Differential Rotation of Close Binary Stars: Application to HR 1099

Abstract

We propose a new method for estimating differentialrotation in binary stars, for which only moderate to poor phase coverage can be obtained (rotation period of order of a few days), preventing the use of conventionalcross-correl ation methods. Assuming a solar-like differential rotation law with two independent parameters (equatorialrotation rate Ω_eq and rotationalshear between pole and equator dΩ), we reconstruct Doppler images for different values of the two parameters, and derive the optimal Ω_eq, dΩ and associated error bars from the corresponding X² map. Simulations show that Ω_eq and dΩ can be recovered with good accuracy, even if the phase coverage per rotation cycle is poor, provided the total data set is long enough. From observations of the HR 1099 K1 subgiant secured in 1998, 1999 and 2000, we obtain that the equator rotates faster than the pole with a rotational shear about 3 times smaller than solar.