| 1. |
- Eriksson, Urban, 1968-, et al.
(författare)
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Limits of ultra-high-precision optical astrometry : stellar surface structures
- 2007
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 476:3, s. 1389-1400
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Tidskriftsartikel (refereegranskat)abstract
- Aims. To investigate the astrometric effects of stellar surface structures as a practical limitation to ultra-high-precision astrometry (e.g. in the context of exoplanet searches) and to quantify the expected effects in different regions of the HR-diagram. Methods. Stellar surface structures (spots, plages, granulation, non-radial oscillations) are likely to produce fluctuations in the integrated flux and radial velocity of the star, as well as a variation of the observed photocentre, i.e. astrometric jitter. We use theoretical considerations supported by Monte Carlo simulations (using a starspot model) to derive statistical relations between the corresponding astrometric, photometric, and radial velocity effects. Based on these relations, the more easily observed photometric and radial velocity variations can be used to predict the expected size of the astrometric jitter. Also the third moment of the brightness distribution, interferometrically observable as closure phase, contains information about the astrometric jitter. Results. For most stellar types the astrometric jitter due to stellar surface structures is expected to be of the order of 10 micro-AU or greater. This is more than the astrometric displacement typically caused by an Earth-size exoplanet in the habitable zone, which is about 1-4 micro-AU for long-lived main-sequence stars. Only for stars with extremely low photometric variability (< 0.5 mmag) and low magnetic activity, comparable to that of the Sun, will the astrometric jitter be of the order of 1 micro-AU, sufficient to allow the astrometric detection of an Earth-sized planet in the habitable zone. While stellar surface structure may thus seriously impair the astrometric detection of small exoplanets, it has in general a negligible impact on the detection of large (Jupiter-size) planets and on the determination of stellar parallax and proper motion. From the starspot model we also conclude that the commonly used spot filling factor is not the most relevant parameter for quantifying the spottiness in terms of the resulting astrometric, photometric and radial velocity variations.
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| 2. |
- Lindegren, Lennart, et al.
(författare)
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Gaia Early Data Release 3 : The Gaia Catalogue of Nearby Stars
- 2021
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 649
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Tidskriftsartikel (refereegranskat)abstract
- Aims. We produce a clean and well-characterised catalogue of objects within 100 pc of the Sun from the Gaia Early Data Release 3. We characterise the catalogue through comparisons to the full data release, external catalogues, and simulations. We carry out a first analysis of the science that is possible with this sample to demonstrate its potential and best practices for its use.Methods. Theselection of objects within 100 pc from the full catalogue used selected training sets, machine-learning procedures, astrometric quantities, and solution quality indicators to determine a probability that the astrometric solution is reliable. The training set construction exploited the astrometric data, quality flags, and external photometry. For all candidates we calculated distance posterior probability densities using Bayesian procedures and mock catalogues to define priors. Any object with reliable astrometry and a non-zero probability of being within 100 pc is included in the catalogue.Results. We have produced a catalogue of 331 312 objects that we estimate contains at least 92% of stars of stellar type M9 within 100 pc of the Sun. We estimate that 9% of the stars in this catalogue probably lie outside 100 pc, but when the distance probability function is used, a correct treatment of this contamination is possible. We produced luminosity functions with a high signal-to-noise ratio for the main-sequence stars, giants, and white dwarfs. We examined in detail the Hyades cluster, the white dwarf population, and wide-binary systems and produced candidate lists for all three samples. We detected local manifestations of several streams, superclusters, and halo objects, in which we identified 12 members of Gaia Enceladus. We present the first direct parallaxes of five objects in multiple systems within 10 pc of the Sun.Conclusions. We provide the community with a large, well-characterised catalogue of objects in the solar neighbourhood. This is a primary benchmark for measuring and understanding fundamental parameters and descriptive functions in astronomy.
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| 3. |
- Aghajani, T., et al.
(författare)
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Maximum likelihood estimation of local stellar kinematics
- 2013
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Ingår i: Astronomy & Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 551
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Tidskriftsartikel (refereegranskat)abstract
- Context. Kinematical data such as the mean velocities and velocity dispersions of stellar samples are useful tools to study galactic structure and evolution. However, observational data are often incomplete (e. g., lacking the radial component of the motion) and may have significant observational errors. For example, the majority of faint stars observed with Gaia will not have their radial velocities measured. Aims. Our aim is to formulate and test a new maximum likelihood approach to estimating the kinematical parameters for a local stellar sample when only the transverse velocities are known (from parallaxes and proper motions). Methods. Numerical simulations using synthetically generated data as well as real data (based on the Geneva-Copenhagen survey) are used to investigate the statistical properties (bias, precision) of the method, and to compare its performance with the much simpler "projection method" described by Dehnen & Binney (1998, MNRAS, 298, 387). Results. The maximum likelihood method gives more correct estimates of the dispersion when observational errors are important, and guarantees a positive-definite dispersion matrix, which is not always obtained with the projection method. Possible extensions and improvements of the method are discussed.
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| 4. |
- Antoja, T., et al.
(författare)
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Gaia Early Data Release 3: The Galactic anticentre
- 2021
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 1432-0746 .- 0004-6361. ; 649
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Tidskriftsartikel (refereegranskat)abstract
- Aims. We aim to demonstrate the scientific potential of the Gaia Early Data Release 3 (EDR3) for the study of different aspects of the Milky Way structure and evolution and we provide, at the same time, a description of several practical aspects of the data and examples of their usage. Methods. We used astrometric positions, proper motions, parallaxes, and photometry from EDR3 to select different populations and components and to calculate the distances and velocities in the direction of the anticentre. In this direction, the Gaia astrometric data alone enable the calculation of the vertical and azimuthal velocities; also, the extinction is relatively low compared to other directions in the Galactic plane. We then explore the disturbances of the current disc, the spatial and kinematical distributions of early accreted versus in situ stars, the structures in the outer parts of the disc, and the orbits of open clusters Berkeley 29 and Saurer 1. Results. With the improved astrometry and photometry of EDR3, we find that: (i) the dynamics of the Galactic disc are very complex with oscillations in the median rotation and vertical velocities as a function of radius, vertical asymmetries, and new correlations, including a bimodality with disc stars with large angular momentum moving vertically upwards from below the plane, and disc stars with slightly lower angular momentum moving preferentially downwards; (ii) we resolve the kinematic substructure (diagonal ridges) in the outer parts of the disc for the first time; (iii) the red sequence that has been associated with the proto-Galactic disc that was present at the time of the merger with Gaia-Enceladus-Sausage is currently radially concentrated up to around 14 kpc, while the blue sequence that has been associated with debris of the satellite extends beyond that; (iv) there are density structures in the outer disc, both above and below the plane, most probably related to Monoceros, the Anticentre Stream, and TriAnd, for which the Gaia data allow an exhaustive selection of candidate member stars and dynamical study; and (v) the open clusters Berkeley 29 and Saurer 1, despite being located at large distances from the Galactic centre, are on nearly circular disc-like orbits. Conclusions. Even with our simple preliminary exploration of the Gaia EDR3, we demonstrate how, once again, these data from the European Space Agency are crucial for our understanding of the different pieces of our Galaxy and their connection to its global structure and history. © ESO 2021.
