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1.
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2.
  • Cumming, Robert, 1967, et al. (författare)
  • Stellar dynamics of blue compact galaxies. II. Further indications of a merger in ESO 338-IG04
  • 2008
  • Ingår i: Astronomy and Astrophysics. - 0004-6361 .- 1432-0746. ; 479:3, s. 725-733
  • Tidskriftsartikel (refereegranskat)abstract
    • Context: Luminous blue compact galaxies, common at ∼ 1 but now relatively rare, show disturbed kinematics in emission lines. Aims: As part of a programme to understand their formation and evolution, we have investigated the stellar dynamics of a number of nearby objects in this class. Methods: We obtained long-slit spectra with VLT/FORS2 in the spectral region covering the near-infrared calcium triplet. In this paper we focus on the well-known luminous blue compact galaxy ESO 338-IG04(Tololo 1924-416). A previous investigation, using Fabry-Perot interferometry, showed that this galaxy has a chaotic Hαvelocity field, indicating that either the galaxy is not in dynamical equilibrium or that Hαdoes not trace the gravitational potential due to feedback from star formation. Results: Along the apparent major axis, the stellar and ionised gas velocities for the most part follow each other. The chaotic velocity field must therefore be a sign that the young stellar population in ESO 338-IG04is not in dynamical equilibrium. The most likely explanation, which is also supported by its morphology, is that the galaxy has experienced a merger and that this has triggered the current starburst. Summarising the results of our programme so far, we note that emission-line velocity fields are not always reliable tracers of stellar motions, and go on to assess the implications for kinematic studies of similar galaxies at intermediate redshift. © 2008 ESO.
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3.
  • Östlin, G., et al. (författare)
  • Dynamical masses of two young globular clusters in the blue compact galaxy ESO 338-IG04
  • 2007
  • Ingår i: Astronomy and Astrophysics. - 0004-6361 .- 1432-0746. ; 461:2, s. 471-483
  • Tidskriftsartikel (refereegranskat)abstract
    • We present high-resolution échelle spectroscopy, obtained with the UVES spectrograph on ESO/VLT, of two luminous star clusters in the metal-poor blue compact galaxy ESO 338-IG04 at a distance of 37.5 Mpc. Cross-correlating with template stars, we obtain line-of-sight velocity dispersions of 33 and 17 km s-1. By combining with size estimates from Hubble Space Telescope images we infer dynamical masses of 1.3 × 107 M ⊙ and 4.0 × 106 M⊙ for the two clusters, making them among the most massive known. The less massive cluster is the faintest cluster for which a dynamical mass has yet been obtained. In both clusters we detect Balmer absorption lines which we use to estimate their ages. From the younger (∼6 Myr) and more massive cluster, we detect He II λ4686 emission of intermediate width, indicating the presence of very massive O-stars. Moreover, analysis of the [O III] λ5007 and Hα emission lines from the region near the younger cluster indicates that it is associated with a bubble expanding at ∼40 km s-1. We also see from the Na ID absorption lines indications of neutral gas flows towards the younger cluster. We compare the dynamical masses with those derived from photometry and discuss implications for the stellar initial mass function (IMF) in each cluster. Both clusters are compatible with rather normal IMFs which will favour their long-term survival and evolution into massive bona fide globular clusters. © ESO 2006.
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4.
