Imaging and Modeling Rapidly Rotating Stars: Cephei and Ophiuchi

We present submilliarc second resolution imaging and modeling of two nearby rapid rotators Cephei and 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 ~2000 K temperature gradient, causing their apparent temperatures and luminosities to be dependent on their viewing angles.Our modeling allow us to determine the true effective temperatures and luminosities of Cep and 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 modeling, 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 modeling of Oph. Although incorporating Vsin i has lifted the degeneracy to some extent,higher-resolution observations are still needed to further constrain the parameters independently.