Binary evolution at the extremes of mass-transfer efficiency: Contact, mergers, and population signatures

Vollständiger Abstract

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Binary star evolution remains inherently uncertain, and several physical processes are still not well understood. These open questions in binary physics come in addition to major uncertainties in single-star evolution, such as angular momentum transport and interior mixing. For example, the efficiency of mass transfer -- which is the fraction of transferred mass that is actually accreted -- is still one of the main uncertainties in binary evolution. In this work, we present a new grid of 1D binary evolution models with identical initial conditions to an earlier grid in which mass transfer was limited by the spin-up of the accretors. In this new grid, we employ fully conservative mass transfer, thus allowing us to perform a one-to-one comparison between the two grids and cover the full possible range from highly non-conservative to fully conservative mass transfer. We explored how these two maximally different mass-transfer efficiencies change the occurrence and incidence of contact phases, stellar mergers, and common envelope phases and how they affect the present-day population of (post-)mass-transferring binaries. We find that fully conservative mass transfer increases the incidence of contact systems by roughly a factor of six (from 11% to 62%), and the incidence of stellar mergers is increased by more than a factor of two (from 16% to 38%). We also find the emergence of double-core common-envelope phases, which are absent for lower mass-transfer efficiencies. Comparison of two synthetic binary populations built using the two grids with observed Algol and stripped-star binaries reveals that even though conservative mass transfer is favoured to reproduce the observed stripped-star binaries, the observed population of Algol binaries cannot be explained by a single mass-transfer efficiency. We conclude that the mass-transfer efficiency depends on the configuration of binary systems and cannot be described by a single value. Our work highlights the need for a better understanding of binary mass transfer and indicates that current binary-star models are incomplete.