Extreme mass ratios and fast rotation in three massive binaries

TL;DR

This study investigates three fast-rotating massive stars with low-mass companions, revealing that their rapid rotation is likely not caused by binary interactions, based on combined photometric and spectroscopic analyses.

Contribution

It provides detailed characterization of three massive binaries with low-mass companions, challenging the idea that binary interactions are the main cause of rapid rotation in such stars.

Findings

Companions are low-mass (~1 M_sun) with large radii and low temperatures.

No UV signature of hot subdwarfs, indicating different companion nature.

Stars are young (<20 Myr) and show non-synchronized rotation despite short orbital periods.

Abstract

The origin of rapid rotation in massive stars remains debated, although binary interactions are now often advocated as a cause. However, the broad and shallow lines in the spectra of fast rotators make direct detection of binarity difficult. In this paper, we report on the discovery and analysis of multiplicity for three fast-rotating massive stars: HD25631 (B3V), HD191495 (B0V), and HD46485 (O7V). They display strikingly similar TESS light curves, with two narrow eclipses superimposed on a sinusoidal variation due to reflection effects. We complement these photometric data by spectroscopy from various instruments (X-Shooter, Espadons, FUSE...), to further constrain the nature of these systems. The detailed analyses of these data demonstrates that the companions of the massive OB stars have low masses (~1Msol) with rather large radii (2-4 Rsol) and low temperatures (<15 kK). These companions display no UV signature, which would exclude a hot subdwarf nature, but disentangling of the large set of X-Shooter spectra of HD25631 revealed the typical signature of chromospheric activity in the companion's spectrum. In addition, despite the short orbital periods (P=3-7d), the fast-rotating OB-stars still display non-synchronized rotation and all systems appear young (<20Myr). This suggests that, as in a few other cases, these massive stars are paired in those systems with non-degenerate, low-mass PMS companions, implying that fast rotation would not be a consequence of a past binary interactions in their case.