Rotational Feature of Vega Revealed from Spectral Line Profiles

Abstract

In order to establish Vega's absolute rotational velocity, ve, as distinct from the inclination angle, i, we conducted a detailed profile study on a large number (~200) of weak lines based on very high signal-to-noise ratio spectrum data, making use of the fact that the lines' characteristic shapes may contain information about rotation-induced gravity darkening. To this end, we developed computer programs to simulate the surface of a rapid rotator withlatitude-dependent atmospheric parameters, from which the spectral energy distribution (SED) and the detailed line profile can be computed. Having restricted the freedom of the parameters according to the requirements of the SED, we concluded by comparing the observed and theoretical profiles that ve~=175 km s-1 (with i~=7deg) is the best solution. It was also found that the abundances derived from lines showing peculiar flat-bottom shapes (e.g., Fe I lines) tend to be overestimated by up to ~0.2 dex when theconventional method of analysis using classical model atmospheres is applied, although this effect is less significant for lines showing normal profiles (e.g., high-excitation Fe II lines).