A realistic two-dimensional model of Altair

Abstract

Fast rotation is responsible for important changes in the structure and evolution of stars. Optical long baseline interferometry now permits the study of its effects on the stellar surface, mainly gravity darkening and flattening. We aim to determine the fundamental parameters of the fast-rotating star Altair, in particular its evolutionary stage, mass, and differential rotation, using state-of-the-art stellar interior and atmosphere models together with interferometric, spectroscopic, and asteroseismic observations. We use ESTER 2D stellar models to produce the relevant surface parameters needed to create intensity maps from atmosphere models. Interferometric and spectroscopic observables are computed from these intensity maps and several stellar parameters are then adjusted using the MCMC algorithm Emcee. We determined Altair's equatorial radius to be 2.008 +/- 0.006 Rsun, the position angle 301.1 +/- 0.3 degrees, the inclination 50.7 +/- 1.2 degrees, and the equatorial angular velocity 0.74 +/- 0.01 times the Keplerian angular velocity. This angular velocity leads to a flattening of 0.220 +/- 0.003. We also deduce from the spectroscopically derived vsini ~ 243 km/s, a true equatorial velocity of ~314 km/s corresponding to a rotation period of 7h46m (~3 c/d). The data also impose a strong correlation between mass, metallicity, hydrogen abundance, and core evolution. Thanks to asteroseismic data, we constrain the mass of Altair to 1.86 +/- 0.03 Msun and further deduce its metallicity Z = 0.019 and its core hydrogen mass fraction Xc = 0.71, assuming an initial solar hydrogen mass fraction X = 0.739. These values suggest that Altair is ~100 Myrs old. Finally, the 2D ESTER model also gives the internal differential rotation of Altair, showing that its core rotates approximately 50% faster than the envelope, while the surface differential rotation does not exceed 6%.

20 pages, 20 figures, accepted for publication in A&A

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• Detecting deep axisymmetric toroidal magnetic fields in stars. The traditional approximation of rotation for differentially rotating deep spherical shells with a general azimuthal magnetic field
• Seismology of Altair with MOST
• The Habitable Zones of Rapidly Rotating Main Sequence A/F Stars
• High contrast imaging with Fizeau interferometry: The case of Altair
• The A-shell star Phi Leo revisited: its photospheric and circumstellar spectra
• Asteroseismic Determination of Stellar Rotation: On Synchronization in the Close Eclipsing Binaries AB Cas and OO Dra
• Pulsation and Rotation of the EL CVn-type Eclipsing Binary 1SWASP J024743.37-251549.2