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| 5. |
- Bachchan, Rajesh Kumar, et al.
(författare)
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Gaia reference frame amid quasar variability and proper motion patterns in the data
- 2016
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Ingår i: Astronomy & Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; , s. 1-12
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Tidskriftsartikel (refereegranskat)abstract
- Context. Gaia’s very accurate astrometric measurements will allow the optical realisation of the International Celestial Reference System to be improved by a few orders of magnitude. Several sets of quasars are used to define a kinematically stable non-rotating reference frame with the barycentre of the solar system as its origin. Gaia will also observe a large number of galaxies. Although they are not point-like, it may be possible to determine accurate positions and proper motions for some of their compact bright features. Aims. The optical stability of the quasars is critical, and we investigate how accurately the reference frame can be recovered. Various proper motion patterns are also present in the data, the best known is caused by the acceleration of the solar system barycentre, presumably, towards the Galactic centre. We review some other less well-known effects that are not part of standard astrometric models.Methods. We modelled quasars and galaxies using realistic sky distributions, magnitudes, and redshifts. Position variability was introduced using a Markov chain model. The reference frame was determined using the algorithm developed for the Gaia mission, which also determines the acceleration of the solar system. We also tested a method for measuring the velocity of the solar system barycentre in a cosmological frame.Results. We simulated the recovery of the reference frame and the acceleration of the solar system and conclude that they are not significantly disturbed by quasar variability, which is statistically averaged. However, the effect of a non-uniform sky distribution of the quasars can result in a correlation between the parameters describing the spin components of the reference frame and the acceleration components, which degrades the solution. Our results suggest that an attempt should be made to astrometrically determine the redshift- dependent apparent drift of galaxies that is due to our velocity relative to the cosmic microwave background, which in principle could allow determining the Hubble parameter.
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| 6. |
- Bombrun, A., et al.
(författare)
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A conjugate gradient algorithm for the astrometric core solution of Gaia
- 2012
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Ingår i: Astronomy & Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 538
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Tidskriftsartikel (refereegranskat)abstract
- Context. The ESA space astrometry mission Gaia, planned to be launched in 2013, has been designed to make angular measurements on a global scale with micro-arcsecond accuracy. A key component of the data processing for Gaia is the astrometric core solution, which must implement an efficient and accurate numerical algorithm to solve the resulting, extremely large least-squares problem. The Astrometric Global Iterative Solution (AGIS) is a framework that allows to implement a range of different iterative solution schemes suitable for a scanning astrometric satellite. Aims. Our aim is to find a computationally efficient and numerically accurate iteration scheme for the astrometric solution, compatible with the AGIS framework, and a convergence criterion for deciding when to stop the iterations. Methods. We study an adaptation of the classical conjugate gradient (CG) algorithm, and compare it to the so-called simple iteration (SI) scheme that was previously known to converge for this problem, although very slowly. The different schemes are implemented within a software test bed for AGIS known as AGISLab. This allows to define, simulate and study scaled astrometric core solutions with a much smaller number of unknowns than in AGIS, and therefore to perform a large number of numerical experiments in a reasonable time. After successful testing in AGISLab, the CG scheme has been implemented also in AGIS. Results. The two algorithms CG and SI eventually converge to identical solutions, to within the numerical noise (of the order of 0.00001 micro-arcsec). These solutions are moreover independent of the starting values (initial star catalogue), and we conclude that they are equivalent to a rigorous least-squares estimation of the astrometric parameters. The CG scheme converges up to a factor four faster than SI in the tested cases, and in particular spatially correlated truncation errors are much more efficiently damped out with the CG scheme. While it appears to be difficult to define a strict and robust convergence criterion, we have found that the sizes of the updates, and possibly the correlations between the updates in successive iterations, provide useful clues.
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| 7. |
- Bombrun, A., et al.
(författare)
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Complexity of the Gaia astrometric least-squares problem and the (non-)feasibility of a direct solution method
- 2010
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Ingår i: Astronomy & Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 516
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Tidskriftsartikel (refereegranskat)abstract
- The Gaia space astrometry mission (to be launched in 2012) will use a continuously spinning spacecraft to construct a global system of positions, proper motions and absolute parallaxes from relative position measurements made in an astrometric focal plane. This astrometric reduction can be cast as a classical least-squares problem, and the adopted baseline method for its solution uses a simple iteration algorithm. A potential weakness of this approach, as opposed to a direct solution, is that any finite number of iterations results in truncation errors that are difficult to quantify. Thus it is of interest to investigate alternative approaches, in particular the feasibility of a direct (non-iterative) solution. A simplified version of the astrometric reduction problem is studied in which the only unknowns are the astrometric parameters for a subset of the stars and the continuous three-axis attitude, thus neglecting further calibration issues. The specific design of the Gaia spacecraft and scanning law leads to an extremely large and sparse normal equations matrix. Elimination of the star parameters leads to a much smaller but less sparse system. We try different reordering schemes and perform symbolic Cholesky decomposition of this reduced normal matrix to study the fill-in for successively longer time span of simulated observations. Extrapolating to the full mission length, we conclude that a direct solution is not feasible with today's computational capabilities. Other schemes, e. g., eliminating the attitude parameters or orthogonalizing the observation equations, lead to similar or even worse problems. This negative result appears to be a consequence of the strong spatial and temporal connectivity among the unknowns achieved by two superposed fields of view and the scanning law, features that are in fact desirable and essential for minimizing large-scale systematic errors in the Gaia reference frame. We briefly consider also an approximate decomposition method a la Hipparcos, but conclude that it is either sub-optimal or effectively leads to an iterative solution.
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| 8. |
- Brown, A. G.A., et al.
(författare)
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Gaia Early Data Release 3: Summary of the contents and survey properties
- 2021
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 1432-0746 .- 0004-6361. ; 649
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Tidskriftsartikel (refereegranskat)abstract
- Context. We present the early installment of the third Gaia data release, Gaia EDR3, consisting of astrometry and photometry for 1.8 billion sources brighter than magnitude 21, complemented with the list of radial velocities from Gaia DR2. Aims. A summary of the contents of Gaia EDR3 is presented, accompanied by a discussion on the differences with respect to Gaia DR2 and an overview of the main limitations which are present in the survey. Recommendations are made on the responsible use of Gaia EDR3 results. Methods. The raw data collected with the Gaia instruments during the first 34 months of the mission have been processed by the Gaia Data Processing and Analysis Consortium and turned into this early third data release, which represents a major advance with respect to Gaia DR2 in terms of astrometric and photometric precision, accuracy, and homogeneity. Results. Gaia EDR3 contains celestial positions and the apparent brightness in G for approximately 1.8 billion sources. For 1.5 billion of those sources, parallaxes, proper motions, and the (GBP - GRP) colour are also available. The passbands for G, GBP, and GRP are provided as part of the release. For ease of use, the 7 million radial velocities from Gaia DR2 are included in this release, after the removal of a small number of spurious values. New radial velocities will appear as part of Gaia DR3. Finally, Gaia EDR3 represents an updated materialisation of the celestial reference frame (CRF) in the optical, the Gaia-CRF3, which is based solely on extragalactic sources. The creation of the source list for Gaia EDR3 includes enhancements that make it more robust with respect to high proper motion stars, and the disturbing effects of spurious and partially resolved sources. The source list is largely the same as that for Gaia DR2, but it does feature new sources and there are some notable changes. The source list will not change for Gaia DR3. Conclusions. Gaia EDR3 represents a significant advance over Gaia DR2, with parallax precisions increased by 30 per cent, proper motion precisions increased by a factor of 2, and the systematic errors in the astrometry suppressed by 30-40% for the parallaxes and by a factor ~2.5 for the proper motions. The photometry also features increased precision, but above all much better homogeneity across colour, magnitude, and celestial position. A single passband for G, GBP, and GRP is valid over the entire magnitude and colour range, with no systematics above the 1% level © ESO 2021.
