The companion mass distribution of post common envelope hot subdwarf binaries: evidence for boosted and disrupted magnetic braking?

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 $3.8<P_{\mathrm{o}\mathrm{r}\mathrm{b}}<12$ 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 $M_{\mathrm{M}\mathrm{S}}\approx 0.15M_{\odot }$ and drops off at $M_{\mathrm{M}\mathrm{S}}>0.2M_{\odot }$, with only two systems hosting companions above the fully-convective limit. There is no correlation between $P_{\mathrm{o}\mathrm{r}\mathrm{b}}$ and $M_{\mathrm{M}\mathrm{S}}$ 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.