The evolution of accretor stars in binary systems due to accretion of increasingly helium-rich material

TL;DR

This paper investigates how accreting helium-rich material in close binary systems reshapes the receiving star (the accretor) using a modified Cambridge STARS framework that evolves both stars and their orbit. It introduces variable composition accretion (VCA) and contrasts it with non-variable accretion (NVCA), incorporating thermohaline mixing to model the inverted mean molecular weight profile created by helium-enriched inflows. The key finding is that VCA produces hotter ($ ext{Teff}$ up by about $0.2$ dex) and more luminous ($L$ up by about $0.15$ dex) accretors than NVCA, with thermohaline mixing mitigating the surface gradient and reducing the temperature excess depending on the mixing efficiency $\alpha_{ m th}$. Population synthesis with BPASS v2.2 indicates helium-rich mass transfer is common, with about $23ackslash ext{%}$ of mass-transfer binaries accreting material with He mass fraction $>0.8$, implying significant implications for spectroscopic masses, ionizing photon budgets, and the diversity of supernova progenitors in stellar populations.

Abstract

The recent discovery of examples of intermediate-mass helium stars have offered new insights into interacting binaries. These observations will allow significant improvements in our understanding of helium stars. However, in the creation of these stars their companions may accrete a significant amount of helium-rich stellar material. These creates stars with unusual composition profiles -- stars with helium-rich cores, hydrogen-rich lower envelopes and a helium-rich outer envelope. Thus the mean molecular weight reaches a minimum in the the middle of the star rather than continuously decreasing outwards in mass. To demonstrate this structure we present Cambridge STARS model calculations of an example interacting binary systems where the helium-rich material is transferred, and compare it to one where the composition of the accreted mass is fixed to the companion's surface composition. We show that the helium-rich material leads to the accretor being 0.2 dex hotter and 0.15 dex more luminous than models where the composition is not helium rich. We use a simple BPASS v2.2 population model to estimate that helium-rich mass transfer occurs in 23 per cent of massive binaries that undergo mass transfer. This suggests this is a common process. This binary process has implications for the discrepancy between spectroscopic and gravitational masses of stars, the production of ionizing photons and possibly the modelling of high redshift galaxies.

Figures (7)

• Figure 1: The evolutionary history of a [parse-numbers=false]30+10 binary with an initial period of $\log~P=0.6\days$. Clockwise from the top are the HR diagram, the mass transfer efficiency, the radii, the separation, the mass transfer rates, the masses, and the surface abundances. On the HR diagram, the circles represent every 1M, and the dashed lines signify periods when the respective star is undergoing RLOF. Key phases of the evolution of the binary are shown marked on the plot, and the marked stages have the same meanings as at the beginning of \ref{['sec:response_models']}. All plots except from the HR diagram show only the region surrounding the contact phase (shown in the grey band, lasting $\sim$5k) as this is the region of interest. The mass transfer rate plot includes a black line showing the separation point between fast Case A and slow Case A mass transfer according to Sen:2022.
• Figure 2: The evolution of the radius as a fraction of Roche lobe radius, surface rotation rate as a fraction of critical surface rotation rate, and luminosity of both stars. The grey bar on each subplot denotes the contact phase, which lasts $\sim5k\yr$.
• Figure 3: The Kelvin--Helmholtz time-scales of both stars in the VCA and NVCA regimes. VCA prolongs the final detached phase and shrinks the contact phase. The solid black lines show the length $\tau$ of each phase -- thermal equilibrium is broken if $\tau_\text{KH}>\tau$, i.e., in the first semidetached phase and the contact phase. In the contact and detached phases, the asterisk represents the duration of the NVCA phase.
• Figure 4: The chemical composition of each star in the VCA model as a function of the internal mass for the donor and accretor. Shown are the five key stages of the binary's lifetime as detailed in \ref{['tab:binary_configuration_parameters']}. Letters are as given at the beginning of \ref{['sec:response_models']}. The green lines under the accretor's plot show the regions where the accretor is helium-rich and helium-poor.
• Figure 5: A HR diagram showing the evolution of our VCA and NVCA models. VCA models are hotter than an equivalent binary with NVCA. The point of divergence between the two models is shortly into the contact phase. The two dashed lines on the HR diagram represent models evolved as the final model of the accretor as a single star. The dotted lines have both thermohaline mixing and VCA enabled, with the thermohaline mixing coefficient shown in the legend. In the inset, the blue dot represents the start of the contact phase, and the cross represents its end for the respective track. All thermohaline mixing models were evaluated with 199 meshpoints (whereas the non-thermohaline mixing models were evaluated with 999 meshpoints), and experience breathing pulses post-RGB. Core and surface abundances of the single-star models are given in \ref{['tab:as_single_star_results']}.