The subdwarf B star SB 290 - A fast rotator on the extreme horizontal branch

TL;DR

This paper reports the discovery of SB 290, a single, fast-rotating hot subdwarf B star on the extreme horizontal branch, suggesting a merger origin for such stars.

Contribution

It provides evidence that some apparently single sdB stars may form through mergers, challenging the view that binary interactions are the sole formation channel.

Findings

SB 290 is a fast rotator on the extreme horizontal branch.

SB 290 is an apparently single sdB star.

The star's properties suggest a merger origin.

Abstract

Hot subdwarf B stars (sdBs) are evolved core helium-burning stars with very thin hydrogen envelopes. In order to form an sdB, the progenitor has to lose almost all of its hydrogen envelope right at the tip of the red giant branch. In close binary systems, mass transfer to the companion provides the extraordinary mass loss required for their formation. However, apparently single sdBs exist as well and their formation is unclear since decades. The merger of helium white dwarfs leading to an ignition of core helium-burning or the merger of a helium core and a low mass star during the common envelope phase have been proposed. Here we report the discovery of SB 290 as the first apparently single fast rotating sdB star located on the extreme horizontal branch indicating that those stars may form from mergers.