(ch)/d eta = 3.58 +/- 0.01 (stat.)(-0.12)(+0.12)(syst.) for non-single-diffractive interactions. At 2.36 TeV, we find dN(ch)/d eta = 3.77 +/- 0.01(stat.)(-0.12)(+0.25)(syst.) for inelastic, and dN(ch)/d eta = 4.43 +/- 0.01(stat.)(-0.12)(+0.17)(syst.) for non-single-diffractive collisions. The relative increase in charged-particle multiplicity from the lower to higher energy is 24.7% +/- 0.5%(stat.)(-2.8)(+5.7)%(syst.) for inelastic and 23.7% +/- 0.5%(stat.)(-1.1)(+4.6)%(syst.) for non-single-diffractive interactions. This increase is consistent with that reported by the CMS collaboration for non-single-diffractive events and larger than that found by a number of commonly used models. The multiplicity distribution was measured in different pseudorapidity intervals and studied in terms of KNO variables at both energies. The results are compared to proton-antiproton data and to model predictions.
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8.
  • Aamodt, K., et al. (author)
  • Charged-particle multiplicity measurement in proton-proton collisions at root s=7 TeV with ALICE at LHC
  • 2010
  • In: European Physical Journal C. Particles and Fields. - : Springer Science and Business Media LLC. - 1434-6044. ; 68:3-4, s. 345-354
  • Journal article (peer-reviewed)abstract
    • The pseudorapidity density and multiplicity distribution of charged particles produced in proton-proton collisions at the LHC, at a centre-of-mass energy root s = 7 TeV, were measured in the central pseudorapidity region vertical bar eta vertical bar < 1. Comparisons are made with previous measurements at root s = 0.9 TeV and 2.36 TeV. At root s = 7 TeV, for events with at least one charged particle in |eta vertical bar| < 1, we obtain dN(ch)/d eta = 6.01 +/- 0.01(stat.)(-0.12)(+0.20) (syst.). This corresponds to an increase of 57.6%+/-0.4%(stat.)(-1.8%)(+3.6) (syst.) relative to collisions at 0.9 TeV, significantly higher than calculations from commonly used models. The multiplicity distribution at 7 TeV is described fairly well by the negative binomial distribution.
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9.
  • Aamodt, K., et al. (author)
  • The ALICE experiment at the CERN LHC
  • 2008
  • In: Journal of Instrumentation. - 1748-0221. ; 3:S08002
  • Research review (peer-reviewed)abstract
    • ALICE (A Large Ion Collider Experiment) is a general-purpose, heavy-ion detector at the CERN LHC which focuses on QCD, the strong-interaction sector of the Standard Model. It is designed to address the physics of strongly interacting matter and the quark-gluon plasma at extreme values of energy density and temperature in nucleus-nucleus collisions. Besides running with Pb ions, the physics programme includes collisions with lighter ions, lower energy running and dedicated proton-nucleus runs. ALICE will also take data with proton beams at the top LHC energy to collect reference data for the heavy-ion programme and to address several QCD topics for which ALICE is complementary to the other LHC detectors. The ALICE detector has been built by a collaboration including currently over 1000 physicists and engineers from 105 Institutes in 30 countries, Its overall dimensions are 16 x 16 x 26 m(3) with a total weight of approximately 10 000 t. The experiment consists of 18 different detector systems each with its own specific technology choice and design constraints, driven both by the physics requirements and the experimental conditions expected at LHC. The most stringent design constraint is to cope with the extreme particle multiplicity anticipated in central Pb-Pb collisions. The different subsystems were optimized to provide high-momentum resolution as well as excellent Particle Identification (PID) over a broad range in momentum, up to the highest multiplicities predicted for LHC. This will allow for comprehensive studies of hadrons, electrons, muons, and photons produced in the collision of heavy nuclei. Most detector systems are scheduled to be installed and ready for data taking by mid-2008 when the LHC is scheduled to start operation, with the exception of parts of the Photon Spectrometer (PHOS), Transition Radiation Detector (TRD) and Electro Magnetic Calorimeter (EMCal). These detectors will be completed for the high-luminosity ion run expected in 2010. This paper describes in detail the detector components as installed for the first data taking in the summer of 2008.
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10.
  • Abelev, B., et al. (author)
  • Production of K*(892)(0) and phi(1020) in pp collisions at root s=7 TeV
  • 2012
  • In: European Physical Journal C. Particles and Fields. - : Springer Science and Business Media LLC. - 1434-6044. ; 72:10
  • Journal article (peer-reviewed)abstract
    • The production of K*(892)(0) and phi(1020) in pp collisions at root s = 7 TeV was measured by the ALICE experiment at the LHC. The yields and the transverse momentum spectra d(2)N/dydp(T) at midrapidity vertical bar y vertical bar < 0.5 in the range 0 < p(T) < 6 GeV/c for K*(892)(0) and 0.4 < p(T) < 6 GeV/c for phi(1020) are reported and compared to model predictions. Using the yield of pions, kaons, and Omega baryons measured previously by ALICE at root s = 7 TeV, the ratios K*/K-, phi/K*, phi/ K-, phi/pi(-), and (Omega + <(Omega)over bar>)/phi are presented. The values of the K*/K-, phi/K* and phi/K- ratios are similar to those found at lower centre-of-mass energies. In contrast, the phi/pi(-) ratio, which has been observed to increase with energy, seems to saturate above 200 GeV. The (Omega + (Omega) over bar)/phi ratio in the p(T) range 1-5 GeV/ c is found to be in good agreement with the prediction of the HIJING/B (B) over bar v2.0model with a strong colour field.
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11.
  • Abelev, B., et al. (author)
  • Anisotropic flow of charged hadrons, pions and (anti-)protons measured at high transverse momentum in Pb-Pb collisions at root S-NN=2.76 TeV
  • 2013
  • In: Physics Letters. Section B: Nuclear, Elementary Particle and High-Energy Physics. - : Elsevier BV. - 0370-2693. ; 719:1-3, s. 18-28
  • Journal article (peer-reviewed)abstract
    • The elliptic, v(2), triangular, v(3), and quadrangular, v(4), azimuthal anisotropic flow coefficients are measured for unidentified charged particles, pions, and (anti-)protons in Pb-Pb collisions at root S-NN = 2.76 TeV with the ALICE detector at the Large Hadron Collider. Results obtained with the event plane and four-particle cumulant methods are reported for the pseudo-rapidity range vertical bar eta vertical bar < 0.8 at different collision centralities and as a function of transverse momentum, p(T), out to p(T) = 20 GeV/c. The observed non-zero elliptic and triangular flow depends only weakly on transverse momentum for p(T) > 8 GeV/c. The small p(T) dependence of the difference between elliptic flow results obtained from the event plane and four-particle cumulant methods suggests a common origin of flow fluctuations up to p(T) = 8 GeV/c. The magnitude of the (anti-)proton elliptic and triangular flow is larger than that of pions out to at least p(T) = 8 GeV/c indicating that the particle type dependence persists out to high p(T). (c) 2013 CERN. Published by Elsevier B.V. All rights reserved.
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12.
  • Abelev, B., et al. (author)
  • J/psi production as a function of charged particle multiplicity in pp collisions at root s=7 TeV
  • 2012
  • In: Physics Letters. Section B: Nuclear, Elementary Particle and High-Energy Physics. - : Elsevier BV. - 0370-2693. ; 712:3, s. 165-175
  • Journal article (peer-reviewed)abstract
    • The ALICE Collaboration reports the measurement of the relative J/psi yield as a function of charged particle pseudorapidity density dN(ch)/d eta in pp collisions at root s = 7 TeV at the LHC. J/psi particles are detected for p(t) > 0, in the rapidity interval vertical bar y vertical bar < 0.9 via decay into e(+)e(-), and in the interval 2.5 < y < 4.0 via decay into mu(+)/mu(-) pairs. An approximately linear increase of the J/psi yields normalized to their event average (dN(J/psi)/dy)/(dN(J/psi)/dy) with (dN(ch)/c eta)/(dN(ch)/d eta) is observed in both rapidity ranges, where dN(ch)/d eta is measured within vertical bar eta vertical bar < 1 and p(t) > 0. In the highest multiplicity interval with (dN(ch)/d eta)(bin)) = 24.1, corresponding to four times the minimum bias multiplicity density, an enhancement relative to the minimum bias J/psi yield by a factor of about 5 at 2.5 < y <4 (8 at vertical bar y vertical bar < 0.9) is observed. (C) 2012 CERN. Published by Elsevier B.V. All rights reserved.
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13.
