The Third Signature of Stellar Granulation

Absolute shifts of spectral-line bisectors (the third signature of granulation) are shown to follow the solar pattern, i.e., weaker lines are more blue-shifted than stronger lines, for a selection of stars on the cool side of the granulation boundary in the H–R diagram. These patterns scale to the solar case by multiplicative factors ranging from 0.53 to 2.10, which is a measure of the granulation velocity gradient through the stellar photosphere. The scale factors increase with effective temperature and are larger in giants than in dwarfs. A universal relation is constructed relating line depth to convective blueshift. The scale factors vary in concert with the first and second granulation signatures, macroturbulence, and asymmetry of spectral lines. Matching the stellar to the solar third signature also results in a determination of the apparent absolute radial velocity of the star with errors ranging from 80 to 300 m s −1 , but potentially considerably smaller. The star γ Cyg, which shows reversed-C-shaped bisectors and lies on the hot side of the granulation boundary, also shows weak lines more blue-shifted than stronger lines.