Tidal synchronization of the subdwarf B binary PG 0101+039

TL;DR

This study confirms that the subdwarf B star PG 0101+039 is tidally synchronized with its white dwarf companion, validating the use of this assumption for similar short-period binaries to determine their masses.

Contribution

The paper provides observational evidence supporting tidal synchronization in PG 0101+039, enabling more accurate mass measurements in faint sdB binary systems.

Findings

Tidal synchronization is confirmed for PG 0101+039.

Light variations are due to ellipsoidal deformation.

Synchronization likely applies to all sdB binaries with periods under half a day.

Abstract

Tidally locked rotation is a frequently applied assumption that helps to measure masses of invisible compact companions in close binaries. The calculations of synchronization times are affected by large uncertainties in particular for stars with radiative envelopes calling for observational constraints. We aim at verifying tidally locked rotation for the binary PG 0101+039, a subdwarf B star + white dwarf binary from its tiny (0.025 %) light variations measured with the MOST satellite (Randall et al. 2005). Binary parameters were derived from the mass function, apparent rotation and surface gravity of PG 0101+039 assuming a canonical mass of 0.47 Mo and tidally locked rotation. The light curve was then synthesised and was found to match the observed amplitude well. We verified that the light variations are due to ellipsoidal deformation and that tidal synchronization is established for PG 0101+039. We conclude that this assumption should hold for all sdB binaries with orbital periods of less than half a day. Hence the masses can be derived from systems too faint to measure tiny light variations.