Fundamental properties of two rapidly rotating stars: Rasalhague and Alkaid

Abstract

Abridged : The fundamental parameters of rapidly rotating stars are key quantities to understand the impact of rotation on stellar evolution. A few nearby early-type stars offer the possibility of precise measurements of these parameters, which will help us constrain newly available two-dimensional models. We propose a method to retrieve the fundamental parameters of a fast rotating star (mass, rotation rate, and age), and also the inclination of its rotation axis on the line of sight, using five spectrophotometric observables along with a set of steady 2D-models. Using the temperature derived by the infrared flux method, the Vsini and the apparent luminosity, along with a grid of 2D steady state models, we select models that are compatible with all observational constraints, and derive the most probable mass, rotation rate, core hydrogen mass fraction relative to that of the envelope, and inclination of the rotation axis on the line of sight of the targetted star. We apply this method to two stars: Rasalhague (alpha Oph) and Alkaid (eta UMa). We confirm and improve the fundamental parameters of Rasalhague and provide a new determination of its rotation axis inclination on the line-of-sight, which we find to be $\sim69$ degrees. Concerning Alkaid, we infer a mass of $5.071\pm0.023 M_\odot$, a rotation rate corresponding to an equatorial rotation period of 14.6 hours. We also find an inclination of the rotation axis of $\sim42$ degrees. We show that Alkaid is a very young star, presumably between 2 and 8 Myrs off the Zero-Age Main Sequence. As a side result, using high resolution spectra and the Least Square Deconvolution method, we determined a precise value of the Vsini of Rasalhague, namely $224.3\pm2.6$ km/s. Similarly, we find $V\sin i=154.3\pm9.1$ km/s for Alkaid.