  • Abramowski, A., et al. (författare)
  • H.E.S.S. detection of TeV emission from the interaction region between the supernova remnant G349.7+0.2 and a molecular cloud
  • 2015
  • Ingår i: Astronomy and Astrophysics. - 0004-6361 .- 1432-0746. ; 574, s. 1-7
  • Tidskriftsartikel (refereegranskat)abstract
    • G349.7+0.2 is a young Galactic supernova remnant (SNR) located at the distance of 11.5 kpc and observed across the entire electromagnetic spectrum from radio to high energy (HE; 0.1 GeV < E < 100 GeV) gamma-rays. Radio and infrared observations indicate that the remnant is interacting with a molecular cloud. In this paper, the detection of very high energy (VHE, E > 100 GeV) gamma-ray emission coincident with this SNR with the High Energy Stereoscopic System (HESS.) is reported. This makes it one of the farthest Galactic SNR ever detected in this domain. An integral flux F(E > 400 GeV) = (6.5 +/- 1.1(stat) +/- 1.3(syst)) x 10-11 ph cm(-2) s(-1) corresponding to similar to 0.7% of that of the Crab Nebula and to a luminosity of similar to 10(34) erg s(-1) above the same energy threshold, and a steep photon index Gamma(VHE) = 2.8 +/- 0.27(stat) +/- 0.20(syst) are measured. The analysis of more than 5 yr of Fermi-LAT data towards this source shows a power-law like spectrum with a best-fit photon index Gamma(HE) = 2.2 +/- 0.04.2(stat-0.31sys)(+0.13), The combined gamma-ray spectrum of 0349.7+0.2 can be described by either a broken power law (I3PL) or a power law with exponential (or sub exponential) cutoff (PLC). In the former case, the photon break energy is found at E-br,E-gamma = 551(-30)(+70) GeV, slightly higher than what is usually observed in the HE/VHE gamma-ray emitting middle-aged SNRs known to be interacting with molecular clouds. In the latter case. the exponential (respectively sub-exponential) cutoff energy is measured at E-cat,E-gamma = 1.4(-0.55)(+1.6) (respectively 0.35(-0.21)(+0.75)) TeV. A pion decay process resulting from the interaction of the accelerated protons and nuclei with the dense surrounding medium is clearly the preferred scenario to explain the gamma-ray emission. The BPL with a spectral steepening of 0.5-1 and the PLC provide equally good fits to the data. The product or the average gas density and the total energy content of accelerated protons and nuclei amounts to nu W-p similar to 5 x 10(51) erg cm(-3)
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5.
  • Israel, F.P., et al. (författare)
  • The millimeter wave continuum spectrum of Centaurus A and its nucleus
  • 2008
  • Ingår i: Astronomy and Astrophysics. - 0004-6361 .- 1432-0746. ; 483:3, s. 741-748
  • Tidskriftsartikel (refereegranskat)abstract
    • Aims. We study the radio emission mechanism of the FR-I AGN NGC 5128 (Centaurus A).Methods. We determine the centimeter and millimeter continuum spectrum of the whole Centaurus A radio source and measure at frequencies between 86 GHz (3.5 mm) and 345 GHz (0.85 mm) the continuum emission from the active radio galaxy nucleus at various times between 1989 and 2005.Results. The integral radio source spectrum becomes steeper at frequencies above 5 GHz, where the spectral index changes from alpha(low) = -0.70 to alpha(high) = -0.82. The SW outer lobe has a steeper spectrum than the NE middle and outer lobes (alpha = -1.0 vs. -0.6). Millimeter emission from the core of Centaurus A is variable, a variability that correlates appreciably better with the 20-200 keV X-ray variability than with 2-10 keV variability.Conclusions. In its quiescent state, the core has a spectral index alpha = -0.3, which steepens when the core brightens. The variability appears to be mostly associated with the inner nuclear jet components that have been detected in VLBI measurements. The densest nuclear components are optically thick below 45-80 GHz.
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6.
