The IACOB project XI. No increase of mass-loss rates over the bistability region

TL;DR

This study investigates whether mass-loss rates in blue supergiants increase at the bistability jump and finds no evidence for such an increase, challenging current stellar evolution models.

Contribution

The paper provides the first comprehensive observational upper limits on mass-loss rates across the bistability region for a large sample of Galactic blue supergiants, showing no increase at the jump.

Findings

No increase in mass-loss rates over the bistability jump.

The drop in stellar rotational velocities is not due to enhanced mass loss.

Current stellar evolution models may need revision regarding mass-loss prescriptions.

Abstract

The properties of blue supergiants are key for constraining the end of the main sequence (MS) of massive stars. Whether the observed drop in the relative number of fast-rotating stars below $\sim$21$\,$kK is due to enhanced mass-loss rates at the location of the bistability jump, or the result of the end of the MS is still debated. Here, we combine newly derived estimates of photospheric and wind parameters with Gaia distances and wind terminal velocities from the literature to obtain upper limits on the mass-loss rates for a sample of 116 Galactic luminous blue supergiants. The parameter space covered by the sample ranges between 35-15$\,$kK in $T_{\rm eff}$ and 4.8-5.8$\,$dex in log(L/L$_{\odot}$). Our results show no increase in the mass-loss rates over the bistability jump. Therefore, we argue that the drop in rotational velocities cannot be explained by enhanced mass loss. Since a large jump in the mass-loss rates is commonly included in evolutionary models, we suggest an urgent revision of the currently used default prescriptions.