Surface parameterisation and spectral synthesis of rapidly rotating stars

Abstract

Context. Spectral synthesis is a powerful tool with which to find the fundamental parameters of stars. Models are usually restricted to single values of temperature and gravity, and assume spherical symmetry. This approximation breaks down for rapidly rotating stars. Aims. This paper presents a joint formalism to allow a computation of the stellar structure – namely, the photospheric radius, R, the effective temperature, Teff, and gravity, geff – as a function of the colatitude, θ, for rapid rotators with radiative envelopes, and a subsequent method to build the corresponding synthetic spectrum. Methods. The structure of the star is computed using a semi-analytical approach, which is easy to implement from a computational point of view and which reproduces very accurately the results of much more complex codes. Once R(θ), Teff(θ), and geff(θ) are computed, the suite of codes, ATLAS and SYNTHE, by R. Kurucz are used to synthesise spectra for a mesh of cells in which the star is divided. The appropriate limb-darkening coefficients are also computed, and the final output spectrum is built for a given inclination of the rotation axis with respect to the line of sight. All the geometrical transformations required are described in detail. Results. The combined formalism has been applied to Vega, a rapidly rotating star almost seen pole-on, as a testbed. The structure reproduces the results from interferometric studies and the synthetic spectrum matches the peculiar shape of the spectral lines well. Conclusions. Contexts where this formalism can be applied are outlined in the final sections.