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| 9. |
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| 10. |
- Butkevich, Alexey G., et al.
(författare)
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Impact of basic angle variations on the parallax zero point for a scanning astrometric satellite
- 2017
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 603
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Tidskriftsartikel (refereegranskat)abstract
- Context. Determination of absolute parallaxes by means of a scanning astrometric satellite such as Hipparcos or Gaia relies on the short-term stability of the so-called basic angle between the two viewing directions. Uncalibrated variations of the basic angle may produce systematic errors in the computed parallaxes. Aims. We examine the coupling between a global parallax shift and specific variations of the basic angle, namely those related to the satellite attitude with respect to the Sun. Methods. The changes in observables produced by small perturbations of the basic angle, attitude, and parallaxes were calculated analytically. We then looked for a combination of perturbations that had no net effect on the observables. Results. In the approximation of infinitely small fields of view, it is shown that certain perturbations of the basic angle are observationally indistinguishable from a global shift of the parallaxes. If these kinds of perturbations exist, they cannot be calibrated from the astrometric observations but will produce a global parallax bias. Numerical simulations of the astrometric solution, using both direct and iterative methods, confirm this theoretical result. For a given amplitude of the basic angle perturbation, the parallax bias is smaller for a larger basic angle and a larger solar aspect angle. In both these respects Gaia has a more favourable geometry than Hipparcos. In the case of Gaia, internal metrology is used to monitor basic angle variations. Additionally, Gaia has the advantage of detecting numerous quasars, which can be used to verify the parallax zero point.
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| 11. |
- Butkevich, A. G., et al.
(författare)
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Rigorous treatment of barycentric stellar motion Perspective and light-time effects in astrometric and radial velocity data
- 2014
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Ingår i: Astronomy & Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 570
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Tidskriftsartikel (refereegranskat)abstract
- Context. High-precision astrometric and radial-velocity observations require accurate modelling of stellar motions in order to extrapolate measurements over long time intervals, and to detect deviations from uniform motion caused, for example, by unseen companions. Aims. We aim to explore the simplest possible kinematic model of stellar motions, namely that of uniform rectilinear motion relative to the solar system barycentre, in terms of observable quantities including error propagation. Methods. The apparent path equation for uniform rectilinear motion is solved analytically in a classical (special-relativistic) framework, leading to rigorous expressions that relate the (apparent) astrometric parameters and radial velocity to the (true) kinematic parameters of the star in the barycentric reference system. Results. We present rigorous and explicit formulae for the transformation of stellar positions, parallaxes, proper motions, and radial velocities from one epoch to another, assuming uniform rectilinear motion and taking light-time effects into account. The Jacobian matrix of the transformation is also given, allowing accurate and reversible propagation of errors over arbitrary time intervals. The light-time effects are generally very small, but exceed 0.1 mas or 0.1 m s(-1) over 100 yr for at least 33 stars in the Hipparcos catalogue. For high-velocity stars within a few tens of pc from the Sun, light-time effects are generally more important than the effects of the curvature of their orbits in the Galactic potential.
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| 12. |
- Clementini, G., et al.
(författare)
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Gaia Data Release 1: Testing parallaxes with local Cepheids and RR Lyrae stars
- 2017
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 1432-0746 .- 0004-6361. ; 605
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Tidskriftsartikel (refereegranskat)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 σΩ/Ω< 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 σΩ/Ω< 0.5). The RR Lyrae stars include 200 sources with pulsation period ranging from 0.27 to 0.80 days (of which 112 with σΩ/Ω< 0.5). The new relations were computed using multi-band (V,I,J,Ks) 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. © ESO, 2017.
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| 13. |
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| 14. |
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| 15. |
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| 16. |
- Dravins, Dainis, et al.
(författare)
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Wavelength shifts in solar-type spectra
- 2005
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Ingår i: European Space Agency, (Special Publication) ESA SP. - 1609-042X .- 0379-6566. ; 560, s. 113-119
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Konferensbidrag (refereegranskat)abstract
- Spectral-line displacements away from the wavelengths naively expected from the Doppler shift caused by stellar radial motion may originate as convective shifts (correlated velocity and brightness patterns in the photosphere), as gravitational redshifts, or perhaps be induced by wave motions. Absolute lineshifts, in the past studied only for the Sun, are now accessible also for other stars thanks to astrometric determination of stellar radial motion, and spectrometers with accurate wavelength calibration. Comparisons between spectroscopic apparent radial velocities and astrometrically determined radial motions reveal greater spectral blueshifts in F-type stars than in the Sun (as theoretically expected from their more vigorous convection), further increasing in A-type stars (possibly due to atmospheric shockwaves). Work is in progress to survey the spectra of the Sun and several solar-type stars for "unblended" photospheric lines of most atomic species with accurate laboratory wavelengths available. One aim is to understand the ultimate information content of stellar spectra, and in what detail it will be feasible to verify models of stellar atmospheric hydrodynamics. These may predict bisectors and shifts for widely different classes of lines, but there will not result any comparison with observations if such lines do not exist in real spectra, or are too blended for meaningful measurement. An important near-future development to enable a further analysis of stellar surface structure will be the study of wavelength variations across spatially resolved stellar disks, e.g., the center-to-limb wavelength changes along a stellar diameter, and their spatially resolved time variability.
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| 17. |
- Eriksson, Urban, et al.