  • Abelev, B., et al. (author)
  • Heavy flavour decay muon production at forward rapidity in proton-proton collisions at root s=7 TeV
  • 2012
  • In: Physics Letters. Section B: Nuclear, Elementary Particle and High-Energy Physics. - : Elsevier BV. - 0370-2693. ; 708:3-5, s. 265-275
  • Journal article (peer-reviewed)abstract
    • The production of muons from heavy flavour decays is measured at forward rapidity in proton-proton collisions at root s = 7 TeV collected with the ALICE experiment at the LHC. The analysis is carried out on a data sample corresponding to an integrated luminosity L-int = 16.5 nb(-1). The transverse momentum and rapidity differential production cross sections of muons from heavy flavour decays are measured in the rapidity range 2.5 < y <4, over the transverse momentum range 2 < p(t) < 12 GeV/c. The results are compared to predictions based on perturbative QCD calculations. (C) 2012 CERN. Published by Elsevier B.V. All rights reserved.
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14.
  • Abelev, B., et al. (author)
  • Inclusive J/psi production in pp collisions at root s=2.76 TeV
  • 2012
  • In: Physics Letters. Section B: Nuclear, Elementary Particle and High-Energy Physics. - : Elsevier BV. - 0370-2693. ; 718:2, s. 295-306
  • Journal article (peer-reviewed)abstract
    • The ALICE Collaboration has measured inclusive J/psi production in pp collisions at a center-of-mass energy root s = 2.76 TeV at the LHC. The results presented in this Letter refer to the rapidity ranges vertical bar y vertical bar < 0.9 and 2.5 < y <4 and have been obtained by measuring the electron and muon pair decay channels, respectively. The integrated luminosities for the two channels are L-int(e) = 1.1 nb(-1) and L-int(mu) = 19.9 nb(-1), and the corresponding signal statistics are N-J/psi(e+e-) = 59 +/- 14 and N-J/psi(mu+mu-) = 1364 +/- 53. We present d sigma(J/psi)/dy for the two rapidity regions under study and, for the forward-y range, d(2)sigma(J/psi)/dydp(t) in the transverse momentum domain 0 < p(t) < 8 GeV/c. The results are compared with previously published results at root s = 7 TeV and with theoretical calculations. (C) 2012 CERN. Published by Elsevier B.V. All rights reserved.
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15.
  • Abelev, B., et al. (author)
  • J/psi Suppression at Forward Rapidity in Pb-Pb Collisions at root s(NN)=2.76 TeV
  • 2012
  • In: Physical Review Letters. - 1079-7114. ; 109:7
  • Journal article (peer-reviewed)abstract
    • The ALICE experiment has measured the inclusive J/psi production in Pb-Pb collisions at root s(NN) = 2.76 TeV down to zero transverse momentum in the rapidity range 2.5 < y < 4. A suppression of the inclusive J/psi yield in Pb-Pb is observed with respect to the one measured in pp collisions scaled by the number of binary nucleon-nucleon collisions. The nuclear modification factor, integrated over the 0%-80% most central collisions, is 0.545 +/- 0.032(stat) +/- 0.083dsyst_ and does not exhibit a significant dependence on the collision centrality. These features appear significantly different from measurements at lower collision energies. Models including J/psi production from charm quarks in a deconfined partonic phase can describe our data.
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16.
  • Abelev, B., et al. (author)
  • Light vector meson production in pp collisions at root s=7 TeV ALICE Collaboration
  • 2012
  • In: Physics Letters. Section B: Nuclear, Elementary Particle and High-Energy Physics. - : Elsevier BV. - 0370-2693. ; 710:4-5, s. 557-568
  • Journal article (peer-reviewed)abstract
    • The ALICE experiment has measured low-mass dimuon production in pp collisions at root s = 7 TeV in the dimuon rapidity region 2.5 < y < 4. The observed dimuon mass spectrum is described as a superposition of resonance decays (eta, rho, omega, eta', phi) into muons and semi-leptonic decays of charmed mesons. The measured production cross sections for omega and phi are sigma(omega)(1 < p(t) < 5 GeV/c. 2.5 < y < 4) = 5.28 +/- 0.54(stat) +/- 0.49(syst) mb and sigma(phi)(1 < p(t) < 5 GeV/c. 2.5 < y < 4) = 0.940 +/- 0.084(stat) +/- 0.076(syst) mb. The differential cross sections d(2)sigma/dy dp(t) are extracted as a function of p(t) for omega and phi. The ratio between the rho and omega cross section is obtained. Results for the phi are compared with other measurements at the same energy and with predictions by models. (C) 2012 CERN. Published by Elsevier B.V. All rights reserved.
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17.
  • Abelev, B., et al. (author)
  • Measurement of the Cross Section for Electromagnetic Dissociation with Neutron Emission in Pb-Pb Collisions at root s(NN)=2.76 TeV
  • 2012
  • In: Physical Review Letters. - 1079-7114. ; 109:25
  • Journal article (peer-reviewed)abstract
    • The first measurement of neutron emission in electromagnetic dissociation of Pb-208 nuclei at the LHC is presented. The measurement is performed using the neutron zero degree calorimeters of the ALICE experiment, which detect neutral particles close to beam rapidity. The measured cross sections of single and mutual electromagnetic dissociation of Pb nuclei at root s(NN) = 2.76 TeV with neutron emission are sigma(singleEMD) = 187.4 +/- 0.2(stat)(-11.2)(+13.2) (syst) b and sigma(mutualEMD) = 5. 7 +/- 0.1(stat) +/- 0.4(syst) b, respectively. The experimental results are compared to the predictions from a relativistic electromagnetic dissociation model. DOI: 10.1103/PhysRevLett.109.252302
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18.
  • Abelev, B., et al. (author)
  • Neutral pion and eta meson production in proton-proton collisions at root s=0.9 TeV and root s=7 TeV
  • 2012
  • In: Physics Letters. Section B: Nuclear, Elementary Particle and High-Energy Physics. - : Elsevier BV. - 0370-2693. ; 717:1-3, s. 162-172
  • Journal article (peer-reviewed)abstract
    • The first measurements of the invariant differential cross sections of inclusive pi(0) and eta meson production at mid-rapidity in proton-proton collisions root s = 0.9 TeV and root s = 7 TeV are reported. The pi(0) measurement covers the ranges 0.4 < p(T) < 7 GeV/c and 0.3 < p(T) < 25 GeV/c for these two energies, respectively. The production of eta mesons was measured at root s = 7 TeV in the range 0.4 < p(T) < 15 GeV/c. Next-to-Leading Order perturbative QCD calculations, which are consistent with the pi(0) spectrum at root s = 0.9 TeV, overestimate those of pi(0) and eta mesons at root s = 7 TeV, but agree with the measured eta/pi(0) ratio at root s = 7 TeV. (C) 2012 CERN. Published by Elsevier B.V. All rights reserved.
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19.
  • Abelev, B., et al. (author)
  • Measurement of event background fluctuations for charged particle jet reconstruction in Pb-Pb collisions at root s(NN)=2.76 TeV
  • 2012
  • In: Journal of High Energy Physics. - 1029-8479. ; :3
  • Journal article (peer-reviewed)abstract
    • The effect of event background fluctuations on charged particle jet reconstruction in Pb-Pb collisions at root s(NN) = 2.76 TeV has been measured with the ALICE experiment. The main sources of non-statistical fluctuations are characterized based purely on experimental data with an unbiased method, as well as by using single high p(t) particles and simulated jets embedded into real Pb-Pb events and reconstructed with the anti-k(t) jet finder. The influence of a low transverse momentum cut-off on particles used in the jet reconstruction is quantified by varying the minimum track p(t) between 0.15 GeV/c and 2 GeV/c. For embedded jets reconstructed from charged particles with p(t) > 0.15 GeV/c, the uncertainty in the reconstructed jet transverse momentum due to the heavy-ion background is measured to be 11.3 GeV/c (standard deviation) for the 10% most central Pb-Pb collisions, slightly larger than the value of 11.0 GeV/c measured using the unbiased method. For a higher particle transverse momentum threshold of 2 GeV/c, which will generate a stronger bias towards hard fragmentation in the jet finding process, the standard deviation of the fluctuations in the reconstructed jet transverse momentum is reduced to 4.8-5.0 GeV/c for the 10% most central events. A non-Gaussian tail of the momentum uncertainty is observed and its impact on the reconstructed jet spectrum is evaluated for varying particle momentum thresholds, by folding the measured fluctuations with steeply falling spectra.
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20.
  • Abelev, B., et al. (author)
  • J/psi Polarization in pp Collisions at root s=7 TeV
  • 2012
  • In: Physical Review Letters. - 1079-7114. ; 108:8
  • Journal article (peer-reviewed)abstract
    • The ALICE Collaboration has studied J/psi production in pp collisions at root s = 7 TeV at the LHC through its muon pair decay. The polar and azimuthal angle distributions of the decay muons were measured, and results on the J/psi polarization parameters lambda(theta) and lambda(phi) were obtained. The study was performed in the kinematic region 2: 5 < y < 4, 2 < p(t) < 8 GeV/c, in the helicity and Collins-Soper reference frames. In both frames, the polarization parameters are compatible with zero, within uncertainties.
