The evolutionary and asteroseismic imprints of mass accretion. The 10 M$_\odot$ $β$ Cep case study

TL;DR

This paper addresses how mass accretion in close binaries reshapes the internal structure and oscillation spectra of a $10\,M_\odot$ star and how these changes can be used to identify post-interaction objects. The authors employ non-rotating MESA models of conservative Roche lobe overflow to produce an accretor that is then evolved like a single star, and they perform adiabatic pulsation analyses with GYRE, including weight-function diagnostics, to compare with isolated $10\,M_\odot$ stars. They find a persistent density bump above the convective-core boundary, an extended and mixed core, and the emergence of off-centre convective zones, all of which imprint distinct $g$- and $p$-mode signatures: irregular $\Delta P$ patterns, larger $p$-mode frequency shifts (up to ~0.5%), and periodic modulations in $\delta\nu_{02}/\Delta\nu$ tied to core-boundary discontinuities. These seismic diagnostics offer a powerful means to identify post-interaction stars and to improve age and structure determinations in binary systems, with observable implications for Kepler, TESS, and PLATO data.

Abstract

We investigate the structural and asteroseismic consequences of mass accretion in massive stars within close binary systems. Using MESA, we model the evolution of the 10 M$_{\odot}$ accretor through and after a Roche lobe overflow phase. In addition to changing the surface composition of the star, mass accretion also significantly modifies the internal structure by expanding the convective core and altering chemical stratification near the core-envelope boundary. This partial core rejuvenation creates a distinct mean molecular weight gradient and leaves a persistent local density modulation. In the late stages of mass transfer, changes in density and sound-speed profiles become apparent and influence stellar oscillations. We analyse the asteroseismic properties of the post-mass transfer models compared to single stars of the same mass and central hydrogen abundance. In the gravity mode regime, the altered Brunt-Väisälä frequency leads to period spacing patterns with larger amplitudes and phase shifts. For low and intermediate-order pressure modes, we find systematic frequency deviations linked to changes in the sound-speed profile. Weight function analyses confirm that these differences arise primarily from structural modifications near the convective core boundary. Furthermore, small frequency separations, sensitive to localised sound-speed gradients, reveal periodic variations attributable to the density discontinuity at the convective core edge. The accretor exhibits a larger sound-speed gradient integral and a longer acoustic radius ratio compared to the single star, consistent with its expanded core. Our results demonstrate that mass accretion imprints measurable asteroseismic signatures on both gravity and pressure modes. These signatures provide powerful diagnostics for identifying post-interaction stars, for refining stellar age and structure estimates in binary systems.

Figures (12)

• Figure 1: The Hertzsprung–Russell diagram with the evolutionary tracks of the binary system components. The donor star is shown with the purple line, and the accretor with the gray line, both plotted alongside single-star reference models for different masses (dashed lines). The mass transfer/accretion rate $\log \dot{|M|}$ is overplotted along the tracks using colour coding, as indicated by the colour bar. We mark key evolutionary changes: the donor fills its Roche lobe and begins mass transfer (I$_\mathrm{don}$), the accretor starts to gain mass and departs from its original track (I$_\mathrm{acc}$), full mass ratio reversal when the accretor reaches 10 M$_{\odot}$ and the following detachment of the system (II$_\mathrm{acc}$ and II$_\mathrm{don}$), thermal readjustment of the accretor (III$_\mathrm{acc}$), and the accretor’s single-star evolution until the end of the main sequence (IV$_\mathrm{acc}$).
• Figure 2: The evolution of mass of the convective core $M_\mathrm{cc}$ in a function of central hydrogen abundance $X_c$ for the single-star (orange lines) and the accretor (blue lines) models. We mark key evolutionary changes of the accretor according to \ref{['fig:HR']}.
• Figure 3: The mixing profiles $D$ during the evolution of the accretor. Left: Evolution from $X_c = 0.7$ to 0.0 in steps of 0.1. Right: Profiles at selected evolutionary stages, before, during and after the RLOF. The shaded areas indicate the regions over which the mass varies, with the current mass indicated by dashed lines. The black lines, as denoted on the right-hand ordinates of the right panels show the corresponding profiles of hydrogen abundances.
• Figure 4: The density $\log \rho$ profiles at selected central hydrogen abundances $X_c$, for the single-star (orange lines) and the accretor (blue lines) models. The local bump discussed in the text is clearly visible in the binary models.
• Figure 5: Hydrogen abundances $X$ in a function of a mass for different evolutionary stages of 10 M$_{\odot}$ single (orange lines) and accretor (blue lines) models. The vertical lines mark features corresponding to Figure \ref{['fig:grad_prop_diagrams']}. For clarity, we only show a fraction of the inner mass coordinate.