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| Paper: |
Radial Velocities without Spectroscopy |
| Volume: |
185, Precise Stellar Radial Velocities, IAU Colloquium 170 |
| Page: |
77 |
| Authors: |
Madsen, S.; Lindegren, L.; Dravins, D. |
| Abstract: |
Accuracies in space astrometry now permit accurate determination of stellar radial velocity without using spectroscopy or invoking the Doppler principle. Already Hipparcos data permit accuracies of 100 m/s in some cases, while future space astrometry missions will enable such determinations for a broad range of stars. Fundamental radial-velocity standards have hitherto been limited to solar-system objects, in particular asteroids, whose space motions can be derived with very high accuracy without the use of spectroscopic data. Astrometric techniques are now extending the realm of such geometrically determined radial velocities to many nearby stars. Among astrometric measures for radial-velocity determination, the most direct is the secular change in trigonometric parallax due to the radial displacement of a star. Although this requires extremely accurate measurements over years or decades, it should become feasible with planned space missions. For Barnard's star (parallax 549 mas, V_r = -110 km/s), the expected parallax change is 34 microarcsec/year. Assuming that a star moves uniformly through space, its velocity can also be derived from the secular change in its proper motion (which varies due to the observer). For astrometric missions now being planned, this method should yield space velocities to better than 100 m/s for several nearby high-velocity stars. A third astrometric method that already has been applied using data from the Hipparcos mission, concerns the secular change of the angular extent of moving star clusters. Since all cluster stars share the same (average) velocity vector, the cluster's apparent size changes as it moves in the radial direction. This relative change (revealed by the proper-motion vectors towards the cluster apex) corresponds to the relative change in distance. Since the individual stellar distances are known from parallaxes, their radial velocities follow. Applying this moving-cluster method to Hipparcos data, radial velocities have now been derived for many stars in the Hyades and in the Ursa Major clusters, reaching accuracies between 100-400 m/s. The comparison of these values with precise spectroscopic measurements reveals wavelength shifts not caused by stellar motion, as discussed elsewhere in this colloquium. |
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