| 1. |
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| 2. |
- 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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| 3. |
- 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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| 4. |
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| 5. |
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| 6. |
- 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
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Bokkapitel (övrigt vetenskapligt/konstnärligt)
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| 7. |
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| 8. |
- 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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| 9. |
- 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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| 10. |
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| 11. |
- Lindegren, Lennart, et al.
(författare)
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The fundamental definition of "radial velocity''
- 2003
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Ingår i: Astronomy & Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 401:3, s. 1185-1201
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Tidskriftsartikel (refereegranskat)abstract
- Accuracy levels of metres per second require the fundamental concept of``radial velocity'' for stars and other distant objects to be examined,both as a physical velocity, and as measured by spectroscopic andastrometric techniques. Already in a classical (non-relativistic)framework the line-of-sight velocity component is an ambiguous concept,depending on whether, e.g., the time of light emission (at the object)or that of light detection (by the observer) is used for recording thetime coordinate. Relativistic velocity effects and spectroscopicmeasurements made inside gravitational fields add further complications,causing wavelength shifts to depend, e.g., on the transverse velocity ofthe object and the gravitational potential at the source. Aiming atdefinitions that are unambiguous at accuracy levels of 1 ms-1, we analyse different concepts of radial velocity andtheir interrelations. At this accuracy level, a strict separation mustbe made between the purely geometric concepts on one hand, and thespectroscopic measurement on the other. Among the geometric concepts wedefine kinematic radial velocity, which corresponds most closely to the``textbook definition'' of radial velocity as the line-of-sightcomponent of space velocity; and astrometric radial velocity, which canbe derived from astrometric observations. Consistent with thesedefinitions, we propose strict definitions also of the complementarykinematic and astrometric quantities, namely transverse velocity andproper motion. The kinematic and astrometric radial velocities depend onthe chosen spacetime metric, and are accurately related by simplecoordinate transformations. On the other hand, the observationalquantity that should result from accurate spectroscopic measurements isthe barycentric radial-velocity measure. This is independent of themetric, and to first order equals the line-of-sight velocity. However,it is not a physical velocity, and cannot be accurately transformed to akinematic or astrometric radial velocity without additional assumptionsand data in modelling the process of light emission from the source, thetransmission of the signal through space, and its recording by theobserver. For historic and practical reasons, the spectroscopicradial-velocity measure is expressed in velocity units asczB, where c is the speed of light and zB is theobserved relative wavelength shift reduced to the solar-systembarycentre, at an epoch equal to the barycentric time of light arrival.The barycentric radial-velocity measure and the astrometric radialvelocity are defined by recent resolutions adopted by the InternationalAstronomical Union (IAU), the motives and consequences of which areexplained in this paper.
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| 12. |
- Madsen, Søren, et al.
(författare)
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Astrometric radial velocities. III. Hipparcos measurements of nearby star clusters and associations
- 2002
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Ingår i: Astronomy & Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 381, s. 446-463
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Tidskriftsartikel (refereegranskat)abstract
- Radial motions of stars in nearby moving clusters are determined fromaccurate proper motions and trigonometric parallaxes, without any use ofspectroscopy. Assuming that cluster members share the same velocityvector (apart from a random dispersion), we apply a maximum-likelihoodmethod on astrometric data from Hipparcos to compute radial and spacevelocities (and their dispersions) in the Ursa Major, Hyades, ComaBerenices, Pleiades, and Praesepe clusters, and for theScorpius-Centaurus, alpha Persei, and ``HIP 98321'' associations. Theradial motion of the Hyades cluster is determined to within 0.4 kms-1 (standard error), and that of its individual stars towithin 0.6 km s-1. For other clusters, Hipparcos data yieldastrometric radial velocities with typical accuracies of a few kms-1. A comparison of these astrometric values withspectroscopic radial velocities in the literature shows a good generalagreement and, in the case of the best-determined Hyades cluster, alsopermits searches for subtle astrophysical differences, such as evidencefor enhanced convective blueshifts of F-dwarf spectra, and decreasedgravitational redshifts in giants. Similar comparisons for the ScorpiusOB2 complex indicate some expansion of its associations, albeit slowerthan expected from their ages. As a by-product from the radial-velocitysolutions, kinematically improved parallaxes for individual stars areobtained, enabling Hertzsprung-Russell diagrams with unprecedentedaccuracy in luminosity. For the Hyades (parallax accuracy 0.3 mas), itsmain sequence resembles a thin line, possibly with wiggles in it.Although this main sequence has underpopulated regions at certaincolours (previously suggested to be ``Böhm-Vitense gaps''), suchare not visible for other clusters, and are probably spurious. Futurespace astrometry missions carry a great potential for absoluteradial-velocity determinations, insensitive to the complexities ofstellar spectra. Based on observations by the ESA Hipparcos satellite.Extended versions of Tables ef{tab1} and ef{tab2} are available inelectronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr(130.79.125.8) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/381/446
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| 13. |
- Madsen, Søren, et al.
(författare)
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Intrinsic spectral blueshifts in rapidly rotating stars?