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21.
  • Abelev, B., et al. (author)
  • Measurement of charm production at central rapidity in proton-proton collisions at root s=7 TeV
  • 2012
  • In: Journal of High Energy Physics. - 1029-8479. ; :1
  • Journal article (peer-reviewed)abstract
    • The p(t)-differential inclusive production cross sections of the prompt charmed mesons D-0, D+, and D*(+) in the rapidity range vertical bar y vertical bar < 0.5 were measured in proton-proton collisions at root s = 7 TeV at the LHC using the ALICE detector. Reconstructing the decays D-0 -> K-pi(+), D+ -> K-pi(+)pi(+), D*(+) -> D-0 pi(+), and their charge conjugates, about 8,400 D-0, 2,900 D+, and 2,600 D*(+) mesons with 1 < p(t) < 24 GeV/c were counted, after selection cuts, in a data sample of 3.14 x 10(8) events collected with a minimum-bias trigger (integrated luminosity L-int = 5 nb(-1)). The results are described within uncertainties by predictions based on perturbative QCD.
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22.
  • Aamodt, K., et al. (author)
  • Harmonic decomposition of two particle angular correlations in Pb-Pb collisions at root s(NN)=2.76 TeV
  • 2012
  • In: Physics Letters. Section B: Nuclear, Elementary Particle and High-Energy Physics. - : Elsevier BV. - 0370-2693. ; 708:3-5, s. 249-264
  • Journal article (peer-reviewed)abstract
    • Angular correlations between unidentified charged trigger (t) and associated (a) particles are measured by the ALICE experiment in Pb-Pb collisions at root s(NN) = 2.76 TeV for transverse momenta 0.25 < p(T)(t,a) < 15 GeV/c, where p(T)(t) > p(T)(a). The shapes of the pair correlation distributions are studied in a variety of collision centrality classes between 0 and 50% of the total hadronic cross section for particles in the pseudorapidity interval |eta| < 1.0. Distributions in relative azimuth Delta phi equivalent to phi(t) - phi(a) are analyzed for |Delta eta| equivalent to |eta(t) - eta(a)| > 0.8, and are referred to as "long-range correlations". Fourier components V-n Delta equivalent to < cos(n Delta phi)> are extracted from the long-range azimuthal correlation functions. If particle pairs are correlated to one another through their individual correlation to a common symmetry plane, then the pair anisotropy V-n Delta (p(T)(t), p(T)(a)) is fully described in terms of single-particle anisotropies v(n)(p(T)) as V-n Delta(p(T)(t), p(T)(a)) = v(n)(p(T)(t))v(n)(p(T)(a)). This expectation is tested for 1 <= n <= 5 by applying a global fit of all V-n Delta(p(T)(t), p(T)(a)) to obtain the best values v(n){GF}(p(T)). It is found that for 2 <= n <= 5, the fit agrees well with data up to p(T)(a) similar to 3-4 GeV/c, with a trend of increasing deviation as p(T)(t) and p(T)(a) are increased or as collisions become more peripheral. This suggests that no pair correlation harmonic can be described over the full 0.25 < p(T) < 15 GeV/c range using a single v(n)(p(T)) curve: such a description is however approximately possible for 2 <= n <= 5 when p(T)(a) < 4 GeV/c. For the n = 1 harmonic, however, a single v(1)(p(T)) curve is not obtained even within the reduced range p(T)(a) < 4 GeV/c. (C) 2012 CERN. Published by Elsevier B.V. All rights reserved.
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23.
  • Aamodt, K., et al. (author)
  • Particle-Yield Modification in Jetlike Azimuthal Dihadron Correlations in Pb-Pb Collisions at root S-NN=2.76 TeV
  • 2012
  • In: Physical Review Letters. - 1079-7114. ; 108:9
  • Journal article (peer-reviewed)abstract
    • The yield of charged particles associated with high-pt trigger particles (8 < p(t) < 15 GeV/c) is measured with the ALICE detector in Pb-Pb collisions at root S-NN = 2.76 TeV relative to proton-proton collisions at the same energy. The conditional per-trigger yields are extracted from the narrow jetlike correlation peaks in azimuthal dihadron correlations. In the 5% most central collisions, we observe that the yield of associated charged particles with transverse momenta p(t) > 3 GeV/c on the away side drops to about 60% of that observed in pp collisions, while on the near side a moderate enhancement of 20%-30% is found.
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24.
  • Aamodt, K., et al. (author)
  • Charged-Particle Multiplicity Density at Midrapidity in Central Pb-Pb Collisions at root s(NN)=2.76 TeV
  • 2010
  • In: Physical Review Letters. - 1079-7114. ; 105:25
  • Journal article (peer-reviewed)abstract
    • The first measurement of the charged-particle multiplicity density at midrapidity in Pb-Pb collisions at a center-of-mass energy per nucleon pair root s(NN) = 2.76 TeV is presented. For an event sample corresponding to the most central 5% of the hadronic cross section, the pseudorapidity density of primary charged particles at midrapidity is 1584 +/- 4(stat) +/- 76(syst), which corresponds to 8.3 +/- 0.4(syst) per participating nucleon pair. This represents an increase of about a factor 1.9 relative to pp collisions at similar collision energies, and about a factor 2.2 to central Au-Au collisions at root s(NN) = 0.2 TeV. This measurement provides the first experimental constraint for models of nucleus-nucleus collisions at LHC energies.
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25.
  • Aamodt, K., et al. (author)
  • Elliptic Flow of Charged Particles in Pb-Pb Collisions at root s(NN)=2.76 TeV
  • 2010
  • In: Physical Review Letters. - 1079-7114. ; 105:25
  • Journal article (peer-reviewed)abstract
    • We report the first measurement of charged particle elliptic flow in Pb-Pb collisions at root s(NN) p = 2.76 TeV with the ALICE detector at the CERN Large Hadron Collider. The measurement is performed in the central pseudorapidity region (vertical bar eta vertical bar < 0.8) and transverse momentum range 0.2 < p(t) < 5.0 GeV/c. The elliptic flow signal v(2), measured using the 4-particle correlation method, averaged over transverse momentum and pseudorapidity is 0.087 +/- 0.002(stat) +/- 0.003(syst) in the 40%-50% centrality class. The differential elliptic flow v(2)(p(t)) reaches a maximum of 0.2 near p(t) = 3 GeV/c. Compared to RHIC Au-Au collisions at root s(NN) = 200 GeV, the elliptic flow increases by about 30%. Some hydrodynamic model predictions which include viscous corrections are in agreement with the observed increase.
  •  
26.
  • Aamodt, K., et al. (author)
  • Femtoscopy of pp collisions at root s=0.9 and 7 TeV at the LHC with two-pion Bose-Einstein correlations
  • 2011
  • In: Physical Review D (Particles, Fields, Gravitation and Cosmology). - 1550-2368. ; 84:11
  • Journal article (peer-reviewed)abstract
    • We report on the high statistics two-pion correlation functions from pp collisions at root s = 0.9 TeV and root s = 7 TeV, measured by the ALICE experiment at the Large Hadron Collider. The correlation functions as well as the extracted source radii scale with event multiplicity and pair momentum. When analyzed in the same multiplicity and pair transverse momentum range, the correlation is similar at the two collision energies. A three-dimensional femtoscopic analysis shows an increase of the emission zone with increasing event multiplicity as well as decreasing homogeneity lengths with increasing transverse momentum. The latter trend gets more pronounced as multiplicity increases. This suggests the development of space-momentum correlations, at least for collisions producing a high multiplicity of particles. We consider these trends in the context of previous femtoscopic studies in high-energy hadron and heavy-ion collisions and discuss possible underlying physics mechanisms. Detailed analysis of the correlation reveals an exponential shape in the outward and longitudinal directions, while the sideward remains a Gaussian. This is interpreted as a result of a significant contribution of strongly decaying resonances to the emission region shape. Significant nonfemtoscopic correlations are observed, and are argued to be the consequence of "mini-jet"-like structures extending to low p(t). They are well reproduced by the Monte-Carlo generators and seen also in pi(+)pi(-) correlations.
  •  
27.
