Rotational properties of single and wide binary subdwarf B stars

TL;DR

This study measures the rotational velocities of single and wide binary subdwarf B stars, finding most are slow rotators except one possibly formed by a merger, which has implications for their formation mechanisms.

Contribution

It provides the first comprehensive comparison of rotational velocities between single and wide binary sdB stars, highlighting the role of mergers in their formation.

Findings

Most sdB stars are slow rotators with vsini<10 km/s.

One single sdB star shows a high rotational velocity of 163 km/s.

Merger events are a plausible formation channel for rapidly rotating sdB 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.