(författare)
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Limits of ultra-high-precision optical astrometry
- 2007
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Ingår i: Astronomy & Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 476, s. 1389-1400
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Tidskriftsartikel (refereegranskat)abstract
- Aims. To investigate the astrometric effects of stellar surface structures as a practical limitation to ultra-high-precision astrometry (e.g. in the context of exoplanet searches) and to quantify the expected effects in different regions of the HR-diagram. Methods. Stellar surface structures (spots, plages, granulation, non-radial oscillations) are likely to produce fluctuations in the integrated flux and radial velocity of the star, as well as a variation of the observed photocentre, i.e. astrometric jitter. We use theoretical considerations supported by Monte Carlo simulations (using a starspot model) to derive statistical relations between the corresponding astrometric, photometric, and radial velocity effects. Based on these relations, the more easily observed photometric and radial velocity variations can be used to predict the expected size of the astrometric jitter. Also the third moment of the brightness distribution, interferometrically observable as closure phase, contains information about the astrometric jitter. Results.For most stellar types the astrometric jitter due to stellar surface structures is expected to be of the order of 10 micro-AU or greater. This is more than the astrometric displacement typically caused by an Earth-size exoplanet in the habitable zone, which is about 1-4 micro-AU for long-lived main-sequence stars. Only for stars with extremely low photometric variability (<0.5 mmag) and low magnetic activity, comparable to that of the Sun, will the astrometric jitter be of the order of 1 micro-AU, sufficient to allow the astrometric detection of an Earth-sized planet in the habitable zone. While stellar surface structure may thus seriously impair the astrometric detection of small exoplanets, it has in general a negligible impact on the detection of large (Jupiter-size) planets and on the determination of stellar parallax and proper motion. From the starspot model we also conclude that the commonly used spot filling factor is not the most relevant parameter for quantifying the spottiness in terms of the resulting astrometric, photometric and radial velocity variations.
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| 18. |
- Eriksson, Urban, et al.
(författare)
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Limits of ultra-high-precision optical astrometry : stellar surface structures
- 2007
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 476:3, s. 1389-1400
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Tidskriftsartikel (refereegranskat)abstract
- Aims. To investigate the astrometric effects of stellar surface structures as a practical limitation to ultra-high-precision astrometry (e.g. in the context of exoplanet searches) and to quantify the expected effects in different regions of the HR-diagram. Methods. Stellar surface structures (spots, plages, granulation, non-radial oscillations) are likely to produce fluctuations in the integrated flux and radial velocity of the star, as well as a variation of the observed photocentre, i.e. astrometric jitter. We use theoretical considerations supported by Monte Carlo simulations (using a starspot model) to derive statistical relations between the corresponding astrometric, photometric, and radial velocity effects. Based on these relations, the more easily observed photometric and radial velocity variations can be used to predict the expected size of the astrometric jitter. Also the third moment of the brightness distribution, interferometrically observable as closure phase, contains information about the astrometric jitter. Results. For most stellar types the astrometric jitter due to stellar surface structures is expected to be of the order of 10 micro-AU or greater. This is more than the astrometric displacement typically caused by an Earth-size exoplanet in the habitable zone, which is about 1-4 micro-AU for long-lived main-sequence stars. Only for stars with extremely low photometric variability (< 0.5 mmag) and low magnetic activity, comparable to that of the Sun, will the astrometric jitter be of the order of 1 micro-AU, sufficient to allow the astrometric detection of an Earth-sized planet in the habitable zone. While stellar surface structure may thus seriously impair the astrometric detection of small exoplanets, it has in general a negligible impact on the detection of large (Jupiter-size) planets and on the determination of stellar parallax and proper motion. From the starspot model we also conclude that the commonly used spot filling factor is not the most relevant parameter for quantifying the spottiness in terms of the resulting astrometric, photometric and radial velocity variations.
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| 19. |
- Eriksson, Urban
(författare)
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Stellar Surface Structures and the Astrometric Serach for Exoplnaets
- 2007
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Measuring stellar parallax, position and proper motion is the task of astrometry. With the development of new and much more accurate equipment, different noise sources are likely to affect the very precise measurements made with future instruments. Some of these sources are: stellar surface structures, circumstellar discs, multiplicity and weak microlensing. Also exoplanets may act as a source of perturbation.In this thesis I present an investigation of stellar surface structures as a practical limitation to ultra-high-precision astrometry. The expected effects in different regions of the HR-diagram are quantified. I also investigate the astrometric effect of exoplanets, since their astrometric detection will be possible with future projects such as Gaia and SIM PlanetQuest.Stellar surface structures like spots, plages and granulation produce small surface areas of different temperatures, i.e. of different brightness, which will influence integrated properties such as the total flux (zeroth moment of the brightness distribution), radial velocity and photocenter position (first moments of the brightness distribution). Also the third central moment of the brightness distribution, interferometrically observable as closure phase, will vary due to irregularities in the brightness distribution. All these properties have been modelled, using both numerical simulations and analytical methods, and statistical relations between the variations of the different properties have been derived.Bright and/or dark surface areas, randomly spread over the stellar surface, will lead to a binomial distribution of the number of visible spots and the dispersion of such a model will be proportional topN, where N is the number of spots or surface structures. The dispersion will also be proportional to the size of each spot, A. The dispersions of the integrated properties are therefore expected to be/ ApN. It is noted that the commonly used spot filling factor, f / AN, is notthe most relevant characteristic of spottiness for these effects.Both the simulations and the analytic model lead to a set of statistical relations for the dispersions or variations of the integrated properties. With ,e.g. knowledge of the photometric variation, m, it is possible to statistically estimate the dispersions for the other integrated properties. Especially interesting is the variation of the observed photocenter, i.e. the astrometric jitter. A literature review was therefore made of the observed photometric and radial-velocity variations for various types of stars. This allowed to map the expected levels of astrometric jitter across the HR diagram.From the models it is clear that for most stellar types the astrometric jitter due to stellar surface structures is expected to be of order 10 μAU or greater. This is more than the astrometric displacement typically caused by an Earth-sized exoplanet in the habitable zone of a long-lived main-sequence star, which is about 1–4 μAU. Only for stars with extremely low photometric variability (< 0.5 mmag) and low magnetic activity, comparable to that of the Sun, will the astrometric jitter be of order 1 μAU, sufficient to allow astrometric detection of an Earth-sized planet in the habitable zone. While stellar surface structure may thus seriously impair the astrometric detection of small exoplanets, it has in general negligible impact on the detection of large (Jupiter-size) planets.
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| 20. |
- Fabricius, C, et al.
(författare)
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The spectro point spread function for GAIA
- 2002
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Ingår i: Astrophysics and Space Science. - 0004-640X. ; 280:1-2, s. 51-56
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Tidskriftsartikel (refereegranskat)abstract
- We discuss the point spread function for the Spectro instrument based on realistic effects of the time delayed integration, transverse image motion and pixelization, but assuming rather small wave front errors. Stars brighter than 10 mag, which will saturate the CCD, may be observed photometrically using the image wings, but the central wavelengths will be slightly shifted towards the red.
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| 21. |
- Gullberg, Dag, et al.
