Astronomy and Astrophysics – Astrophysics
Scientific paper
Feb 1990
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1990a%26a...228..203d&link_type=abstract
Astronomy and Astrophysics, Vol. 228, NO. 1/FEB(I), P. 203, 1990
Astronomy and Astrophysics
Astrophysics
74
Convection, Lines: Formation, Lines: Profile, Radial Velocities, Stars: Atmospheres Of, Stars: Late-Type, Stars: Rotation Of
Scientific paper
Numerical simulations of stellar photo spheric structure have provided line profiles at different positions across stellar disks. Using these data, synthetic Fe line profiles in disk-integrated flux are computed (including their asymmetries and wavelength shifts) for models corresponding to Procyon (F 5 IV-V), α Cen A (G2V), β Hyi (G2IV) and α Cen B (K1V). The line profiles are computed without any adjustable physical parameters besides that of stellar rotation, and the model atmospheres contain no classical parameters such as "mixing-length" nor "turbulence". Since line strength, width, asymmetry, rotational broadening, and limb darkening change with disk position, the disk-integrated profiles reflect these properties in a complex manner. This intercoupling might allow determinations of not only stellar rotation, but also line profile variations across stellar disks, using observations of similar stars with different rotation. Grids of "observed" synthetic line profiles and bisectors illustrate effects of finite spectral resolution. Comparisons with observations show good agreement, and the stellar rotation can be independently determined from the symmetric line broadening, and from the bisector patterns. For the well observed stars Procyon and α Cen A, we estimate V sin i≃2.9 and 1.8 km s-1, respectively. For the solar near-twin α Cen A, the profile and bisector fits are almost perfect, and permit the identification of subtle differences against the Sun, apparently reflecting changes in solar-type granulation during some billion years of stellar evolution. The bisector fit for Procyon is excellent, but some absorption is missing in the flanks of the intensity profiles outside about ±5 km s-1. This, and a similar effect in the subgiant β Hyi, is believed to be an artifact of the hydrodynamically anelastic atmospheric model, which excludes sound waves and absorption by features moving at near-sonic speeds. Different stars have different line asymmetries, and in each star there is a systematic dependence on line-strength. The excitation-potential and wavelength-region dependences are smaller. The convective blueshift of spectral lines ranges between ≃200 km s-1 in K dwarfs to ≃1000 m s-1 in F stars. Such effects may limit the accuracies possible in spectroscopic determinations of stellar radial velocities.
Dravins Dainis
Nordlund Aake
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