The Limited Influence of Pressure Gradients on Late‐Type Stellar Line Asymmetries

Abstract

Line asymmetries and shifts are powerful tools for studying velocity fields in the stellar photospheres. Other effects, however, could also generate asymmetries, blurring the information of the velocity patterns. We have studied the shifts and asymmetries induced in the profiles of spectral lines by pressure effects. The best theoretical and experimental data on line broadening and shifts caused by collisions with atomic hydrogen were used to analyze the Na I D and three Ca I lines. Line bisectors of synthetic spectra computed with accurate data for the Na I and Ca I lines are compared with very high resolution, high signal-to-noise ratio solar spectra and indicate that pressure broadening reproduces the wings of the observed lines, but pressure shifts introduce neither asymmetries nor shifts comparable to the observed ones.