* Astronomy

Title: Imaging and Modelling Rapidly Rotating Stars: Alpha Cephei and Alpha Ophiuchi
Authors: M. Zhao (1), J. D. Monnier (1), E. Pedretti (2), N. Thureau (2), A. Merand (3,4), T. ten Brummelaar (3), H. McAlister (3), S. T. Ridgway (5), N. Turner (3), J. Sturmann (3), L. Sturmann (3), C. Farrington (3), P. J. Goldfinger (3) ((1). University of Michigan, (2). University of St. Andrews, Scotland, UK, (3). The CHARA Array, Georgia State University, (4). European Southern Observatory, (5).National Optical Astronomy Observatory, NOAO)

We present sub-milliarcseond resolution imaging and modelling of two nearby rapid rotators Alpha Cephei and Alpha Ophiuchi, obtained with the CHARA array - the largest optical/IR interferometer in the world. Incorporating a gravity darkening model, we are able to determine the inclination, the polar and equatorial radius and temperature, as well as the fractional rotation speed of the two stars with unprecedented precision. The polar and equatorial regions of the two stars have ~2000K temperature gradient, causing their apparent temperatures and luminosities to be dependent on their viewing angles. Our modelling allow us to determine the true effective temperatures and luminosities of Alpha Cep and Alpha Oph, permitting us to investigate their true locations on the H-R diagram. These properties in turn give us estimates of the masses and ages of the two stars within a few percent of error using stellar evolution models. Also, based on our gravity darkening modelling, we propose a new method to estimate the masses of single stars in a more direct way through Vsin(i) measurements and precise geometrical constraint. Lastly, we investigate the degeneracy between the inclination and the gravity darkening coefficient, which especially affects the modelling of Alpha Oph. Although incorporating Vsin(i) has lifted the degeneracy to some extent, higher resolution observations are still needed to further constrain the parameters independently.

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