The single star path to Be stars

Abstract

Context. Be stars are rapidly rotating B main sequence stars, which show line emission due to an outflowing disc. By studying the evolution of rotating single star models, we can assess their contribution to the observed Be star populations. Aims. We identify the main effects which are responsible for single stars to approach critical rotation as functions of initial mass and metallicity, and predict the properties of populations of rotating single stars. Methods. We perform population synthesis with single star models of initial masses ranging between 3 and 30 M , initial equatorial rotation velocities between 0 and 600 km s −1 at compositions representing the Milky Way, Large and Small Magellanic Clouds. These models include efficient core-envelope coupling mediated by internal magnetic fields and correspond to the maximum efficiency of Be star production. We predict Be star fractions and the positions of fast rotating stars in the colour-magnitude diagram. Results. We identify stellar wind mass-loss and the convective core mass fraction as the key parameters which determine the time dependance of the stellar rotation rates. Using empirical distributions of initial rotational velocities, our single star models can reproduce the trends observed in Be star fractions with mass and metallicity. However, they fail to produce a significant number of stars rotating very close to critical. We also find that rapidly rotating Be stars in the Magellanic Clouds should have significant surface nitrogen enrichments, which may be in conflict with abundance determinations of Be stars.Conclusions. Single star evolution may explain the high number of Be stars if 70 to 80% of critical rotation would be sufficient to produce the Be phenomenon. However even in this case, the unexplained presence of many Be stars far below the cluster turn-off indicates the importance of the binary channel for Be star production.

Centrifugally driven mass-loss and outbursts of massive stars

“…Some aspects of Be stars remain difficult to explain with our single star models. For instance, like Hastings et al (2019), our single star models predict that Be stars are much more common near the TAMS, whereas only a mild increase is observed. Additionally, we may expect higher levels of nitrogen enrichment than observed.…”

“…Since we now know that AM transport in radiative regions of stars is even more efficient than the TS dynamo predicts (Cantiello et al 2014), a larger fraction of Be stars should be produced. Because our AM transport prescription and the TS dynamo predict nearly rigid rotation during the MS, we expect to find similar results to Hastings et al (2019) for MS stars, though we have not performed a full population synthesis calculation. Like Hastings et al 2019, we find a 'goldilocks' range of ≈ 15-25 M (at solar metallicity) where Be star formation is most efficient, the lower bound arising from a smaller core mass and the upper bound from larger wind AM losses.…”

“…Ekström et al (2008) agrees that inclusion of magnetic torques via the Tayler-Spruit (TS) dynamo (Spruit 2002) would increase AM transport and Be star formation. In the late stages of preparation of this manuscript, Hastings, Wang & Langer (2019) examined Be star formation in models including AM transport via the TS dynamo. They indeed find a higher rate of Be star formation, approximately consistent with observations if Be star are formed when v rot /v crit 0.75.…”

“…Unlike most prior analyses (e.g. Ekström et al 2008;Hastings et al 2019), we incorporate very efficient angular momentum (AM) transport within our stellar models as required by asteroseismic observations (e.g. Mosser et al 2012;Cantiello et al 2014;Gehan et al 2018;Fuller et al 2019).…”

A dearth of young and bright massive stars in the Small Magellanic Cloud

“…B.10. Emission features make it likely that these are late-MS stars that evolved toward critical velocity in isolation (Ekström et al 2008;Hastings et al 2020) or were aided by a binary companion (e.g., Gies et al 1998;Wang et al 2020) rather than He-burning blue supergiants. The offset of 0.05 − 0.1 dex is in line with a shift in T eff of 0.05 dex at critical rotation (Paxton et al 2019), uncertainties in overshooting (see Schootemeijer et al 2019), and modest errors.…”