  • Aamodt, K., et al. (author)
  • Rapidity and transverse momentum dependence of inclusive J/psi production in pp collisions at root s=7 TeV
  • 2011
  • In: Physics Letters. Section B: Nuclear, Elementary Particle and High-Energy Physics. - : Elsevier BV. - 0370-2693. ; 704:5, s. 442-455
  • Journal article (peer-reviewed)abstract
    • The ALICE experiment at the LHC has studied inclusive J/psi production at central and forward rapidities in pp collisions at root s = 7 TeV. In this Letter, we report on the first results obtained detecting the J/psi through the dilepton decay into e(+)e(-) and mu(+)mu(-) pairs in the rapidity ranges vertical bar y vertical bar < 0.9 and 2.5 < y < 4, respectively, and with acceptance down to zero PT. In the dielectron channel the analysis was carried Out on a data sample corresponding to an integrated luminosity L-int = 5.6 nb(-1) and the number of signal events is N-J/psi = 352 +/- 32 (stat.) +/- 28 (syst.): the corresponding figures in the dimuon channel are L-int = 15.6 nb(-1) and N-J/psi = 1924 +/- 77 (stat.) +/- 144 (syst.). The measured production cross sections are sigma(J/psi) (vertical bar y vertical bar < 0.9) = 10.7 +/- 1.0 (stat.) +/- 1.6 (syst.)(-2.3)(+1.6) (syst.pol.) mu b and sigma(J/psi) (2.5 < y < 4) = 6.31 +/- 0.25 (stat.) +/- 0.76 (syst.)(-1.96)(+0.95) (syst.pol.) mu b. The differential cross sections, in transverse momentum and rapidity, of the J/psi were also measured. (C) 2011 CERN. Published by Elsevier B.V. All rights reserved.
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28.
  • Aamodt, K., et al. (author)
  • Strange particle production in proton-proton collisions at root s=0.9 TeV with ALICE at the LHC
  • 2011
  • In: European Physical Journal C. Particles and Fields. - : Springer Science and Business Media LLC. - 1434-6044. ; 71:3
  • Journal article (peer-reviewed)abstract
    • The production of mesons containing strange quarks (K-S(0), phi) and both singly and doubly strange baryons (Lambda, (Lambda) over bar, and Xi(-) + (Xi) over bar (+)) are measured at mid-rapidity in pp collisions at root s = 0.9 TeV with the ALICE experiment at the LHC. The results are obtained from the analysis of about 250 k minimum bias events recorded in 2009. Measurements of yields (dN/dy) and transverse momentum spectra at mid-rapidity for inelastic pp collisions are presented. For mesons, we report yields (< dN/dy >) of 0.184 +/- 0.002(stat.) +/- 0.006(syst.) for K-S(0) and 0.021 +/- 0.004(stat.) +/- 0.003(syst.) for phi. For baryons, we find < dN/dy > = 0.048 +/- 0.001(stat.) +/- 0.004(syst.) for Lambda, 0.047 +/- 0.002(stat.) +/- 0.005(syst.) for (Lambda) over bar and 0.0101 +/- 0.0020(stat.) +/- 0.0009(syst.) for Xi(-) + (Xi) over bar (+). The results are also compared with predictions for identified particle spectra from QCD-inspired models and provide a baseline for comparisons with both future pp measurements at higher energies and heavy-ion collisions.
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29.
  • Aamodt, K., et al. (author)
  • Centrality Dependence of the Charged-Particle Multiplicity Density at Midrapidity in Pb-Pb Collisions at root s(NN)=2.76 TeV
  • 2011
  • In: Physical Review Letters. - 1079-7114. ; 106:3
  • Journal article (peer-reviewed)abstract
    • The centrality dependence of the charged-particle multiplicity density at midrapidity in Pb-Pb collisions at root s(NN) = 2: 76 TeV is presented. The charged-particle density normalized per participating nucleon pair increases by about a factor of 2 from peripheral (70%-80%) to central (0%-5%) collisions. The centrality dependence is found to be similar to that observed at lower collision energies. The data are compared with models based on different mechanisms for particle production in nuclear collisions.
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30.
  • Aamodt, K., et al. (author)
  • Suppression of charged particle production at large transverse momentum in central Pb-Pb collisions at root s(NN)=2.76 TeV
  • 2011
  • In: Physics Letters. Section B: Nuclear, Elementary Particle and High-Energy Physics. - : Elsevier BV. - 0370-2693. ; 696:1-2, s. 30-39
  • Journal article (peer-reviewed)abstract
    • Inclusive transverse momentum spectra of primary charged particles in Pb-Pb collisions at root s(NN) = 2.76 TeV have been measured by the ALICE Collaboration at the LHC. The data are presented for central and peripheral collisions, corresponding to 0-5% and 70-80% of the hadronic Pb-Pb cross section. The measured charged particle spectra in |eta| < 0.8 and 0.3 < p(T) < 20 GeV/c are compared to the expectation in pp collisions at the same root s(NN), scaled by the number of underlying nucleon-nucleon collisions. The comparison is expressed in terms of the nuclear modification factor R-AA. The result indicates only weak medium effects (R-AA approximate to 0.7) in peripheral collisions. In central collisions, R-AA reaches a minimum of about 0.14 at p(T) = 6-7 GeV/c and increases significantly at larger p(T). The measured suppression of high-p(T) particles is stronger than that observed at lower collision energies, indicating that a very dense medium is formed in central Pb-Pb collisions at the LHC. (C) 2010 CERN. Published by Elsevier B.V. All rights reserved.
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31.
  • Aamodt, K., et al. (author)
  • Two-pion Bose-Einstein correlations in central Pb-Pb collisions at root(NN)-N-S=2.76 TeV
  • 2011
  • In: Physics Letters. Section B: Nuclear, Elementary Particle and High-Energy Physics. - : Elsevier BV. - 0370-2693. ; 696:4, s. 328-337
  • Journal article (peer-reviewed)abstract
    • The first measurement of two-pion Bose-Einstein correlations in central Pb-Pb collisions at root(NN)-N-S = 2.76 TeV at the Large Hadron Collider is presented. We observe a growing trend with energy now not only for the longitudinal and the outward but also for the sideward pion source radius. The pion homogeneity volume and the decoupling time are significantly larger than those measured at RHIC. (C) 2010 CERN. Published by Elsevier B.V. All rights reserved.
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32.
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33.
  • Prusti, T., et al. (author)
  • The Gaia mission
  • 2016
  • In: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 595
  • Journal article (peer-reviewed)abstract
    • Gaia is a cornerstone mission in the science programme of the European Space Agency (ESA). The spacecraft construction was approved in 2006, following a study in which the original interferometric concept was changed to a direct-imaging approach. Both the spacecraft and the payload were built by European industry. The involvement of the scientific community focusses on data processing for which the international Gaia Data Processing and Analysis Consortium (DPAC) was selected in 2007. Gaia was launched on 19 December 2013 and arrived at its operating point, the second Lagrange point of the Sun-Earth-Moon system, a few weeks later. The commissioning of the spacecraft and payload was completed on 19 July 2014. The nominal five-year mission started with four weeks of special, ecliptic-pole scanning and subsequently transferred into full-sky scanning mode. We recall the scientific goals of Gaia and give a description of the as-built spacecraft that is currently (mid-2016) being operated to achieve these goals. We pay special attention to the payload module, the performance of which is closely related to the scientific performance of the mission. We provide a summary of the commissioning activities and findings, followed by a description of the routine operational mode. We summarise scientific performance estimates on the basis of in-orbit operations. Several intermediate Gaia data releases are planned and the data can be retrieved from the Gaia Archive, which is available through the Gaia home page.
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34.
  • Brown, A. G. A., et al. (author)
  • Gaia Data Release 1 Summary of the astrometric, photometric, and survey properties
  • 2016
  • In: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 595
  • Journal article (peer-reviewed)abstract
    • Context. At about 1000 days after the launch of Gaia we present the first Gaia data release, Gaia DR1, consisting of astrometry and photometry for over 1 billion sources brighter than magnitude 20.7. Aims. A summary of Gaia DR1 is presented along with illustrations of the scientific quality of the data, followed by a discussion of the limitations due to the preliminary nature of this release. Methods. The raw data collected by Gaia during the first 14 months of the mission have been processed by the Gaia Data Processing and Analysis Consortium (DPAC) and turned into an astrometric and photometric catalogue. Results. Gaia DR1 consists of three components: a primary astrometric data set which contains the positions, parallaxes, and mean proper motions for about 2 million of the brightest stars in common with the HIPPARCOS and Tycho-2 catalogues - a realisation of the Tycho-Gaia Astrometric Solution (TGAS) - and a secondary astrometric data set containing the positions for an additional 1.1 billion sources. The second component is the photometric data set, consisting of mean G-band magnitudes for all sources. The G-band light curves and the characteristics of similar to 3000 Cepheid and RR Lyrae stars, observed at high cadence around the south ecliptic pole, form the third component. For the primary astrometric data set the typical uncertainty is about 0.3 mas for the positions and parallaxes, and about 1 mas yr(-1) for the proper motions. A systematic component of similar to 0.3 mas should be added to the parallax uncertainties. For the subset of similar to 94 000 HIPPARCOS stars in the primary data set, the proper motions are much more precise at about 0.06 mas yr(-1). For the secondary astrometric data set, the typical uncertainty of the positions is similar to 10 mas. The median uncertainties on the mean G-band magnitudes range from the mmag level to similar to 0.03 mag over the magnitude range 5 to 20.7. Conclusions. Gaia DR1 is an important milestone ahead of the next Gaia data release, which will feature five-parameter astrometry for all sources. Extensive validation shows that Gaia DR1 represents a major advance in the mapping of the heavens and the availability of basic stellar data that underpin observational astrophysics. Nevertheless, the very preliminary nature of this first Gaia data release does lead to a number of important limitations to the data quality which should be carefully considered before drawing conclusions from the data.
