Astrometric versus Spectroscopic Radial Velocities

Abstract

The apparent radial velocity of a star, as deduced from wavelength shifts, comprises not merely its true velocity, but also components arising from dynamics in the star’s atmosphere, gravitational redshift, and other effects. For the Sun, such phenomena can be segregated since the relative Sun-Earth motion is known from planetary system dynamics. This is now becoming possible also for other stars, whose true radial motions are determined through space astrometry. A study of the differences between accurate astrometric velocities (from Hipparcos), and precise spectroscopic values (from ELODIE) is in progress. Data for cool stars in the Hyades indicate a tendency of relative blueshifts among earlier main-sequence F-type stars, and in giants. This is theoretically expected: an increased convective blueshift due to the more vigorous convection in F-stars, and a decreased gravitational redshift in giants.

Hyades dynamics fromN-body simulations: Accuracy of astrometric radial velocities from Hipparcos

“…Such lineshifts are caused for instance by convective motions and gravitational redshift in the stellar atmospheres (Dravins et al 1999a). Absolute lineshifts could previously only be observed in the solar spectrum, but are now within reach for a range of spectral types through the use of astrometric radial velocities.…”

Differential Fe I Line Shifts as Convective Signatures in R = 40 000 Échelle Spectra?

“…This would suggest that the supergiant is on the early side of the granulation boundary with inverted C-shaped bisectors, as could be expected for a GOIb star (Gray & Toner 1986). We get a convective signature for Procyon (F5IV-V) of-54 m s _ 1 with an estimated error of 80 m s _ 1 , based on three observations.…”

Exactly What Is Stellar ‘Radial Velocity’?

“…The term containing $ i accounts for the gravitational blueshift due to the potential at the observer, while v 2 includes the transverse Doppler effect from the motion of the observer; each term contributes ~ -3 m s _ 1 for an observer on the Earth, u • vi is the component of the observer's motion along the line of sight, which for a terrestrial observer may amount to ±30 000 m s _ 1 . Similarly, u • vo represents the radial velocity of the star, 3>o determines its gravitational redshift (~ +300 to 1000 m s _ 1 for main-sequence stars, but ranging from +30 to 30 000 m s _ 1 for other stellar types; Dravins et al 1998), and v^ its transverse Doppler effect (~ +100 m s _ 1 for fast-moving stars). Only the first factor on the right-hand-side of Eq.…”

“…Dravins 1998;Dravins et al 1998). In this paper we are not concerned with shifts caused for instance by stellar convection, but with the fundamental question how the concept 'radial velocity' may be defined.…”