  • Olofsson, Henrik, 1972, et al. (författare)
  • A spectral line survey of Orion KL in the bands 486-492 and 541-577 GHz with the Odin satellite. I. The observational data
  • 2007
  • Ingår i: Astronomy and Astrophysics. - 0004-6361 .- 1432-0746. ; 476:number 2, December III, s. 791-806
  • Tidskriftsartikel (refereegranskat)abstract
    • Aims.Spectral line surveys are useful since they allow identification of new molecules and new lines in uniformly calibrated data sets. The subsequent multi-transition analysis will provide improved knowledge of molecular abundances, cloud temperatures and densities, and may also reveal previously unsuspected blends of molecular lines, which otherwise may lead to erroneous conclusions. Nonetheless, large portions of the sub-millimetre spectral regime remain unexplored due to severe absorptions by H2O and O2 in the terrestrial atmosphere. The purpose of the measurements presented here is to cover wavelength regions at and around 0.55 mm - regions largely unobservable from the ground.Methods.Using the Odin astronomy/aeronomy satellite, we performed the first spectral survey of the Orion KL molecular cloud core in the bands 486-492 and 541-576 GHz with rather uniform sensitivity (22-25 mK baseline noise). Odin's 1.1 m size telescope, equipped with four cryo-cooled tuneable mixers connected to broad band spectrometers, was used in a satellite position-switching mode. Two mixers simultaneously observed different 1.1 GHz bands using frequency steps of 0.5 GHz (25 h each). An on-source integration time of 20 h was achieved for most bands. The entire campaign consumed ~1100 orbits, each containing one hour of serviceable astro-observation.Results.We identified 280 spectral lines from 38 known interstellar molecules (including isotopologues) having intensities in the range 80 to 0.05 K. An additional 64 weak lines remain unidentified. Apart from the ground state rotational 11,0-10,1 transitions of ortho-H2O, H218O and H217O, the high energy 62,4-71,7 line of para-H2O (Eu=867$\,$K) and the HDO(20,2-11,1) line have been observed, as well as the 10-01 lines from NH3 and its rare isotopologue 15NH3. We suggest assignments for some unidentified features, notably the new interstellar molecules ND and SH-. Severe blends have been detected in the line wings of the H218O, H217O and 13CO lines changing the true linewidths of the outflow emission.
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7.
  • Persson, Carina, 1964, et al. (författare)
  • Water and ammonia abundances in S140 with the Odin satellite
  • 2009
  • Ingår i: Astronomy and Astrophysics. - 0004-6361 .- 1432-0746. ; 2:494, s. 637-646
  • Tidskriftsartikel (refereegranskat)abstract
    • We investigate the effect of the physical environment on water and ammonia abundances across the S140 photodissociation region (PDR) with an embedded outflow.We used the Odin satellite to obtain strip maps of the ground-state rotational transitions of ortho-water and ortho-ammonia, as well as CO(5-4) and 13co(5-4) across the PDR, and H_2^18O in the central position. A physi-chemicalinhomogeneous PDR model was used to compute the temperature and abundance distributions for water, ammonia, and CO. A multi-zone escape probability method then calculated the level populations and intensity distributions. These results are compared to a homogeneous model computed with an enhanced version of the RADEX code.H_2O, NH_3, and ^13CO show emission from an extended PDR with a narrow line width of ~3 km/s. Like CO, the water line profile is dominated by outflow emission, but mainly in the red wing. Even though CO shows strong self-absorption, no signs of self-absorption are seen in the water line. The H_2^18O molecule is not detected.The PDR model suggests that the water emission arises mainly from the surfaces of optically thick, high-density clumps with n(H_2)>10^6 cm^-3 and a clump water abundance, with respect to H_2, of 5*10^-8. The mean water abundance in the PDR is 5*10^-9 and between ~4*10^-8 - 4*10^-7 in the outflow derived from a simple two-level approximation.The RADEX model points to a somewhat higher average PDR water abundance of 1*10^-8. At low temperatures deep in the cloud, the water emission is weaker, likely due to adsorption onto dust grains, while ammonia is still abundant. Ammonia is also observed in the extended clumpy PDR, likely from the same high density and warm clumps as water. The average ammonia abundance is about the same as for water: 4*10^-9 and 8*10^-9 given by the PDR model and RADEX, respectively. The differences between the models most likely arise from uncertainties in density,beam-filling, and volume-filling of clumps. The similarity of water and ammonia PDR emission is also seen in the almost identical line profiles observed close to the bright rim. Around the central position, ammonia also shows some outflow emission, although weaker than water in the red wing. Predictions of the H_2O 1(1,0)-1(0,1) and 1(1,1)-0(0,0) antenna temperatures across the PDR are estimated with our PDR model for the forthcoming observations with the Herschel Space Observatory.
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8.
