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