Minimum and maximum mass-luminosity relations for stripped stars

TL;DR

This paper develops new mass-luminosity relations for partially-stripped helium stars, improving models for their evolution and wind properties, which are crucial for understanding their role in supernovae and compact object formation.

Contribution

It introduces synthetic models of partially-stripped helium stars with chemical gradients, providing new mass-luminosity relations that account for partial envelope stripping, unlike previous fully-stripped or homogeneous models.

Findings

Maximum luminosity occurs at intermediate hydrogen gradient slopes.

Partially-stripped stars can exceed pure-He star luminosities.

New fit relations enable better predictions of star properties.

Abstract

Envelope stripping, whether through single-star wind mass loss or binary mass transfer, is a key evolutionary pathway for the formation of classical Wolf-Rayet stars and lower-mass stripped helium (He) stars. However, to study the evolution of these objects into black holes, neutron stars, and stripped-envelope supernovae, we need appropriate input models for the core-He burning phase without relying on the uncertain evolution into this evolved phase. Reliable mass-luminosity relations (MLRs) for He stars are needed for stellar wind and evolution studies, but the MLRs currently in literature are either for fully-stripped or chemically homogeneous stars, neither of which reflect the important and recently also observationally confirmed stage of partial stripping. We alleviate this drawback by computing sets of MESA synthetic structure models with partially-stripped chemical profiles, consisting of a pure-He core and a hydrogen (H)-depleted envelope with an H/He chemical gradient left behind from the receding convective core during the main sequence. As the H slope increases from 0 (full chemical homogeneity) to $\infty$ (pure-He stars) in our synthetic models, we find the luminosity to initially increase before eventually decreasing. The maximum luminosity for a given mass is reached for an intermediate H-profile slope corresponding to a partially-stripped structure, exceeding even the values documented for pure-He stars, primarily due to the H shell disproportionately dominating the total luminosity budget. We also provide convenient mass-luminosity fit relations to predict the minimum, maximum, and pure-He luminosities for a given mass -- and vice versa -- while accounting for structures achievable through partial stripping. We also explore the impact of the higher luminosity on the wind properties of partially-stripped configurations using hydrodynamically consistent atmosphere models.