  • Röllig, M., et al. (författare)
  • A photon dominated region code comparison study
  • 2007
  • Ingår i: Astronomy and Astrophysics. - 0004-6361 .- 1432-0746. ; 467:No. 1 (May III 2007), s. 187-206
  • Tidskriftsartikel (refereegranskat)abstract
    • Aims.We present a comparison between independent computer codes, modeling the physics and chemistry of interstellar photon dominated regions (PDRs). Our goal was to understand the mutual differences in the PDR codes and their effects on the physical and chemical structure of the model clouds, and to converge the output of different codes to a common solution.Methods. A number of benchmark models have been created, covering low and high gas densities n = 103,105.5 cm-3 and far ultraviolet intensities $\chi$ = 10, 105 in units of the Draine field (FUV: 6
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9.
  • Adén, Daniel, et al. (författare)
  • An abundance study of red-giant-branch stars in the Hercules dwarf spheroidal galaxy
  • 2011
  • Ingår i: Astronomy & Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 525
  • Tidskriftsartikel (refereegranskat)abstract
    • Context. Dwarf spheroidal galaxies are some of the most metal-poor, and least luminous objects known. Detailed elemental abundance analysis of stars in these faint objects is key to our understanding of star formation and chemical enrichment in the early universe, and may provide useful information on how larger galaxies form. Aims. Our aim is to provide a determination of [Fe/H] and [Ca/H] for confirmed red-giant branch member stars of the Hercules dwarf spheroidal galaxy. Based on this we explore the ages of the prevailing stellar populations in Hercules, and the enrichment history from supernovae. Additionally, we aim to provide a new simple metallicity calibration for Stromgren photometry for metal-poor, red giant branch stars. Methods. High-resolution, multi-fibre spectroscopy and Stromgren photometry are combined to provide as much information on the stars as possible. From this we derive abundances by solving the radiative transfer equations through marcs model atmospheres. Results. We find that the red-giant branch stars of the Hercules dSph galaxy are more metal-poor than estimated in our previous study that was based on photometry alone. From this, we derive a new metallicity calibration for the Stromgren photometry. Additionally, we find an abundance trend such that [Ca/Fe] is higher for more metal-poor stars, and lower for more metal-rich stars, with a spread of about 0.8 dex. The [Ca/Fe] trend suggests an early rapid chemical enrichment through supernovae of type II, followed by a phase of slow star formation dominated by enrichment through supernovae of type Ia. A comparison with isochrones indicates that the red giants in Hercules are older than 10 Gyr.
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10.
  • Babusiaux, C., et al. (författare)
  • Observational Hertzsprung-Russell diagrams
  • 2018
  • Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 616
  • Tidskriftsartikel (refereegranskat)abstract
    • Context. Gaia Data Release 2 provides high-precision astrometry and three-band photometry for about 1.3 billion sources over the full sky. The precision, accuracy, and homogeneity of both astrometry and photometry are unprecedented. Aims. We highlight the power of the Gaia DR2 in studying many fine structures of the Hertzsprung-Russell diagram (HRD). Gaia allows us to present many different HRDs, depending in particular on stellar population selections. We do not aim here for completeness in terms of types of stars or stellar evolutionary aspects. Instead, we have chosen several illustrative examples. Methods. We describe some of the selections that can be made in Gaia DR2 to highlight the main structures of the Gaia HRDs. We select both field and cluster (open and globular) stars, compare the observations with previous classifications and with stellar evolutionary tracks, and we present variations of the Gaia HRD with age, metallicity, and kinematics. Late stages of stellar evolution such as hot subdwarfs, post-AGB stars, planetary nebulae, and white dwarfs are also analysed, as well as low-mass brown dwarf objects. Results. The Gaia HRDs are unprecedented in both precision and coverage of the various Milky Way stellar populations and stellar evolutionary phases. Many fine structures of the HRDs are presented. The clear split of the white dwarf sequence into hydrogen and helium white dwarfs is presented for the first time in an HRD. The relation between kinematics and the HRD is nicely illustrated. Two different populations in a classical kinematic selection of the halo are unambiguously identified in the HRD. Membership and mean parameters for a selected list of open clusters are provided. They allow drawing very detailed cluster sequences, highlighting fine structures, and providing extremely precise empirical isochrones that will lead to more insight in stellar physics. Conclusions. Gaia DR2 demonstrates the potential of combining precise astrometry and photometry for large samples for studies in stellar evolution and stellar population and opens an entire new area for HRD-based studies.
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