- 2003
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Ingår i: Astronomy & Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 411:3, s. 581-586
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Tidskriftsartikel (refereegranskat)abstract
- Spectroscopic radial velocities for several nearby open clusters suggestthat spectra of (especially earlier-type) rapidly rotating stars aresystematically blueshifted by 3 km s-1 or more, relative tothe spectra of slowly rotating ones. Comparisons with astrometricallydetermined radial motions in the Hyades suggests this to be an absoluteblueshift, relative to wavelengths naively expected from stellar radialmotion and gravitational redshift. Analogous trends are seen also inmost other clusters studied (Pleiades, Coma Berenices, Praesepe, alphaPersei, IC 2391, NGC 6475, IC 4665, NGC 1976 and NGC 2516). Possiblemechanisms are discussed, including photospheric convection, stellarpulsation, meridional circulation, and shock-wave propagation, as wellas effects caused by template mismatch in determining wavelengthdisplacements. For early-type stars, a plausible mechanism is shock-wavepropagation upward through the photospheric line-forming regions. Suchwavelength shifts thus permit studies of certain types of stellaratmospheric dynamics and - irrespective of their cause - may influencededuced open-cluster membership (when selected from common velocity) anddeduced cluster dynamics (some types of stars might show fortuitousvelocity patterns).
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| 14. |
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| 15. |
- Perryman, M. A. C., et al.
(författare)
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GAIA: Composition, formation and evolution of the Galaxy
- 2001
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Ingår i: Astronomy & Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 369, s. 339-363
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Tidskriftsartikel (refereegranskat)abstract
- The GAIA astrometric mission has recently been approved as one of thenext two ``cornerstones'' of ESA's science programme, with a launch datetarget of not later than mid-2012. GAIA will provide positional andradial velocity measurements with the accuracies needed to produce astereoscopic and kinematic census of about one billion stars throughoutour Galaxy (and into the Local Group), amounting to about 1 percent ofthe Galactic stellar population. GAIA's main scientific goal is toclarify the origin and history of our Galaxy, from a quantitative censusof the stellar populations. It will advance questions such as when thestars in our Galaxy formed, when and how it was assembled, and itsdistribution of dark matter. The survey aims for completeness to V=20mag, with accuracies of about 10 mu as at 15 mag. Combined withastrophysical information for each star, provided by on-boardmulti-colour photometry and (limited) spectroscopy, these data will havethe precision necessary to quantify the early formation, and subsequentdynamical, chemical and star formation evolution of our Galaxy.Additional products include detection and orbital classification of tensof thousands of extra-Solar planetary systems, and a comprehensivesurvey of some 105-106 minor bodies in our SolarSystem, through galaxies in the nearby Universe, to some 500 000 distantquasars. It will provide a number of stringent new tests of generalrelativity and cosmology. The complete satellite system was evaluated aspart of a detailed technology study, including a detailed payloaddesign, corresponding accuracy assesments, and results from a prototypedata reduction development.
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| 16. |
- Quist, Carl F., et al.
(författare)
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Binarity of Hipparcos Main Sequence Survey Stars
- 2001
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Ingår i: The Formation of Binary Stars, Proceedings of IAU Symp. 200, held 10-15 April 2000, in Potsdam, Germany, Edited by Hans Zinnecker and Robert D. Mathieu, 2001, p. 64.. ; 200, s. 64-64
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Konferensbidrag (refereegranskat)abstract
- Hipparcos observations of double stars, mainly A and F primaries, aremodelled in order to check the statistics of double-star solutions inthe Hipparcos Catalogue and to set quantitative limits on binarydistributions. The number of companions per primary is estimated to 0.25+/-0.05 for semi-major axes from 1 to 10 AU, and to 0.43 +/-0.20 for therange 10--100 AU. The total multiplicity found is between 0.9 and 1.2.
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| 17. |
- Soffel, M., et al.
(författare)
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The IAU 2000 Resolutions for Astrometry, Celestial Mechanics, and Metrology in the Relativistic Framework: Explanatory Supplement
- 2003
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Ingår i: Astrophysical Journal. - : American Astronomical Society. - 0004-637X. ; 126:6, s. 2687-2706
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Tidskriftsartikel (refereegranskat)abstract
- We discuss the IAU resolutions B1.3, B1.4, B1.5, and B1.9 that wereadopted during the 24th General Assembly in Manchester, 2000, andprovides details on and explanations for these resolutions. It isexplained why they present significant progress over the correspondingIAU 1991 resolutions and why they are necessary in the light of presentaccuracies in astrometry, celestial mechanics, and metrology. In fact,most of these resolutions are consistent with astronomical models andsoftware already in use. The metric tensors and gravitational potentialsof both the Barycentric Celestial Reference System and the GeocentricCelestial Reference System are defined and discussed. The necessity andrelevance of the two celestial reference systems are explained. Thetransformations of coordinates and gravitational potentials arediscussed. Potential coefficients parameterizing the post-Newtoniangravitational potentials are expounded. Simplified versions of the timetransformations suitable for modern clock accuracies are elucidated.Various approximations used in the resolutions are explicated andjustified. Some models (e.g., for higher spin moments) that serve thepurpose of estimating orders of magnitude have actually never beenpublished before.
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