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

TL;DR

This study investigates how a case B merger between an 11 M_sun primary and a 6.6 M_sun secondary, occurring during the Hertzsprung gap, reshapes the internal structure and subsequent evolution of the product. Using MESA in 1D, it compares three merger-construction schemes—rapid accretion with surface-like composition, rapid accretion with secondary-average composition, and entropy-sorted mixing—evolving the remnants to carbon ignition. All merger products become extended blue supergiants with undermassive helium cores and low carbon masses, but the internal evolution and CO-core properties depend on the construction method, particularly for entropy-sorted models. The results imply that merger history strongly influences core structure and late-stage evolution, with significant implications for supernova progenitors and observational signatures, and motivate building a broader grid of entropy-sorted merger models across masses and mass ratios.

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.

Figures (9)

• Figure 1: The period evolution and mass transfer/mass loss rate for the primary star in the 11 + 6.6 $\mathrm{M}_\odot$ binary for the last $\sim$10 000 years prior to the contact phase. As the mass transfer initiates and rapidly increases, the period shrinks.
• Figure 2: Top left: The entropy profiles of both the primary and secondary immediately preceding the merger. The structure of the merger product will be determined by new composition profiles rank ordered by entropy. Top right: The entropy-sorted $^4$He profile of the merger product compared to the profiles of the stars individually. The combined profile is colored based on which star contributed the helium in each zone. Vertical dashed lines mark the maximum extent of the helium core and base of the convective envelope at carbon ignition. Bottom left: The helium profiles of the entropy-sorted (dark red), rapid-accretion (pink), and composition-averaged rapid-accretion (navy) models immediately post-merger. Bottom right: The helium profiles of all three merger models after they have thermally relaxed. The spikes in the entropy-sorted model have settled into a more coherent distribution of helium.
• Figure 3: The thermal structure of the merger at the end of each of the first three steps in the creation of the merger product, with ('+ $\tau_{KH}$') and without thermal relaxation of roughly a thermal time (3500 years) in between the mass relaxation and the composition relaxation. The thermal profile of the primary is also shown for comparison. If the thermal relaxation is not included, the temperature gradient is steeper and the temperature outside of the core is an order of magnitude cooler.
• Figure 4: The distribution of $X_\mathrm{C}$ and $M_\mathrm{CO}$ of our models compared to MESA models of single and stripped stars from Sch21. Both the rapid accretion and the entropy sorted merger models have significantly lower $X_\mathrm{C}$ than the single and stripped stars.
• Figure 5: Evolution in central density $\rho_\mathrm{C}$ and temperature T$_\mathrm{C}$ of the merger and single star models. Central helium abundances of 0.9, 0.5, and 0.1 are denoted by squares, circles, and triangles respectively. The rapid-accretion merger models overlap.