Minimum and maximum mass-luminosity relations for stripped stars

¹ Armagh Observatory and Planetarium, College Hill, Armagh, BT61 9DG Northern Ireland, UK
² Zentrum für Astronomie der Universität Heidelberg, Astronomisches Rechen-Institut, Mönchhofstr. 12-14, 69120 Heidelberg, Germany
³ Interdisziplinäres Zentrum für Wissenschaftliches Rechnen, Universität Heidelberg, Im Neuenheimer Feld 225, 69120 Heidelberg, Germany

^⋆ Corresponding author: gauthamns96@gmail.com

Received: 29 March 2025
Accepted: 13 August 2025

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 require 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 available in the literature either refer to 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-profile slope increases from 0 (full chemical homogeneity) to ∞ (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; this is 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 have also explored the impact of the higher luminosity on the wind properties of partially stripped configurations using hydrodynamically consistent atmosphere models.