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35.
  • Clementini, G., et al. (author)
  • Testing parallaxes with local Cepheids and RR Lyrae stars
  • 2017
  • In: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 605
  • Journal article (peer-reviewed)abstract
    • Context. Parallaxes for 331 classical Cepheids, 31 Type II Cepheids, and 364 RR Lyrae stars in common between Gaia and the HIPPARCOS and Tycho-2 catalogues are published in Gaia Data Release 1 (DR1) as part of the Tycho-Gaia Astrometric Solution (TGAS). Aims. In order to test these first parallax measurements of the primary standard candles of the cosmological distance ladder, which involve astrometry collected by Gaia during the initial 14 months of science operation, we compared them with literature estimates and derived new period-luminosity (PL), period-Wesenheit (PW) relations for classical and Type II Cepheids and infrared PL, PL-metallicity (PLZ), and optical luminosity-metallicity (MV-[Fe/H]) relations for the RR Lyrae stars, with zero points based on TGAS.Methods. Classical Cepheids were carefully selected in order to discard known or suspected binary systems. The final sample comprises 102 fundamental mode pulsators with periods ranging from 1.68 to 51.66 days (of which 33 with sigma(omega)/omega < 0 : 5). The Type II Cepheids include a total of 26 W Virginis and BL Herculis stars spanning the period range from 1.16 to 30.00 days (of which only 7 with sigma(omega)/omega 0 : 5). The RR Lyrae stars include 200 sources with pulsation period ranging from 0.27 to 0.80 days (of which 112 with sigma(omega)/omega < 0 : 5). The new relations were computed using multi- band (V; I; J; K-s) photometry and spectroscopic metal abundances available in the literature, and by applying three alternative approaches: (i) linear least-squares fitting of the absolute magnitudes inferred from direct transformation of the TGAS parallaxes; (ii) adopting astrometry-based luminosities; and (iii) using a Bayesian fitting approach. The last two methods work in parallax space where parallaxes are used directly, thus maintaining symmetrical errors and allowing negative parallaxes to be used. The TGAS-based PL; PW; PLZ, and MV [Fe/H] relations are discussed by comparing the distance to the Large Magellanic Cloud provided by different types of pulsating stars and alternative fitting methods.Results. Good agreement is found from direct comparison of the parallaxes of RR Lyrae stars for which both TGAS and HST measurements are available. Similarly, very good agreement is found between the TGAS values and the parallaxes inferred from the absolute magnitudes of Cepheids and RR Lyrae stars analysed with the Baade-Wesselink method. TGAS values also compare favourably with the parallaxes inferred by theoretical model fitting of the multi-band light curves for two of the three classical Cepheids and one RR Lyrae star, which were analysed with this technique in our samples. The K-band PL relations show the significant improvement of the TGAS parallaxes for Cepheids and RR Lyrae stars with respect to the HIPPARCOS measurements. This is particularly true for the RR Lyrae stars for which improvement in quality and statistics is impressive.Conclusions. TGAS parallaxes bring a significant added value to the previous HIPPARCOS estimates. The relations presented in this paper represent the first Gaia-calibrated relations and form a work-in-progress milestone report in the wait for Gaia-only parallaxes of which a first solution will become available with Gaia Data Release 2 (DR2) in 2018.
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36.
  • van Leeuwen, F., et al. (author)
  • Gaia Data Release 1 : Open cluster astrometry: Performance, limitations, and future prospects
  • 2017
  • In: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 601
  • Journal article (peer-reviewed)abstract
    • Context. The first Gaia Data Release contains the Tycho-Gaia Astrometric Solution (TGAS). This is a subset of about 2 million stars for which, besides the position and photometry, the proper motion and parallax are calculated using Hipparcos and Tycho-2 positions in 1991.25 as prior information. Aims. We investigate the scientific potential and limitations of the TGAS component by means of the astrometric data for open clusters. Methods. Mean cluster parallax and proper motion values are derived taking into account the error correlations within the astrometric solutions for individual stars, an estimate of the internal velocity dispersion in the cluster, and, where relevant, the effects of the depth of the cluster along the line of sight. Internal consistency of the TGAS data is assessed. Results. Values given for standard uncertainties are still inaccurate and may lead to unrealistic unit-weight standard deviations of least squares solutions for cluster parameters. Reconstructed mean cluster parallax and proper motion values are generally in very good agreement with earlier Hipparcos-based determination, although the Gaia mean parallax for the Pleiades is a significant exception. We have no current explanation for that discrepancy. Most clusters are observed to extend to nearly 15 pc from the cluster centre, and it will be up to future Gaia releases to establish whether those potential cluster-member stars are still dynamically bound to the clusters. Conclusions. The Gaia DR1 provides the means to examine open clusters far beyond their more easily visible cores, and can provide membership assessments based on proper motions and parallaxes. A combined HR diagram shows the same features as observed before using the Hipparcos data, with clearly increased luminosities for older A and F dwarfs.
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37.
  • Recio-Blanco, A., et al. (author)
  • Gaia Data Release 3: Chemical cartography of the Milky Way
  • 2023
  • In: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 674
  • Journal article (peer-reviewed)abstract
    • Context. The motion of stars has been used to reveal details of the complex history of the Milky Way, in constant interaction with its environment. Nevertheless, to reconstruct the Galactic history puzzle in its entirety, the chemo-physical characterisation of stars is essential. Previous Gaia data releases were supported by a smaller, heterogeneous, and spatially biased mixture of chemical data from ground-based observations.Aims. Gaia Data Release 3 opens a new era of all-sky spectral analysis of stellar populations thanks to the nearly 5.6 million stars observed by the Radial Velocity Spectrometer (RVS) and parametrised by the GSP-Spec module. In this work, we aim to demonstrate the scientific quality of Gaia s Milky Way chemical cartography through a chemo-dynamical analysis of disc and halo populations.Methods. Stellar atmospheric parameters and chemical abundances provided by Gaia DR3 spectroscopy are combined with DR3 radial velocities and EDR3 astrometry to analyse the relationships between chemistry and Milky Way structure, stellar kinematics, and orbital parameters.Results. The all-sky Gaia chemical cartography allows a powerful and precise chemo-dynamical view of the Milky Way with unprecedented spatial coverage and statistical robustness. First, it reveals the strong vertical symmetry of the Galaxy and the flared structure of the disc. Second, the observed kinematic disturbances of the disc seen as phase space correlations and kinematic or orbital substructures are associated with chemical patterns that favour stars with enhanced metallicities and lower [α/Fe] abundance ratios compared to the median values in the radial distributions. This is detected both for young objects that trace the spiral arms and older populations. Several α, iron-peak elements and at least one heavy element trace the thin and thick disc properties in the solar cylinder. Third, young disc stars show a recent chemical impoverishment in several elements. Fourth, the largest chemo-dynamical sample of open clusters analysed so far shows a steepening of the radial metallicity gradient with age, which is also observed in the young field population. Finally, the Gaia chemical data have the required coverage and precision to unveil galaxy accretion debris and heated disc stars on halo orbits through their [α/Fe] ratio, and to allow the study of the chemo-dynamical properties of globular clusters. Conclusions. Gaia DR3 chemo-dynamical diagnostics open new horizons before the era of ground-based wide-field spectroscopic surveys. They unveil a complex Milky Way that is the outcome of an eventful evolution, shaping it to the present day.
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38.
