On the Rarity of X-Ray Binaries with Naked Helium Donors

TL;DR

This paper investigates the scarcity of observed high-mass X-ray binaries with naked helium donors, proposing that binary mergers during common-envelope phases are common and consistent with current observations, thus constraining common-envelope physics.

Contribution

It demonstrates through stellar evolution models that binary mergers during common-envelope phases explain the rarity of He-HMXBs and aligns with observed populations.

Findings

Binary mergers during CE events are likely common.

The scenario aligns with standard CE energy parameterization.

Future observations can further constrain CE ejection efficiency.

Abstract

The paucity of known High-Mass X-Ray Binaries (HMXB) with naked He donor stars (hereafter He star) in the Galaxy has been noted over the years as a surprising fact, given the significant number of Galactic HMXBs containing H-rich donors, which are expected to be their progenitors. This contrast has further sharpened in light of recent observations uncovering a preponderance of HMXBs hosting loosely bound Be donors orbiting neutron stars (NS), which would be expected to naturally evolve into He-HMXBs through dynamical mass transfer onto the NS and a common-envelope (CE) phase. Hence, reconciling the large population of Be-HMXBs with the observation of only one He-HMXB can help constrain the dynamics of CE physics. Here, we use detailed stellar structure and evolution models and show that binary mergers of HMXBs during CE events must be common in order to resolve the tension between these observed populations. We find that, quantitatively, this scenario remains consistent with the typically adopted energy parameterization of CE evolution, yielding expected populations which are not at odds with current observations. However, future observations which better constrain the underlying population of loosely bound O/B-NS binaries are likely to place significant constraints on the efficiency of CE ejection.