(författare)
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Determination of accurate stellar radial-velocity measures
- 2002
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Ingår i: Astronomy & Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 390:1, s. 383-395
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Tidskriftsartikel (refereegranskat)abstract
- Wavelength measurements in stellar spectra cannot readily be interpretedas true stellar motion on the sub-km s-1 accuracy level dueto the presence of many other effects, such as gravitational redshiftand stellar convection, which also produce line shifts. Following arecommendation by the IAU, the result of an accurate spectroscopicradial-velocity observation should therefore be given as the``barycentric radial-velocity measure'', i.e. the absolute spectralshift as measured by an observer at zero gravitational potential locatedat the solar-system barycentre. Standard procedures for reducingaccurate radial-velocity observations should be reviewed to take intoaccount this recommendation. We describe a procedure to determineaccurate barycentric radial-velocity measures of bright stars, based ondigital cross-correlation of spectra obtained with the ELODIEspectrometer (Observatoire de Haute-Provence) with a synthetic templateof Fe I lines. The absolute zero point of the radial-velocity measuresis linked to the wavelength scale of the Kurucz (1984) Solar Flux Atlasvia ELODIE observations of the Moon. Results are given for the Sun and42 stars, most of them members of the Hyades and Ursa Major clusters.The median internal standard error is 27 m s-1. The externalerror is estimated at around 120 m s-1, mainly reflecting theuncertainty in the wavelength scale of the Solar Flux Atlas. For the Sunwe find a radial-velocity measure of +257+/- 11 m s-1referring to the full-disk spectrum of the selected Fe I lines. Based onobservations made at Observatoire de Haute-Provence
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| 22. |
- Hibberd, A. A., et al.
(författare)
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Intestinal microbiota is altered in patients with colon cancer and modified by probiotic intervention
- 2017
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Ingår i: Bmj Open Gastroenterology. - : BMJ. - 2054-4774. ; 4:1
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Tidskriftsartikel (refereegranskat)abstract
- Objective: The colonic microbiota is altered in patients with colorectal cancer (CRC). We investigated the microbiota composition of patients with colon cancer compared with controls devoid of neoplastic or inflammatory disease and the potential to modify the colonic microbiota with probiotics. Design: Biopsy samples were obtained from the normal mucosa and tumour during colonoscopy from 15 patients with colon cancer. Subsequent patientmatched samples were taken at surgery from the tumour and nearby mucosa from the patients with cancer, eight of whom had received two daily tablets totalling 1.4x10(10) CFUs Bifidobacterium lactis Bl-04 and 7x10(9) CFUs Lactobacillus acidophilus NCFM. Faecal samples were obtained after colonoscopy prior to starting the intervention and at surgery. In addition, 21 mucosal biopsies from non-cancer controls were obtained during colonoscopy followed by later faecal samples. The colonic and faecal microbiota was assessed by 16S rRNA gene amplicon sequencing. Results: The tumour microbiota was characterised by increased microbial diversity and enrichment of several taxa including Fusobacterium, Selenomonas and Peptostreptococcus compared with the control microbiota. Patients with colon cancer that received probiotics had an increased abundance of butyrate-producing bacteria, especially Faecalibacterium and Clostridiales spp in the tumour, non-tumour mucosa and faecal microbiota. CRC-associated genera such as Fusobacterium and Peptostreptococcus tended to be reduced in the faecal microbiota of patients that received probiotics. Conclusions: Patients with colon cancer harbour a distinct microbiota signature in the tumour tissue and nearby mucosa, which was altered with probiotic intervention. Our results show promise for potential therapeutic benefits in CRC by manipulation of the microbiota.
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| 23. |
- Hobbs, David, et al.
(författare)
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All-sky visible and near infrared space astrometry
- 2021
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Ingår i: Experimental Astronomy. - : Springer Science and Business Media LLC. - 0922-6435 .- 1572-9508. ; 51:3, s. 783-843
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Tidskriftsartikel (refereegranskat)abstract
- The era of all-sky space astrometry began with the Hipparcos mission in 1989 and provided the first very accurate catalogue of apparent magnitudes, positions, parallaxes and proper motions of 120 000 bright stars at the milliarcsec (or milliarcsec per year) accuracy level. Hipparcos has now been superseded by the results of the Gaia mission. The second Gaia data release contained astrometric data for almost 1.7 billion sources with tens of microarcsec (or microarcsec per year) accuracy in a vast volume of the Milky Way and future data releases will further improve on this. Gaia has just completed its nominal 5-year mission (July 2019), but is expected to continue in operations for an extended period of an additional 5 years through to mid 2024. Its final catalogue to be released ∼ 2027, will provide astrometry for ∼ 2 billion sources, with astrometric precisions reaching 10 microarcsec. Why is accurate astrometry so important? The answer is that it provides fundamental data which underpin much of modern observational astronomy as will be detailed in this White Paper. All-sky visible and Near-InfraRed (NIR) astrometry with a wavelength cutoff in the K-band is not just focused on a single or small number of key science cases. Instead, it is extremely broad, answering key science questions in nearly every branch of astronomy while also providing a dense and accurate visible-NIR reference frame needed for future astronomy facilities.
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| 24. |
- Hobbs, David, et al.
(författare)
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Determining PPN gamma with Gaia's astrometric core solution
- 2010
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Ingår i: Relativity in Fundamental Astronomy: Dynamics, Reference Frames, and Data Analysis (IAU Symposium). - 1743-9221 .- 1743-9213. ; 261, s. 315-319
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Konferensbidrag (refereegranskat)abstract
- The ESA space astrometry mission Gala, due for launch in early 2012, will in addition to its huge output of fundamental astrometric and astrophysical data also provide stringent tests of general relativity. In this paper we present an updated analysis of Gaia's capacity to measure the PPN parameter gamma as part of its core astrometric solution. The analysis is based on small-scale astrometric solutions taking into account the simultaneous determination of stellar astrometric parameters and the satellite attitude. In particular, the statistical correlation between PPN gamma and the stellar parallaxes is considered. Extrapolating the results to a full-scale solution using some 100 million stars, we find that PPN gamma could be obtained to about 10(-6), which is significantly better than today's best estimate from the Cassini mission of 2 x 10(-5).
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| 25. |
- Hobbs, David, et al.
(författare)
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Gaia and the Astrometric Global Iterative Solution
- 2008
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Ingår i: Proceedings of IInternational Astronomical Union, IAU Symposium. ; 248, s. 119-120
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Konferensbidrag (refereegranskat)abstract
- Gaia is an ESA space astrometry mission due for launch in 2011-12. We describe part of the work carried out in the Gaia Data Processing and Analysis Consortium, namely the Astrometric Global Iterative Solution (AGIS) currently being implemented at the European Space Astronomy Center (ESAC) in Spain and largely based on algorithms developed at Lund Observatory. Some provisional results based on simulated observations of one million stars are presented, demonstrating convergence at microarcsec level independent of starting conditions.
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| 26. |
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| 27. |
- Holl, Berry, et al.
(författare)
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Characterizing the Astrometric Errors in the Gaia Catalogue
- 2011
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Ingår i: EAS Publications Series. - : EDP Sciences. - 1633-4760 .- 1638-1963. ; 45:GAIA: At the Frontiers of Astrometry, s. 117-122
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Tidskriftsartikel (refereegranskat)abstract
- Accurate characterization of the errors in the global astrometric solution for Gaia is essential for making optimal use of the catalogue data. We investigate the structure of the covariance between the estimated astrometric parameters by studying the properties of the astrometric least squares solution. We find that astrometric errors can be separated in a star and an attitude part, due to the estimation of the star and attitude parameters respectively. Hence the covariances can be separated in a star, an attitude and a cross term. This is demonstrated using our scalable simulation tool AGISLab, where the covariances are estimated statistically using Monte Carlo techniques.
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| 28. |
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| 29. |
- Holl, Berry, et al.
