Astrometric radial velocities (Dravins, Madsen, Lindegren):
Accurate astrometric measurements of stars
allow their space motions, including the radial component, to be determined
without using the spectroscopic Doppler effect. For a few fast-moving
nearby stars accuracies of the order of 10 km/s have been achieved by combining
Hipparcos results with old ground-based observations. This method
needs Gaia to provide astrophysically interesting accuracies (better than
1 km/s). With currently available (Hipparcos) data, such accuracies
are in fact obtained with another method based on the parallel space motions
of stars in moving clusters. We have used Hipparcos measurements
of nearby clusters like Ursa Major and the Hyades to determine astrometric
radial velocities with accuracies of a few hundred m/s.
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Main references:
Dravins, Lindegren & Madsen (A&A 348, 1040, 1999),
Lindegren, Madsen & Dravins (A&A 356, 1119, 2000),
Madsen, Dravins & Lindegren (A&A 382, 446, 2002).
The fundamental definition of radial velocity (Dravins, Lindegren):
The need to accurately compare astrometric and spectroscopic radial velocities
has forced us to examine the fundamental definition of 'radial velocity'.
Two resolutions providing stringent definitions of the spectroscopic quantity,
called the barycentric radial-velocity measure, and the geometric
quantity, called astrometric radial velocity, were adopted by the IAU
General Assembly in Manchester, August 2000.
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Main reference:
Lindegren & Dravins (A&A 401, 1185, 2003).
Spectroscopic radial-velocity measures (Dravins, Gullberg, Lindegren):
Measurements of stellar spectra on an absolute wavelength
scale were obtained during 16 nights of observing in 1996-97, using the
ELODIE spectrometer at Observatoire de Haute-Provence. These provide
spectroscopic radial velocity measures which are not equivalent
to the astrometric ones, due to spectroscopic wavelength shifts from
gravitation, stellar surface convection, etc.
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Main reference:
Gullberg & Lindegren (A&A 390, 383, 2002).
Stellar surface structure (Dravins et al.):
In a programme aiming at understanding stellar surface
structures we compare the astrometric radial velocities with spectroscopic
measures, which include the effects of gravitational redshift and convective
motions in the stellar atmospheres. The observed wavelength shifts
for different atomic species will be compared with synthetic shifts computed
from a grid of hydrodynamic models of stellar photospheres across the HR
diagram.
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