The mass range for hot subdwarf B stars from MESA simulations : dependence on metallicity and overshooting

Hot subdwarf B (sdB) stars are helium core burning stars that have lost almost their entire hydrogen envelope due to binary interaction. Their assumed canonical mass of MsdB ∼ 0.47 M⊙ has recently been debated given a broad range found both from observations as well as from the simulations. The mass range for sdBs as a function of initial mass was derived two decades ago by Han et al. (2002), using the Eggleton code, for two different metallicities (Z =0.02 and Z = 0.004). Here, I revised and refined these calculations, using the stellar evolution code MESA. An excellent agreement was obtained for low-mass progenitors, up to ∼ 2.0 M⊙. For more massive progenitors, a direct comparison was not possible due to the different prescription for overshooting these authors used, which is not available in MESA. However, I found that in general the MESA models result in a wider mass range compared to the simulations performed by Han et al. (2002) with the Eggleton code, for more massive stars. The effects of metallicity and the inclusion of core overshooting during the main sequence were also analysed. I found that the lower metallicity models predict, on average, slightly more massive sdBs (0.01 − 0.02 M⊙ larger). The inclusion of core overshooting during the main sequence mostly affected progenitors more massive than ∼ 1.5 M⊙, as expected, decreasing the maximum initial mass for which the core becomes degenerate during the red giant branch phase, and increasing the sdB mass for progenitors that ignite helium under non-degenerate conditions. The duration of the sdB phase was also calculated, finding a strong anti-correlation with the sdB mass. Finally, I discussed several factors that might affect the sdB mass distribution and should be considered in binary population synthesis models that aim to compare with observational samples.