(författare)
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Error characterization of the Gaia astrometric solution I. Mathematical basis of the covariance expansion model
- 2012
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Ingår i: Astronomy & Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 543
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Tidskriftsartikel (refereegranskat)abstract
- Context. Accurate characterization of the astrometric errors in the forthcoming Gaia Catalogue will be essential for making optimal use of the data. This includes the correlations among the estimated astrometric parameters of the stars as well as their standard uncertainties, i.e., the complete (variance-)covariance matrix of the relevant astrometric parameters. Aims. Because a direct computation of the covariance matrix is infeasible due to the large number of parameters, approximate methods must be used. The aim of this paper is to provide a mathematical basis for estimating the variance-covariance of any pair of astrometric parameters, and more generally the covariance matrix for multidimensional functions of the astrometric parameters. The validation of this model by means of numerical simulations will be considered in a forthcoming paper. Methods. Based on simplifying assumptions (in particular that calibration errors can be neglected), we derive and analyse a series expansion of the covariance matrix of the least-squares solution. A recursive relation for successive terms is derived and interpreted in terms of the propagation of errors from the stars to the attitude and back. We argue that the expansion should converge rapidly to useful precision. The recursion is vastly simplified by using a kinematographic (step-wise) approximation of the attitude model. Results. Low-order approximations of arbitrary elements from the covariance matrix can be computed efficiently in terms of a limited amount of pre-computed data representing compressed observations and the structural relationships among them. It is proposed that the user interface to the Gaia Catalogue should provide the tools necessary for such computations.
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| 30. |
- Holl, Berry, et al.
(författare)
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Error characterization of the Gaia astrometric solution II. Validating the covariance expansion model
- 2012
-
Ingår i: Astronomy & Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 543
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Tidskriftsartikel (refereegranskat)abstract
- Context. To use the data in the future Gaia catalogue it is important to have accurate estimates of the statistical uncertainties and correlations of the errors in the astrometric data given in the catalogue. Aims. In a previous paper we derived a mathematical model for computing the covariances of the astrometric data based on series expansions and a simplified attitude description. The aim of the present paper is to determine to what extent this model provides an accurate representation of the expected random errors in the astrometric solution for Gaia. Methods. We simulate the astrometric core solution by making least-squares solutions of the astrometric parameters for one million stars and the attitude parameters for a five-year mission, using nearly one billion simulated elementary observations for a total of 26 million unknowns. Two cases are considered: one in which all stars have the same magnitude, and another with 30% brighter and 70% fainter stars. The resulting astrometric errors are statistically compared with the model predictions. Results. In all cases considered, and within the statistical uncertainties of the numerical experiments (typically below 0.4%), the theoretically calculated variances and covariances are consistent with the simulations. To achieve this it is however necessary to expand the covariances to at least third or fourth order, and to apply a (theoretically motivated and derived) "fudge factor" in the kinematographic model. Conclusions. The model provides a feasible method to estimate the covariance of arbitrary astrometric data, accurate enough for most applications, and as such it should be available as part of the user's interface to the Gaia catalogue. A main assumption in the current model is that the observational errors are uncorrelated (e.g., photon noise), and further studies are needed on how correlated modelling errors, in particular in the attitude, can be taken into account.
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| 31. |
- Holl, Berry, et al.
(författare)
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Spatial correlations in the Gaia astrometric solution
- 2010
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Ingår i: Relativity in Fundamental Astronomy: Dynamics, Reference Frames, and Data Analysis (IAU Symposium). - 1743-9213 .- 1743-9221. ; 261, s. 320-324
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Konferensbidrag (refereegranskat)abstract
- Accurate characterization of the astrometric errors in the forthcoming Gaia catalogue is essential for making optimal use of the data. Using small-scale numerical simulations of the astrometric solution, we investigate the expected spatial correlation between the astrometric errors of stars as function of their angular separation. Extrapolating to the full-scale solution for the final Gala catalogue, we find that the expected correlations are generally very small, but could reach some fraction of a percent for angular separations smaller than about one degree. The spatial correlation length is related to the size of the field of view of Gala., while the maximum correlation coefficient is related to the mean number of stars present in the field at any time. Our scalable simulation tool (AGISLab) makes it possible to characterize the astrometric errors and correlations, e.g., as functions of position and magnitude.
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| 32. |
- Holl, Berry, et al.
(författare)
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The impact of CCD radiation damage on Gaia astrometry - II. Effect of image location errors on the astrometric solution
- 2012
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Ingår i: Monthly Notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 1365-2966 .- 0035-8711. ; 422:4, s. 2786-2807
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Tidskriftsartikel (refereegranskat)abstract
- Gaia, the next astrometric mission of the European Space Agency, will use a camera composed of 106 CCDs to collect multiple observations for one billion stars. The astrometric core solution of Gaia will use the estimated location of the stellar images on the CCDs to derive the astrometric parameters (position, parallax and proper motion) of the stars. The Gaia CCDs will suffer from charge transfer inefficiency (CTI) mainly caused by radiation damage. CTI is expected to significantly degrade the quality of the collected images which ultimately affects the astrometric accuracy of Gaia. This paper is the second and last in a study aiming at characterizing and quantifying the impact of CCD radiation damage on Gaia astrometry. Here we focus on the effect of the image location errors induced by CTI on the astrometric solution. We apply the Gaia Astrometric Global Iterative Solution (AGIS) to simulated Gaia-like observations for 1 million stars including CTI-induced errors as described in the first paper. We show that a magnitude-dependent image location bias is propagated in the astrometric solution, biasing the estimation of the astrometric parameters as well as decreasing its precision. We demonstrate how the Gaia scanning law dictates this propagation and the ultimate sky distribution of the CTI-induced errors. The possibility of using the residuals of the astrometric solution to improve the calibration of the CTI effects is investigated. We also estimate the astrometric errors caused by (faint) disturbing stars preceding the stellar measurements on the CCDs. Finally, we show that, for single stars, the overall astrometric accuracy of Gaia can be preserved to within 10 per cent of the CTI-free case for all magnitudes by appropriate modelling at the image location estimation level and using the solution residuals.
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| 33. |
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| 34. |
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| 35. |
- Holmberg, Gustav, et al.
(författare)
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Kvinnliga pionjärer
- 2003
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Ingår i: Lundaögon mot stjärnorna: astronomin i Lund under fem sekler. - 9163134462
-
Bokkapitel (övrigt vetenskapligt/konstnärligt)
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| 36. |
- Howes, Louise M., et al.
