Stellar Lineshifts Induced by Photospheric Convection

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Abstract
Effects of stellar atmospheres on measured radial velocities are examined. Surface convection (“stellar granulation”) causes photo-spheric line asymmetries and wavelength shifts of ≃ 100 - 500 m/s. Cyclic changes in the convection patterns, such as observed during the solar 11-year cycle, may mimic radial velocity variations of perhaps 30 m/s. The study of stellar atmospheres would benefit from accurate (< 100 m/s) differential radial velocity measurements among lines of different parameters (strength, excitation potential, wavelength region) in the same star. To minimize convective perturbations, the flanks of very strong photospheric lines should probably be used, at least in stars with weak chromospheres. To disentangle cyclic variations due to stellar motion, several different groups of lines should be monitored, together with the Ca II K chromospheric emission.