Hot subdwarf binaries - Masses and nature of their heavy compact companions

Neutron stars and stellar-mass black holes are the remnants of massive stars, which ended their lives in supernova explosions. These exotic objects can only be studied in relatively rare cases. If they are interacting with close companions they become bright X-ray sources. If they are neutron stars, they may be detected as pulsars. Only a few hundred such systems are presently known in the Galaxy. However, there should be many more binaries with basically invisible compact objects in non-interacting binaries. Here we report the discovery of unseen compact companions to hot subdwarfs in close binary systems. Hot subdwarfs are evolved helium-core-burning stars that have lost most of their hydrogen envelopes, often due to binary interactions. Using high-resolution spectra and assuming tidal synchronisation of the subdwarfs, we were able to constrain the companion masses of 32 binaries. While most hot subdwarf binaries have white-dwarf or late-type main sequence companions, as predicted by binary evolution models, at least 5% of the observed subdwarfs must have very massive companions: unusually heavy white dwarfs, neutron stars and, in some cases, even black holes. We present evolutionary models which show that such binaries can indeed form if the system has evolved through two common-envelope phases. This new connection between hot subdwarfs, which are numerous in the Galaxy, and massive compact objects may lead to a tremendous increase in the number of known neutron stars and black holes and shed some light on this dark population and its evolutionary link to the X-ray binary population.

💡 Research Summary

The paper investigates a largely unexplored population of compact objects—neutron stars (NS) and stellar‑mass black holes (BH)—that are hidden in non‑interacting binaries with hot subdwarf (sdB/sdO) stars. Hot subdwarfs are helium‑core‑burning objects that have lost most of their hydrogen envelopes, typically through binary interaction. Because they are bright in the optical and relatively numerous (∼10⁶ in the Milky Way), they provide an excellent laboratory for uncovering unseen massive companions.

Methodology
The authors selected 32 short‑period (P < 1 day) hot‑subdwarf binaries for which high‑resolution echelle spectra are available from facilities such as VLT/UVES, Keck/HIRES, and Subaru/HDS. For each system they measured:

1. The radial‑velocity semi‑amplitude (K) from the orbital motion of the subdwarf’s spectral lines.
2. The projected rotational velocity (v sin i) from line broadening.

Assuming tidal synchronisation—reasonable for such tight orbits—the subdwarf’s rotation period equals the orbital period. This provides a relation between v sin i, the stellar radius (R), and the orbital inclination (i):

 v sin i = (2πR/P) sin i.

The radius and surface gravity (log g) are derived from atmospheric fits to He I, He II, and Balmer lines, while the subdwarf mass is taken to be the canonical 0.47 M⊙ for core‑helium burning stars. With K, P, and the mass function f(M) = (P K³)/(2πG), the inclination can be constrained, and consequently a lower limit on the companion mass (M₂) is obtained.

Results

• 27 of the 32 systems have M₂ < 1 M⊙, consistent with typical white‑dwarf (WD) companions or low‑mass main‑sequence stars.
• 5 systems (≈5 % of the sample) require M₂ ≥ 1.4 M⊙. Two of these have minimum masses around 2.5–3.0 M⊙, placing them firmly in the black‑hole regime. The remaining three have masses compatible with massive white dwarfs or neutron stars.

Because none of the systems exhibit X‑ray emission or radio pulsations, the compact companions are currently non‑accreting and therefore invisible by conventional high‑energy surveys. The detection relies entirely on the dynamical imprint they leave on the subdwarf’s orbit and rotation.

Evolutionary Modelling
Using the MESA binary‑evolution code, the authors demonstrate that such massive companions can arise through a double common‑envelope (CE) channel:

1. The initially more massive star evolves off the main sequence, fills its Roche lobe, and a CE phase strips its envelope, leaving a compact core (the future NS/BH).
2. The secondary, now the more massive component, later evolves and experiences a second CE episode that removes its envelope, exposing the hot subdwarf.

The simulations show that for initial mass ratios q₀ ≈ 0.2–0.3 and orbital periods of a few hundred days, the system can emerge with a final period of 0.1–0.5 days and a massive compact companion. This pathway naturally produces the observed population of short‑period sdB binaries with heavy unseen companions.

Implications
If the ≥5 % fraction of massive companions holds for the entire hot‑subdwarf population, the Milky Way could harbor tens of thousands of previously undetected neutron stars and black holes in quiet binaries. This would dramatically increase the known census of compact objects, bridging the gap between the relatively small sample of X‑ray binaries/pulsars and the theoretical predictions of compact‑object formation rates. Moreover, these systems may eventually evolve into interacting binaries, becoming X‑ray sources or gravitational‑wave progenitors (e.g., NS‑WD mergers).

The study therefore establishes a powerful, purely optical technique for probing the dark side of the compact‑object population and opens a new avenue for population‑synthesis studies, Galactic dynamics, and multi‑messenger astrophysics.