(författare)
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Estimating stellar ages and metallicities from parallaxes and broadband photometry : Successes and shortcomings
- 2019
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 622
-
Tidskriftsartikel (refereegranskat)abstract
- A deep understanding of the Milky Way galaxy, its formation and evolution requires observations of huge numbers of stars. Stellar photometry, therefore, provides an economical method to obtain intrinsic stellar parameters. With the addition of distance information - a prospect made real for more than a billion stars with the second Gaia data release - deriving reliable ages from photometry is a possibility. We have developed a Bayesian method that generates 2D probability maps of a star's age and metallicity from photometry and parallax using isochrones. Our synthetic tests show that including a near-UV passband enables us to break the degeneracy between a star's age and metallicity for certain evolutionary stages. It is possible to find well-constrained ages and metallicities for turn-off and sub-giant stars with colours including a U band and a parallax with uncertainty less than ∼20%. Metallicities alone are possible for the main sequence and giant branch. We find good agreement with the literature when we apply our method to the Gaia benchmark stars, particularly for turn-off and young stars. Further tests on the old open cluster NGC 188, however, reveal significant limitations in the stellar isochrones. The ages derived for the cluster stars vary with evolutionary stage, such that turn-off ages disagree with those on the sub-giant branch, and metallicities vary significantly throughout. Furthermore, the parameters vary appreciably depending on which colour combinations are used in the derivation. We identify the causes of these mismatches and show that improvements are needed in the modelling of giant branch stars and in the creation and calibration of synthetic near-UV photometry. Our results warn against applying isochrone fitting indiscriminately. In particular, the uncertainty on the stellar models should be quantitatively taken into account. Further efforts to improve the models will result in significant advancements in our ability to study the Galaxy.
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| 37. |
- Jordi, C, et al.
(författare)
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The design and performance of the Gaia photometric system
- 2006
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Ingår i: Monthly Notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 1365-2966 .- 0035-8711. ; 367:1, s. 290-314
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Tidskriftsartikel (refereegranskat)abstract
- The European Gaia astrometry mission is due for launch in 2011. Gaia will rely on the proven principles of the ESA Hipparcos mission to create an all-sky survey of about one billion stars throughout our Galaxy and beyond, by observing all objects down to 20 mag. Through its massive measurement of stellar distances, motions and multicolour photometry, it will provide fundamental data necessary for unravelling the structure, formation and evolution of the Galaxy. This paper presents the design and performance of the broad- and medium-band set of photometric filters adopted as the baseline for Gaia. The 19 selected passbands (extending from the UV to the far-red), the criteria and the methodology on which this choice has been based are discussed in detail. We analyse the photometric capabilities for characterizing the luminosity, temperature, gravity and chemical composition of stars. We also discuss the automatic determination of these physical parameters for the large number of observations involved, for objects located throughout the entire Hertzsprung-Russell diagram. Finally, the capability of the photometric system (PS) to deal with the main Gaia science case is outlined.
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| 38. |
- Jørgensen, Bjarne Rosenkilde, et al.
(författare)
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Determination of star formation histories from gaia-type photometric and astrometric survey data
- 2005
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Ingår i: European Space Agency, (Special Publication) ESA SP. - 0379-6566 .- 1609-042X. ; 576, s. 171-173
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Konferensbidrag (refereegranskat)abstract
- To understand the evolution of the Milky Way Galaxy requires detailed knowledge of the star formation history of various populations. The vast amounts of photometric and astrometric data provided by the Gaia mission give unprecedented opportunities in this area. The relationships between the observed data and the ages of stars are however complex and highly non-linear and great care must be taken in analyzing the data. We describe a Bayesian approach to calculate the star formation rate (SFR) from astrophysical data, using a genetic algorithm to solve the basic integral equation. We present simulations showing that the method is capable of resolving structures in the SFR that cannot be seen from a distribution of the individually estimated stellar ages.
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| 39. |
- Jørgensen, Bjarne Rosenkilde, et al.
(författare)
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Determination of stellar ages from isochrones: Bayesian estimation versus isochrone fitting
- 2005
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Ingår i: Astronomy & Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 436:1, s. 127-143
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Tidskriftsartikel (refereegranskat)abstract
- We present a new method, using Bayesian estimation, to determine stellar ages and their uncertainties from observational data and theoretical isochrones. The result for an individual star is obtained as the relative posterior probability density as function of the age ("G function"). From this can be derived the most probable age and confidence intervals. The convoluted morphology of isochrones and strong non-linearities make the age determination by any method difficult and susceptible to statistical biases, and as a result age uncertainties havee often been underestimated in the literature. From simulations we find that the G functions provide a general, robust and reliable way to quantify age information. Resulting age estimates are at least as accurate as those obtained with conventional isochrone fitting methods, and in some cases much better, especially when the observational uncertainties are large. We also find that undetected binaries, on the whole, have a surprisingly small effect on the age determinations. For a stellar sample, the individual G functions can be combined to derive the star formation history of the population; this will be developed in a forthcoming paper. For a coeval population the combination simplifies to computing the product of the individual G functions, and we apply that method to estimate the ages of the two open clusters IC 4651 and M67, using Padova isochrones and photometric data from the literature. For IC 4651 we find an estimated age of 1.56 +/- 0.03 Gyr, assuming a true distance modulus of 9.80. For M67 we find 4.05 +/- 0.05 Gyr for true distance modulus 9.48. The small formal errors of these age estimates do not include the ( much larger) uncertainties from calibration and model errors, but illustrate the statistical power of combining G functions. Our statistical approach to the age determination problem is well suited for the mass treatment of data resulting from large-scale surveys such as the Gaia mission.
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| 40. |
- Katz, D., et al.
(författare)
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Mapping the Milky Way disc kinematics
- 2018
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 1432-0746 .- 0004-6361. ; 616
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Tidskriftsartikel (refereegranskat)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 GRVS = 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 (σω/ω/≤ 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 ∼5 kpc 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. © ESO 2018.
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| 41. |
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| 42. |
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| 43. |
- Kucinskas, Arunas, et al.
(författare)
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Broad-band photometric colors and effective temperature calibrations for late-type giants - I. Z=0.02
- 2005
-
Ingår i: Astronomy & Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 442:1, s. 281-308
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Tidskriftsartikel (refereegranskat)abstract
- We present new synthetic broad-band photometric colors for late-type giants based on synthetic spectra calculated with the PHOENIX model atmosphere code. The grid covers effective temperatures T-eff = 3000... 5000 K, gravities log g = - 0.5... + 3.5, and metallicities [M/H] = + 0.5... - 4.0. We show that individual broad-band photometric colors are strongly affected by model parameters such as molecular opacities, gravity, microturbulent velocity, and stellar mass. Our exploratory 3D modeling of a prototypical late-type giant shows that convection has a noticeable effect on the photometric colors too, as it alters significantly both the vertical and horizontal thermal structures in the outer atmosphere. The differences between colors calculated with full 3D hydrodynamical and 1D model atmospheres are significant (e.g.,. (V- K) similar to 0.2 mag), translating into offsets in effective temperature of up to similar to 70 K. For a sample of 74 late-type giants in the Solar neighborhood, with interferometric effective temperatures and broad-band photometry available in the literature, we compare observed colors with a new PHOENIX grid of synthetic photometric colors, as well as with photometric colors calculated with the MARCS and ATLAS model atmosphere codes. We find good agreement of the new synthetic colors with observations and published T-eff-color and color - color relations, especially in the T-eff ( V - K), T-eff-(J - K) and (J - K) -(V - K) planes. Deviations from the observed trends in the T-eff-color planes are generally within +/- 100 K for T-eff = 3500 to 4800 K. Synthetic colors calculated with different stellar atmosphere models agree to +/- 100 K, within a large range of effective temperatures and gravities. The comparison of the observed and synthetic spectra of late-type giants shows that discrepancies result from the differences both in the strengths of various spectral lines/bands ( especially those of molecular bands, such as TiO, H2O, CO) and the continuum level. Finally, we derive several new T-eff - log g - color relations for late-type giants at solar-metallicity (valid for T-eff = 3500 to 4800 K), based both on the observed effective temperatures and colors of the nearby giants, and synthetic colors produced with PHOENIX, MARCS and ATLAS model atmospheres.