  • Vallenari, A., et al. (author)
  • Gaia Data Release 3: Summary of the content and survey properties
  • 2023
  • In: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 674
  • Journal article (peer-reviewed)abstract
    • Context. We present the third data release of the European Space Agency's Gaia mission, Gaia DR3. This release includes a large variety of new data products, notably a much expanded radial velocity survey and a very extensive astrophysical characterisation of Gaia sources.Aims. We outline the content and the properties of Gaia DR3, providing an overview of the main improvements in the data processing in comparison with previous data releases (where applicable) and a brief discussion of the limitations of the data in this release.Methods. The Gaia DR3 catalogue is the outcome of the processing of raw data collected with the Gaia instruments during the first 34 months of the mission by the Gaia Data Processing and Analysis Consortium.Results. The Gaia DR3 catalogue contains the same source list, celestial positions, proper motions, parallaxes, and broad band photometry in the G, GBP, and G(RP) pass-bands already present in the Early Third Data Release, Gaia EDR3. Gaia DR3 introduces an impressive wealth of new data products. More than 33 million objects in the ranges G(RVS) < 14 and 3100 < T-eff < 14 500, have new determinations of their mean radial velocities based on data collected by Gaia. We provide GRVS magnitudes for most sources with radial velocities, and a line broadening parameter is listed for a subset of these. Mean Gaia spectra are made available to the community. The Gaia DR3 catalogue includes about 1 million mean spectra from the radial velocity spectrometer, and about 220 million low-resolution blue and red prism photometer BP /RP mean spectra. The results of the analysis of epoch photometry are provided for some 10 million sources across 24 variability types. Gaia DR3 includes astrophysical parameters and source class probabilities for about 470 million and 1500 million sources, respectively, including stars, galaxies, and quasars. Orbital elements and trend parameters are provided for some 800 000 astrometric, spectroscopic and eclipsing binaries. More than 150 000 Solar System objects, including new discoveries, with preliminary orbital solutions and individual epoch observations are part of this release. Reflectance spectra derived from the epoch BP /RP spectral data are published for about 60 000 asteroids. Finally, an additional data set is provided, namely the Gaia Andromeda Photometric Survey, consisting of the photometric time series for all sources located in a 5:5 degree radius field centred on the Andromeda galaxy.Conclusions. This data release represents a major advance with respect to Gaia DR2 and Gaia EDR3 because of the unprecedented quantity, quality, and variety of source astrophysical data. To date this is the largest collection of all-sky spectrophotometry, radial velocities, variables, and astrophysical parameters derived from both low- and high-resolution spectra and includes a spectrophotometric and dynamical survey of SSOs of the highest accuracy. The non-single star content surpasses the existing data by orders of magnitude. The quasar host and galaxy light profile collection is the first such survey that is all sky and space based. The astrophysical information provided in Gaia DR3 will unleash the full potential of Gaia's exquisite astrometric, photometric, and radial velocity surveys.
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39.
  • Klioner, S. A., et al. (author)
  • Gaia Early Data Release 3: The celestial reference frame (Gaia-CRF3)
  • 2022
  • In: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 667
  • Journal article (peer-reviewed)abstract
    • Context. Gaia-CRF3 is the celestial reference frame for positions and proper motions in the third release of data from the Gaia mission, Gaia DR3 (and for the early third release, Gaia EDR3, which contains identical astrometric results). The reference frame is defined by the positions and proper motions at epoch 2016.0 for a specific set of extragalactic sources in the (E)DR3 catalogue.Aims. We describe the construction of Gaia-CRF3 and its properties in terms of the distributions in magnitude, colour, and astrometric quality.Methods. Compact extragalactic sources in Gaia DR3 were identified by positional cross-matching with 17 external catalogues of quasi-stellar objects (QSO) and active galactic nuclei (AGN), followed by astrometric filtering designed to remove stellar contaminants. Selecting a clean sample was favoured over including a higher number of extragalactic sources. For the final sample, the random and systematic errors in the proper motions are analysed, as well as the radio-optical offsets in position for sources in the third realisation of the International Celestial Reference Frame (ICRF3).Results. Gaia-CRF3 comprises about 1.6 million QSO-like sources, of which 1.2 million have five-parameter astrometric solutions in Gaia DR3 and 0.4 million have six-parameter solutions. The sources span the magnitude range G = 13-21 with a peak density at 20.6 mag, at which the typical positional uncertainty is about 1 mas. The proper motions show systematic errors on the level of 12 μas yr-1 on angular scales greater than 15 deg. For the 3142 optical counterparts of ICRF3 sources in the S/X frequency bands, the median offset from the radio positions is about 0.5 mas, but it exceeds 4 mas in either coordinate for 127 sources. We outline the future of Gaia-CRF in the next Gaia data releases. Appendices give further details on the external catalogues used, how to extract information about the Gaia-CRF3 sources, potential (Galactic) confusion sources, and the estimation of the spin and orientation of an astrometric solution.
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40.
  • De Ridder, J., et al. (author)
  • Gaia Data Release 3: Pulsations in main sequence OBAF-type stars
  • 2023
  • In: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 674
  • Journal article (peer-reviewed)abstract
    • Context. The third Gaia data release provides photometric time series covering 34 months for about 10 million stars. For many of those stars, a characterisation in Fourier space and their variability classification are also provided. This paper focuses on intermediate- to high-mass (IHM) main sequence pulsators (M ≥  1.3 M⊙) of spectral types O, B, A, or F, known as β Cep, slowly pulsating B (SPB), δ Sct, and γ Dor stars. These stars are often multi-periodic and display low amplitudes, making them challenging targets to analyse with sparse time series.Aims. We investigate the extent to which the sparse Gaia DR3 data can be used to detect OBAF-type pulsators and discriminate them from other types of variables. We aim to probe the empirical instability strips and compare them with theoretical predictions. The most populated variability class is that of the δ Sct variables. For these stars, we aim to confirm their empirical period-luminosity (PL) relation, and verify the relation between their oscillation amplitude and rotation.Methods. All datasets used in this analysis are part of the Gaia DR3 data release. The photometric time series were used to perform a Fourier analysis, while the global astrophysical parameters necessary for the empirical instability strips were taken from the Gaia DR3 gspphot tables, and the v sin i data were taken from the Gaia DR3 esphs tables. The δ Sct PL relation was derived using the same photometric parallax method as the one recently used to establish the PL relation for classical Cepheids using Gaia data.Results. We show that for nearby OBAF-type pulsators, the Gaia DR3 data are precise and accurate enough to pinpoint them in the Hertzsprung-Russell (HR) diagram. We find empirical instability strips covering broader regions than theoretically predicted. In particular, our study reveals the presence of fast rotating gravity-mode pulsators outside the strips, as well as the co-existence of rotationally modulated variables inside the strips as reported before in the literature. We derive an extensive period–luminosity relation for δ Sct stars and provide evidence that the relation features different regimes depending on the oscillation period. We demonstrate how stellar rotation attenuates the amplitude of the dominant oscillation mode of δ Sct stars.Conclusions. The Gaia DR3 time-series photometry already allows for the detection of the dominant (non-)radial oscillation mode in about 100 000 intermediate- and high-mass dwarfs across the entire sky. This detection capability will increase as the time series becomes longer, allowing the additional delivery of frequencies and amplitudes of secondary pulsation modes.
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41.
  • Drimmel, R., et al. (author)
  • Gaia Data Release 3: Mapping the asymmetric disc of the Milky Way
  • 2023
  • In: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 674
  • Journal article (peer-reviewed)abstract
    • Context. With the most recent Gaia data release, the number of sources with complete 6D phase space information (position and velocity) has increased to well over 33 million stars, while stellar astrophysical parameters are provided for more than 470 million sources, and more than 11 million variable stars are identified.Aims. Using the astrophysical parameters and variability classifications provided in Gaia DR3, we selected various stellar populations to explore and identify non-axisymmetric features in the disc of the Milky Way in configuration and velocity space.Methods. Using more about 580 000 sources identified as hot OB stars, together with 988 known open clusters younger than 100 Myr, we mapped the spiral structure associated with star formation 4−5 kpc from the Sun. We selected over 2800 Classical Cepheids younger than 200 Myr that show spiral features extending as far as 10 kpc from the Sun in the outer disc. We also identified more than 8.7 million sources on the red giant branch (RGB), of which 5.7 million have line-of-sight velocities. This later sample allows the velocity field of the Milky Way to be mapped as far as 8 kpc from the Sun, including the inner disc.Results. The spiral structure revealed by the young populations is consistent with recent results using Gaia EDR3 astrometry and source lists based on near-infrared photometry, showing the Local (Orion) Arm to be at least 8 kpc long, and an outer arm consistent with what is seen in HI surveys, which seems to be a continuation of the Perseus arm into the third quadrant. The subset of RGB stars with velocities clearly reveals the large-scale kinematic signature of the bar in the inner disc, as well as evidence of streaming motions in the outer disc that might be associated with spiral arms or bar resonances. A local comparison of the velocity field of the OB stars reveals similarities and differences with the RGB sample.Conclusions. This cursory study of Gaia DR3 data shows there is a rich bounty of kinematic information to be explored more deeply, which will undoubtedly lead us to a clearer understanding of the dynamical nature of the non-axisymmetric structures of the Milky Way.
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42.
