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

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.