2024/08/27 by Lisa Blomberg, Kareem El-Badry, Blomberg, Lisa +9
Earth and Planetary Sciences · Physics and Astronomy · #FOS: Physical sciences #Geological and Geophysical Studies #Geophysics and Gravity Measurements #Pulsars and Gravitational Waves Research #Solar and Stellar Astrophysics (astro-ph.SR)
paper · pdf · doi:10.48550/arxiv.2408.15334
openalex publication_date 2024/08/27 · openalex created_date 2024/09/21 · openalex updated_date 2026/07/31
We measure the mass distribution of main-sequence (MS) companions to hot subdwarf B stars (sdBs) in post-common envelope binaries (PCEBs). We carried out a spectroscopic survey of 14 eclipsing systems ("HW Vir binaries") with orbital periods of 3.8 < P\rm orb < 12 hours, resulting in a well-understood selection function and a near-complete sample of HW Vir binaries with G < 16. We constrain companion masses from the radial velocity curves of the sdB stars. The companion mass distribution peaks at M\rm MS≈ 0.15 M\odot and drops off at M\rm MS > 0.2 M\odot, with only two systems hosting companions above the fully-convective limit. There is no correlation between P\rm orb and M\rm MS within the sample. A similar drop-off in the companion mass distribution of white dwarf (WD) + MS PCEBs has been attributed to disrupted magnetic braking (MB) below the fully-convective limit. We compare the sdB companion mass distribution to predictions of binary evolution simulations with a range of MB laws. Because sdBs have short lifetimes compared to WDs, explaining the lack of higher-mass MS companions to sdBs with disrupted MB requires MB to be boosted by a factor of 20-100 relative to MB laws inferred from the rotation evolution of single stars. We speculate that such boosting may be a result of irradiation-driven enhancement of the MS stars' winds. An alternative possibility is that common envelope evolution favors low-mass companions in short-period orbits, but the existence of massive WD companions to sdBs with similar periods disfavors this scenario.