  • Galluccio, L., et al. (author)
  • Gaia Data Release 3: Reflectance spectra of Solar System small bodies
  • 2023
  • In: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 674
  • Journal article (peer-reviewed)abstract
    • Context. The Gaia mission of the European Space Agency (ESA) has been routinely observing Solar System objects (SSOs) since the beginning of its operations in August 2014. The Gaia data release three (DR3) includes, for the first time, the mean reflectance spectra of a selected sample of 60 518 SSOs, primarily asteroids, observed between August 5, 2014, and May 28, 2017. Each reflectance spectrum was derived from measurements obtained by means of the Blue and Red photometers (BP/RP), which were binned in 16 discrete wavelength bands. For every spectrum, the DR3 also contains additional information about the data quality for each band.Aims. We describe the processing of the Gaia spectral data of SSOs, explaining both the criteria used to select the subset of asteroid spectra published in Gaia DR3, and the different steps of our internal validation procedures. In order to further assess the quality of Gaia SSO reflectance spectra, we carried out external validation against SSO reflectance spectra obtained from ground-based and space-borne telescopes and available in the literature; we present our validation approach.Methods. For each selected SSO, an epoch reflectance was computed by dividing the calibrated spectrum observed by the BP/RP at each transit on the focal plane by the mean spectrum of a solar analogue. The latter was obtained by averaging the Gaia spectral measurements of a selected sample of stars known to have very similar spectra to that of the Sun. Finally, a mean of the epoch reflectance spectra was calculated in 16 spectral bands for each SSO.Results. Gaia SSO reflectance spectra are in general agreement with those obtained from a ground-based spectroscopic campaign specifically designed to cover the same spectral interval as Gaia and mimic the illumination and observing geometry characterising Gaia SSO observations. In addition, the agreement between Gaia mean reflectance spectra and those available in the literature is good for bright SSOs, regardless of their taxonomic spectral class. We identify an increase in the spectral slope of S-type SSOs with increasing phase angle. Moreover, we show that the spectral slope increases and the depth of the 1 μm absorption band decreases for increasing ages of S-type asteroid families. The latter can be interpreted as proof of progressive ageing of S-type asteroid surfaces due to their exposure to space weathering effects.
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43.
  • Lindegren, L., et al. (author)
  • Gaia Data Release 3: A golden sample of astrophysical parameters
  • 2023
  • In: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 674
  • Journal article (peer-reviewed)abstract
    • Context. Gaia Data Release 3 (DR3) provides a wealth of new data products for the astronomical community to exploit, including astrophysical parameters for half a billion stars. In this work, we demonstrate the high quality of these data products and illustrate their use in different astrophysical contexts.Aims. We produce homogeneous samples of stars with high-quality astrophysical parameters by exploiting Gaia DR3, while focusing on many regimes across the Hertzsprung-Russell (HR) diagram; spectral types OBA, FGKM, and ultracool dwarfs (UCDs). We also focus on specific subsamples of particular interest to the community: solar analogues, carbon stars, and the Gaia spectrophotometric standard stars (SPSS).Methods. We query the astrophysical parameter tables along with other tables in Gaia DR3 to derive the samples of the stars of interest. We validate our results using the Gaia catalogue itself and by comparison with external data.Results. We produced six homogeneous samples of stars with high-quality astrophysical parameters across the HR diagram for the community to exploit. We first focus on three samples that span a large parameter space: young massive disc stars (OBA; about 3 Million), FGKM spectral type stars (about 3 Million), and UCDs (about 20 000). We provide these sources along with additional information (either a flag or complementary parameters) as tables that are made available in the Gaia archive. We also identify 15 740 bone fide carbon stars and 5863 solar analogues, and provide the first homogeneous set of stellar parameters of the SPSS sample. We demonstrate some applications of these samples in different astrophysical contexts. We use a subset of the OBA sample to illustrate its usefulness in analysing the Milky Way rotation curve. We then use the properties of the FGKM stars to analyse known exoplanet systems. We also analyse the ages of some unseen UCD-companions to the FGKM stars. We additionally predict the colours of the Sun in various passbands (Gaia, 2MASS, WISE) using the solar-analogue sample.Conclusions. Gaia DR3 contains a wealth of new high-quality astrophysical parameters for the community to exploit.
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44.
  • Montegriffo, P., et al. (author)
  • Gaia Data Release 3: The Galaxy in your preferred colours: Synthetic photometry from Gaia low-resolution spectra
  • 2023
  • In: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 674
  • Journal article (peer-reviewed)abstract
    • Gaia Data Release 3 provides novel flux-calibrated low-resolution spectrophotometry for ≅220 million sources in the wavelength range 330 nm ≤ λ ≤ 1050 nm (XP spectra). Synthetic photometry directly tied to a flux in physical units can be obtained from these spectra for any passband fully enclosed in this wavelength range. We describe how synthetic photometry can be obtained from XP spectra, illustrating the performance that can be achieved under a range of different conditions - for example passband width and wavelength range - as well as the limits and the problems affecting it. Existing top-quality photometry can be reproduced within a few per cent over a wide range of magnitudes and colour, for wide and medium bands, and with up to millimag accuracy when synthetic photometry is standardised with respect to these external sources. Some examples of potential scientific application are presented, including the detection of multiple populations in globular clusters, the estimation of metallicity extended to the very metal-poor regime, and the classification of white dwarfs. A catalogue providing standardised photometry for ≅2.2×108 sources in several wide bands of widely used photometric systems is provided (Gaia Synthetic Photometry Catalogue; GSPC) as well as a catalogue of ≅105 white dwarfs with DA/non-DA classification obtained with a Random Forest algorithm (Gaia Synthetic Photometry Catalogue for White Dwarfs; GSPC-WD).
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45.
  • Lindegren, L., et al. (author)
  • Gaia Data Release 3: The extragalactic content
  • 2023
  • In: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 674
  • Journal article (peer-reviewed)abstract
    • The Gaia Galactic survey mission is designed and optimized to obtain astrometry, photometry, and spectroscopy of nearly two billion stars in our Galaxy. Yet as an all-sky multi-epoch survey, Gaia also observes several million extragalactic objects down to a magnitude of G 21 mag. Due to the nature of the Gaia onboard-selection algorithms, these are mostly point-source-like objects. Using data provided by the satellite, we have identified quasar and galaxy candidates via supervised machine learning methods, and estimate their redshifts using the low resolution BP/RP spectra. We further characterise the surface brightness profiles of host galaxies of quasars and of galaxies from pre-defined input lists. Here we give an overview of the processing of extragalactic objects, describe the data products in Gaia DR3, and analyse their properties. Two integrated tables contain the main results for a high completeness, but low purity (50-70%), set of 6.6 million candidate quasars and 4.8 million candidate galaxies. We provide queries that select purer sub-samples of these containing 1.9 million probable quasars and 2.9 million probable galaxies (both 95% purity). We also use high quality BP/RP spectra of 43 thousand high probability quasars over the redshift range 0.05-4.36 to construct a composite quasar spectrum spanning restframe wavelengths from 72 1000 nm.
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46.
  • Schultheis, M., et al. (author)
  • Gaia Data Release 3: Exploring and mapping the diffuse interstellar band at 862 nm
  • 2023
  • In: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 674
  • Journal article (peer-reviewed)abstract
    • Context. Diffuse interstellar bands (DIBs) are common interstellar absorption features in spectroscopic observations but their origins remain unclear. DIBs play an important role in the life cycle of the interstellar medium (ISM) and can also be used to trace Galactic structure.Aims. Here, we demonstrate the capacity of the Gaia-Radial Velocity Spectrometer (RVS) in Gaia DR3 to reveal the spatial distribution of the unknown molecular species responsible for the most prominent DIB at 862 nm in the RVS passband, exploring the Galactic ISM within a few kiloparsecs from the Sun.Methods. The DIBs are measured within the GSP-Spec module using a Gaussian profile fit for cool stars and a Gaussian process for hot stars. In addition to the equivalent widths and their uncertainties, Gaia DR3 provides their characteristic central wavelength, width, and quality flags.Results. We present an extensive sample of 476 117 individual DIB measurements obtained in a homogeneous way covering the entire sky. We compare spatial distributions of the DIB carrier with interstellar reddening and find evidence that DIB carriers are present in a local bubble around the Sun which contains nearly no dust. We characterised the DIB equivalent width with a local density of 0.19 ± 0.04 kpc1 and a scale height of 98.60 8.46+11.10 pc. The latter is smaller than the dust scale height, indicating that DIBs are more concentrated towards the Galactic plane. We determine the rest-frame wavelength with unprecedented precision (?0 = 8620.86 ± 0.019 in air) and reveal a remarkable correspondence between the DIB velocities and the CO gas velocities, suggesting that the 862 nm DIB carrier is related to macro-molecules. Conclusions. We demonstrate the unique capacity of Gaia to trace the spatial structure of the Galactic ISM using the 862 nm DIB.
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47.
