Binary sdB Stars with Massive Compact Companions

TL;DR

This study analyzes subluminous B stars in close binaries to measure companion masses, revealing a higher-than-expected occurrence of massive compact objects like neutron stars or black holes, challenging existing binary evolution models.

Contribution

It provides new measurements of sdB binary systems, identifying several with massive compact companions, and offers insights into their tidal synchronization and inclination constraints.

Findings

Five companions are consistent with white dwarfs or late-type stars.

Four sdBs have companions potentially being heavy white dwarfs, neutron stars, or black holes.

High fraction of massive companions challenges current binary evolution theories.

Abstract

The masses of compact objects like white dwarfs, neutron stars and black holes are fundamental to astrophysics, but very difficult to measure. We present the results of an analysis of subluminous B (sdB) stars in close binary systems with unseen compact companions to derive their masses and clarify their nature. Radial velocity curves were obtained from time resolved spectroscopy. The atmospheric parameters were determined in a quantitative spectral analysis. Based on high resolution spectra we were able to measure the projected rotational velocity of the stars with high accuracy. In the distribution of projected rotational velocities signs of tidal locking with the companions are visible. By detecting ellipsoidal variations in the lightcurve of an sdB binary we were able to show that subdwarf binaries with orbital periods up to 0.6 d are most likely synchronized. In this case, the inclination angles and companion masses of the binaries can be tightly constrained. Five invisible companions have masses that are compatible with that of normal white dwarfs or late type main sequence stars. However, four sdBs have compact companions massive enough to be heavy white dwarfs (>1 Msun), neutron stars or even black holes. Such a high fraction of massive compact companions is not expected from current models of binary evolution.