Rotational properties of single and wide binary subdwarf B stars

Abstract

We measured projected rotational velocities of more than a hundred apparently single sdBs. A comparison is made with sdB stars in binary systems with orbits so wide, that tidal interaction becomes negligible. All of these stars are slow rotators (vsini<10 km/s) with EC 22081-1916 being the only exception. This single star has the highest projected rotational velocity ever measured for an sdB (vsini=163 km/s) and might have been formed by a merger event. The merger of a red-giant core and a low-mass, main-sequence star or substellar object during a common envelope phase fits particularly well with observations. The implications of our results for hot subdwarf formation are briefly discussed.