Stellar Surface Convection, Line Asymmetries, and Wavelength Shifts (original) (raw)

When observed under sufficient resolution, practically all stellar spectral lines prove to be slightly asymmetric. Absorption lines in cooler stars form in inhomogeneous atmospheres, affected by surface convection (stellar granulation). Asymmetries and wavelength shifts originate from correlated velocity and brightness patterns: rising ( = blueshifted) elements are hot (=bright), and convective blueshifts result from a larger contribution of such blueshifted photons than of redshifted ones from the sinking and cooler (=darker) gas. For the Sun, the effect is around 300 m/s. High-excitation lines form predominantly in the hottest elements and show a more pronounced blueshift. The effects are predicted to be greater in F-type stars, and in giants. In the presence of magnetic fields, convection is disturbed and granules do not develop to equally large size or velocity amplitude, resulting in smaller blueshifts (by perhaps 10% or 30 m/s) during the years around activity maximum in the 1...