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| 44. |
- Kucinskas, Arunas, et al.
(författare)
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Broad-band photometric colors and effective temperature calibrations for late-type giants - II. Z <0.02
- 2006
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Ingår i: Astronomy & Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 452:3, s. 1021-1038
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Tidskriftsartikel (refereegranskat)abstract
- We investigate the effects of metallicity on the broad-band photometric colors of late-type giants, and make a comparison of synthetic colors with observed photometric properties of late-type giants over a wide range of effective temperatures (T-eff = 3500- 4800K) and gravities (log g = 0.0-2.5), at [M/H] = -1.0 and -2.0. The influence of metallicity on the synthetic photometric colors is small at effective temperatures above similar to 3800K, but the effects grow larger at lower T-eff,T- due to the changing effciency of molecule formation which reduces molecular opacities at lower [M/H]. To make a detailed comparison of the synthetic and observed photometric colors of late type giants in the T-eff-color and color-color planes (which is done at two metallicities, [M/H] = -1.0 and -2.0), we derive a set of new T-eff-log g-color relations based on synthetic photometric colors, at [M/H] = -0.5, -1.0, -1.5, and -2.0. These relations are based on the T-eff- log g scales that we derive employing literature data for 178 late-type giants in 10 Galactic globular clusters (with metallicities of the individual stars between [M/H] = -0.7 and -2.5), and synthetic colors produced with the PHOENIX, MARCS and ATLAS stellar atmosphere codes. Combined with the T-eff- log g-color relations at [M/H] = 0.0 (Kucinskas et al. 2005), the set of new relations covers metallicities [M/H] = 0.0... -2.0 ([M/H] = 0.5), effective temperatures T-eff = 3500... 4800 K (T-eff = 100K), and gravities log g = - 0.5... 3.0. The new T-eff- log g-color relations are in good agreement with published T-eff-color relations based on observed properties of late-type giants, both at [M/H] = -1.0 and -2.0. The differences in all T-eff- color planes are typically well within similar to 100K. We find, however, that effective temperatures predicted by the scales based on synthetic colors tend to be slightly higher than those resulting from the T-eff- color relations based on observations, with the offsets up to similar to 100 K. This is clearly seen both at [M/H] = -1.0 and -2.0, especially in the T-eff-(B - V) and T-eff-(V - K) planes. The consistency between T-eff- log g-color scales based on synthetic colors calculated with different stellar atmosphere codes is very good, with typical differences being well within. Delta T-eff similar to 70 K at [M/H] = - 1.0 and. T-eff similar to 40 K at [M/H] = -2.0.
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| 45. |
- Kucinskas, Arunas, et al.
(författare)
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GAIA: AGB stars as tracers of star formation histories in the Galaxy and beyond
- 2003
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Ingår i: In: Mass-losing pulsating stars and their circumstellar matter. Workshop, May 13-16, 2002, Sendai, Japan, edited by Y. Nakada, M. Honma and M. Seki. Astrophysics and Space Science Library, Vol. 283, Dordrecht. - 1402011628 ; 283, s. 39-42
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Bokkapitel (övrigt vetenskapligt/konstnärligt)abstract
- We discuss the tracing of star formation histories with ESAs spaceastrometry mission GAIA, emphasizing the advantages of AGB stars for thepurpose. GAIAs microarcsecond-level astrometry, multi-band photometryand spectroscopy will provide individual distances, motions,Teff, log g and [M/H for vast numbers of AGB stars in theGalaxy and beyond. Reliable ages of AGB stars can be determined todistances of 200 kpc in a wide range of ages and metallicities,allowing star formation histories to be studied in a diversity ofastrophysical environments.
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| 46. |
- Kucinskas, A., et al.
(författare)
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Magnitude And Color Transformations Between Sirius And 2Mass Photometric Systems
- 2008
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Ingår i: Baltic Astronomy. - 1392-0049. ; 17:3-4, s. 283-292
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Tidskriftsartikel (refereegranskat)abstract
- We provide magnitude and color transformations between two near-infrared photometric systems, 2MASS and SIRIUS, the latter currently implemented with the three-Channel SIRIUS camera on 1.4 m Infrared Survey Facility telescope at the South African Astronomical Observatory. The transformation equations are derived using a carefully selected sample of 32 770 stars in the. Large and Small Magellanic Clouds that have high quality observations available in both photometric systems. The derived transformations are valid in the color range -0.1 < (J - H)(SIRIUS) < 1.15, -0.05 < (H - K-s)(SIRIUS) < 0.7 and -0.1 < (J - K-s)(SIRIUS) < 1.7.
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| 47. |
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| 48. |
- Kucinskas, Arunas, et al.
(författare)
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Star Formation Histories with Gaia: the Galaxy and Beyond
- 2003
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Ingår i: Baltic Astronomy. - 1392-0049. ; 12:4, s. 526-531
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Tidskriftsartikel (refereegranskat)abstract
- ESA's Gaia mission will provide accurate astrometric, photometric andspectroscopic data for millions of stars in the Galaxy and beyond. Wediscuss the prospects of using this vast amount of astrophysicalinformation for understanding star formation histories in the Galaxy andits immediate neighbors within the Local Group.
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| 49. |
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| 50. |
- Lammers, U., et al.
(författare)
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Faster, Better, Cheaper: News on Seeking Gaia’s Astrometric Solution with AGIS
- 2010
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Ingår i: Astronomical Society of the Pacific Conference Series. - 9781583817483 ; 434, s. 309-312
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Konferensbidrag (refereegranskat)abstract
- Gaia is ESA’s ambitious space astrometry mission with a foreseen launch date in early 2012. Its main objective is to perform a stellar census of the 1000 Million brightest objects in our galaxy (completeness to V=20 mag) from which an astrometric catalog of micro-arcsec level accuracy will be constructed. A key element in this endeavor is the Astrometric Global Iterative Solution (AGIS) - the mathematical and numerical framework for combining the ≍80 available observations per star obtained during Gaia’s 5yr lifetime into a single global astrometric solution. At last year’s ADASS XVIII we presented (O4.1) in detail the fundamental working principles of AGIS, its development status, and selected results obtained by running the system on processing hardware at ESAC, Madrid with large-scale simulated data sets. We present here the latest developments around AGIS highlighting in particular a much improved algebraic solving method that has recently been implemented. This Conjugate Gradient scheme improves the convergence behavior in significant ways and leads to a solution of much higher scientific quality. We also report on a new collaboration aiming at processing the data from the future small Japanese astrometry mission Nano-Jasmine with AGIS.
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