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| Paper: |
Astrometric versus Spectroscopic Radial Velocities |
| Volume: |
185, Precise Stellar Radial Velocities, IAU Colloquium 170 |
| Page: |
41 |
| Authors: |
Dravins, D.; Gullberg, D.; Lindegren, L.; Madsen, S. |
| Abstract: |
The radial velocity of a star, as deduced from wavelength shifts, does not merely contain the true velocity of the stellar center of mass but also components arising from dynamics in the star's atmosphere, gravitational redshifts, and other effects. For the Sun, the segregation of such effects has been possible because the relative Sun-Earth motion is accurately known from planetary system dynamics, and does not have to be deduced from asymmetric and shifted line profiles. For other stars, accurate determinations of their true radial motion have only recently become feasible with space astrometry. Data from Hipparcos permit accurate such determinations for stars in nearby moving clusters such as Ursa Major and the Hyades (Dravins et al., in Proc. Hipparcos - Venice '97, ESA SP-402, p.733, 1997). When a star cluster (whose stars share the same velocity vector) moves in the radial direction, its angular size changes, as measured by stellar proper-motion vectors. This rate of change equals the time derivative of the [known] distance, i.e. the radial velocity. Future astrometric missions will extend astrometric radial-velocity determinations also to individual field stars with measurable changes in parallax and proper motion. For these stars with astrometric radial-velocity determinations, a parallel spectroscopic program has recently been completed at Haute-Provence Observatory, using its ELODIE radial-velocity spectrometer. Almost 100 program stars of many different spectral types were observed under very good signal-to-noise conditions. Work is in progress to compare the spectroscopic radial velocities with the astrometric values, and to search for systematic line shift differences between groups of different spectral lines (with respect to line-strength, excitation potential, or wavelength region). The overall stability of ELODIE spectra reaches 10 m/s; the expected spectroscopic precision for groups of 100 selected lines in any one star is about 50 m/s; the accuracy in astrometric radial velocity reaches 200 m/s, while hydrodynamic models of stellar atmospheres predict differences on the order of 1 km/s in convective line shifts between different stars. Gravitational redshifts are of comparable magnitude. This program thus aims at identifying signatures of stellar surface structure from line shift patterns, at finding differences in gravitational redshift between different spectral types, and at improving the absolute calibration of velocity values for stars of different rotational velocity and spectral complexity. The program includes not only Hyades and Ursa Major stars, but also IAU radial-velocity standards, metal-deficient stars, and others. For a further discussion, see: <A HREF="http://www.astro.lu.se/dainis/HTML/ASTROMET.html"> Discussion </A> |
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