The structure and evolution of a high-mass stellar merger in the Hertzsprung gap

TL;DR

This study models the long-term evolution of high-mass stellar mergers crossing the Hertzsprung gap, revealing common features like blue supergiant phases and core differences based on merger modeling methods.

Contribution

It compares two different methods of modeling stellar mergers and analyzes their impact on the star's internal structure and evolution to carbon ignition.

Findings

All merger products undergo an extended blue supergiant phase.

Merger products have undermassive helium cores and low carbon fractions.

Different modeling methods lead to distinct internal evolution patterns.

Abstract

Post-main-sequence binary mergers are a common evolutionary pathway for massive stars, but the effects of merging on the long-term structure and evolution of the resulting star are a matter of active debate. Furthermore, the way in which merger products are modeled in 1D is not uniform. We present the evolution of an 11 M$_\odot$ and 6.6 M$_\odot$ binary on an 11 day orbit, that merges while the primary is crossing the Hertzsprung gap. We construct the merger product either by rapidly accreting the secondary onto the surface of the primary or by injecting material from the secondary deeper into the primary via entropy sorting. We then evolve them to carbon ignition, comparing their interior structures at this stage. We find that all merger products experience an extended blue supergiant phase and have undermassive helium cores and low carbon mass fractions compared to single and stripped stars. However, the evolution of central density, temperature, and composition in the entropy-sorted model is distinct from those of the rapid-accretion models.