  • Krone-Martins, A., et al. (author)
  • Gaia Focused Product Release: A catalogue of sources around quasars to search for strongly lensed quasars
  • 2024
  • In: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 685
  • Journal article (peer-reviewed)abstract
    • Context. Strongly lensed quasars are fundamental sources for cosmology. The Gaia space mission covers the entire sky with the unprecedented resolution of 0.18âà € ³ in the optical, making it an ideal instrument to search for gravitational lenses down to the limiting magnitude of 21. Nevertheless, the previous Gaia Data Releases are known to be incomplete for small angular separations such as those expected for most lenses.Aims. We present the Data Processing and Analysis Consortium GravLens pipeline, which was built to analyse all Gaia detections around quasars and to cluster them into sources, thus producing a catalogue of secondary sources around each quasar. We analysed the resulting catalogue to produce scores that indicate source configurations that are compatible with strongly lensed quasars.Methods. GravLens uses the DBSCAN unsupervised clustering algorithm to detect sources around quasars. The resulting catalogue of multiplets is then analysed with several methods to identify potential gravitational lenses. We developed and applied an outlier scoring method, a comparison between the average BP and RP spectra of the components, and we also used an extremely randomised tree algorithm. These methods produce scores to identify the most probable configurations and to establish a list of lens candidates.Results. We analysed the environment of 3 760 032 quasars. A total of 4 760 920 sources, including the quasars, were found within 6âà € ³ of the quasar positions. This list is given in the Gaia archive. In 87% of cases, the quasar remains a single source, and in 501 385 cases neighbouring sources were detected. We propose a list of 381 lensed candidates, of which we identified 49 as the most promising ones. Beyond these candidates, the associate tables in this Focused Product Release allow the entire community to explore the unique Gaia data for strong lensing studies further.
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48.
  • Brown, A. G. A., et al. (author)
  • Gaia Data Release 2 Summary of the contents and survey properties
  • 2018
  • In: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 616
  • Journal article (peer-reviewed)abstract
    • Context. We present the second Gaia data release, Gaia DR2, consisting of astrometry, photometry, radial velocities, and information on astrophysical parameters and variability, for sources brighter than magnitude 21. In addition epoch astrometry and photometry are provided for a modest sample of minor planets in the solar system. Aims. A summary of the contents of Gaia DR2 is presented, accompanied by a discussion on the differences with respect to Gaia DR1 and an overview of the main limitations which are still present in the survey. Recommendations are made on the responsible use of Gaia DR2 results. Methods. The raw data collected with the Gaia instruments during the first 22 months of the mission have been processed by the Gaia Data Processing and Analysis Consortium (DPAC) and turned into this second data release, which represents a major advance with respect to Gaia DR1 in terms of completeness, performance, and richness of the data products. Results. Gaia DR2 contains celestial positions and the apparent brightness in G for approximately 1.7 billion sources. For 1.3 billion of those sources, parallaxes and proper motions are in addition available. The sample of sources for which variability information is provided is expanded to 0 : 5 million stars. This data release contains four new elements: broad-band colour information in the form of the apparent brightness in the G(BP) (330-680 nm) and G(RP) (630-1050 nm) bands is available for 1.4 billion sources; median radial velocities for some 7 million sources are presented; for between 77 and 161 million sources estimates are provided of the stellar effective temperature, extinction, reddening, and radius and luminosity; and for a pre-selected list of 14 000 minor planets in the solar system epoch astrometry and photometry are presented. Finally, Gaia DR2 also represents a new materialisation of the celestial reference frame in the optical, the Gaia-CRF2, which is the first optical reference frame based solely on extragalactic sources. There are notable changes in the photometric system and the catalogue source list with respect to Gaia DR1, and we stress the need to consider the two data releases as independent. Conclusions. Gaia DR2 represents a major achievement for the Gaia mission, delivering on the long standing promise to provide parallaxes and proper motions for over 1 billion stars, and representing a first step in the availability of complementary radial velocity and source astrophysical information for a sample of stars in the Gaia survey which covers a very substantial fraction of the volume of our galaxy.
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49.
  • Katz, D., et al. (author)
  • Gaia Data Release 2 Mapping the Milky Way disc kinematics
  • 2018
  • In: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 616
  • Journal article (peer-reviewed)abstract
    • Context. The second Gaia data release (Gaia DR2) contains high-precision positions, parallaxes, and proper motions for 1.3 billion sources as well as line-of-sight velocities for 7.2 million stars brighter than G(RVS) = 12 mag. Both samples provide a full sky coverage. Aims. To illustrate the potential of Gaia DR2, we provide a first look at the kinematics of the Milky Way disc, within a radius of several kiloparsecs around the Sun. Methods. We benefit for the first time from a sample of 6.4 million F-G-K stars with full 6D phase-space coordinates, precise parallaxes (sigma((omega) over bar)/(omega) over bar <= 20%), and precise Galactic cylindrical velocities (median uncertainties of 0.9-1.4 km s(-1) and 20% of the stars with uncertainties smaller than 1 km s(-1) on all three components). From this sample, we extracted a sub-sample of 3.2 million giant stars to map the velocity field of the Galactic disc from similar to 5 kpc to similar to 13 kpc from the Galactic centre and up to 2 kpc above and below the plane. We also study the distribution of 0.3 million solar neighbourhood stars (r < 200 pc), with median velocity uncertainties of 0.4 km s(-1), in velocity space and use the full sample to examine how the over-densities evolve in more distant regions. Results. Gaia DR2 allows us to draw 3D maps of the Galactocentric median velocities and velocity dispersions with unprecedented accuracy, precision, and spatial resolution. The maps show the complexity and richness of the velocity field of the galactic disc. We observe streaming motions in all the components of the velocities as well as patterns in the velocity dispersions. For example, we confirm the previously reported negative and positive galactocentric radial velocity gradients in the inner and outer disc, respectively. Here, we see them as part of a non-axisymmetric kinematic oscillation, and we map its azimuthal and vertical behaviour. We also witness a new global arrangement of stars in the velocity plane of the solar neighbourhood and in distant regions in which stars are organised in thin substructures with the shape of circular arches that are oriented approximately along the horizontal direction in the U - V plane. Moreover, in distant regions, we see variations in the velocity substructures more clearly than ever before, in particular, variations in the velocity of the Hercules stream. Conclusions. Gaia DR2 provides the largest existing full 6D phase-space coordinates catalogue. It also vastly increases the number of available distances and transverse velocities with respect to Gaia DR1. Gaia DR2 offers a great wealth of information on the Milky Way and reveals clear non-axisymmetric kinematic signatures within the Galactic disc, for instance. It is now up to the astronomical community to explore its full potential.
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50.
  • Mignard, F., et al. (author)
  • Gaia Data Release 2 The celestial reference frame (Gaia-CRF2)
  • 2018
  • In: Astronomy and Astrophysics. - : EDP SCIENCES S A. - 0004-6361 .- 1432-0746. ; 616
  • Journal article (peer-reviewed)abstract
    • Context: The second release of Gaia data (Gaia DR2) contains the astrometric parameters for more than half a million quasars. This set defines a kinematically non-rotating reference frame in the optical domain. A subset of these quasars have accurate VLBI positions that allow the axes of the reference frame to be aligned with the International Celestial Reference System (ICRF) radio frame.Aims: We describe the astrometric and photometric properties of the quasars that were selected to represent the celestial reference frame of Gaia DR2 (Gaia-CRF2), and to compare the optical and radio positions for sources with accurate VLBI positions.Methods: Descriptive statistics are used to characterise the overall properties of the quasar sample. Residual rotation and orientation errors and large-scale systematics are quantified by means of expansions in vector spherical harmonics. Positional differences are calculated relative to a prototype version of the forthcoming ICRF3.Results: Gaia-CRF2 consists of the positions of a sample of 556 869 sources in Gaia DR2, obtained from a positional cross-match with the ICRF3-prototype and AllWISE AGN catalogues. The sample constitutes a clean, dense, and homogeneous set of extragalactic point sources in the magnitude range G similar or equal to 16 to 21 mag with accurately known optical positions. The median positional uncertainty is 0.12 mas for G < 18 mag and 0.5 mas at G = 20 mag. Large-scale systematics are estimated to be in the range 20 to 30 mu as. The accuracy claims are supported by the parallaxes and proper motions of the quasars in Gaia DR2. The optical positions for a subset of 2820 sources in common with the ICRF3-prototype show very good overall agreement with the radio positions, but several tens of sources have significantly discrepant positions.Conclusions: Based on less than 40% of the data expected from the nominal Gaia mission, Gaia-CRF2 is the first realisation of a non-rotating global optical reference frame that meets the ICRS prescriptions, meaning that it is built only on extragalactic sources. Its accuracy matches the current radio frame of the ICRF, but the density of sources in all parts of the sky is much higher, except along the Galactic equator.
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