A search for magnetic fields in cool sdB stars

TL;DR

This study investigates the presence of magnetic fields in cool sdB stars to understand their rotation and abundance anomalies, finding no strong large-scale fields but suggesting possible weaker fields in some stars.

Contribution

It provides the first search for magnetic fields in cool sdB stars, offering insights into their magnetic properties and potential influence on stellar rotation and surface abundances.

Findings

Large-scale magnetic fields of kG order are generally absent.

Possible magnetic fields of a few hundred Gauss cannot be ruled out.

Marginal detection of a magnetic field in SB 290, requiring further confirmation.

Abstract

Hot cluster Horizontal Branch (HB) stars and field subdwarf B (sdB) stars are core helium burning stars that exhibit abundance anomalies that are believed to be due to atomic diffusion. Diffusion can be effective in these stars because they are slowly rotating. In particular, the slow rotation of the hot HB stars (T_eff > 11000K), which show abundance anomalies, contrasts with the fast rotation of the cool HB stars, where the observed abundances are consistent with those of red giants belonging to the same cluster. The reason why sdB stars and hot HB stars are rotating slowly is unknown. In order to assess the possible role of magnetic fields on abundances and rotation, we investigated the occurrence of such fields in sdB stars with T_eff < 30000K, whose temperatures overlap with those of the hot HB stars. We conclude that large-scale organised magnetic fields of kG order are not generally present in these stars but at the achieved accuracy, the possibility that they have fields of a few hundred Gauss remains open. We report the marginal detection of such a field in SB 290; further observations are needed to confirm it.