Spectroscopic Effects of Rotation in Extended Stellar Atmospheres

Abstract

Rotation can markedly impact the interpretation of spectral diagnostics from hot stars with winds. Using a newly developed formal solution code, and a semianalytic model for the effects of rotation on the wind structure, we investigate the effects of rotation on the observed spectrum. Nonphotospheric lines formed in extended, optically thick atmospheres do not show just the simple classical Doppler line broadening. In such cases the traditional method of convolving the line profile with a rotation profile is insufficient. To handle these situations, we start with a sophisticated one-dimensional model of a stellar atmosphere and use a two-dimensional observer's-frame code to calculate the observed effect on line profiles under various assumptions about the two-dimensional velocity field and density distribution. Because it should generally be a small effect, rotational distortion of the stellar surface is currently ignored. Gravity darkening is also not handled at this time. Importantly, the code handles arbitrary line profiles and line blends, and lines formed at any depth in the stellar photosphere or wind. We apply the code to models that were used to investigate the Small Magellanic Cloud O7 Iaf+ supergiant AV 83. We find that the distortions in the velocity field caused by rotation can have a significant influence on the profile shape, while modifications to the density structure primarily influence the line strength. These affect the quality of the spectroscopic fits to the data, and consequently, the derived velocity field and other stellar parameters might be significantly biased for rapidly rotating stars with extended winds. The code will made available with CMFGEN and can be used to examine the influence of rotation on spectroscopic fitting and derived stellar parameters.