Asteroseismic Imprints of Mass Transfer in Binary Stars: Probing the Interiors of Donors and Accretors with Gravity and Acoustic Modes

Abstract

Context. The synergy between close binary stars and asteroseismology enables constraints on mass-transfer episodes and their consequences for internal structure, rotation profiles, and oscillation modes. Aims. We investigate how mass accretion and donation in close binaries affects the internal structure and oscillation modes of main-sequence stars. Methods. Building on the established relation between the Brunt-Vaisala (buoyancy) glitch and the Fourier spectra of g-mode period spacings, we quantitatively explain the origins of the g-mode period-spacing differences between single-star and mass-accretion/donation models of intermediate-mass stars (M = 2.0, 3.0, and 4.5 Msun). In particular, the hydrogen mass fraction profiles X of the donor model show two chemical gradient regions, which results in a double-peaked Brunt-Vaisala profile. The presence of additional buoyancy glitches gives rise to further periodic modulations in the g-mode period spacings. Results. Mass-accretion induced changes in the chemical profile create sharp features in the buoyancy frequency, which modify both the amplitudes and frequencies of the g-mode period-spacing variations. This behavior resembles that produced by multiple chemical transition zones in compact pulsators such as white dwarfs and sub-dwarf B stars. Similarly, for acoustic modes in the M = 1 Msun solar-like models, we attribute the differences in frequency-separation ratios between single-star and mass-donor models to the variations in the internal sound-speed gradient (acoustic glitches). We discuss future prospects for using asteroseismology to discover the mass-transfer products and constrain the mass-transfer processes in binary star evolution.

Figures (6)

• Figure 1: Left: a1) Evolutionary tracks of four different scenarios of high-mass stars on the HR diagram. A1 are A4 are single-star models ($M=4.5$ and $M=3.0~{\rm M_{\odot}}$), and A2 and A3 are mass-accreting and mass-donating models ($M=3.0 \rightarrow 4.5~{\rm M_{\odot}}$ and $M=4.5\rightarrow 3.0~{\rm M_{\odot}}$). a2) The asymptotic period spacing $\Delta P$ as a function of the central hydrogen mass fraction $X_c$. a3) The profiles of hydrogen mass fraction $X$ for selected models are labeled by different symbols and colors in the corresponding upper and middle panels. Right: b1) Evolutionary tracks of four different scenarios of low-mass stars on the HR diagram. b2) Asymptotic period spacing $\Delta P$ as a function of $X_c$. b3) The profiles of hydrogen mass fraction $X$ in the stellar interior for selected models are labeled by different symbols and colors in b1) and b2).
• Figure 2: Stellar structure and oscillations of a $4.5~\rm M_{\odot}$ star from the single-star model (red) and the mass-accretion model (black). Left panels: The profiles of the buoyancy frequency $N$ (solid lines: left y-axis) and the hydrogen mass fraction $X$ (dashed lines: right y-axis). From the top to bottom, the central hydrogen $X_c$ values are 0.4, 0.3, 0.2, 0.1, 0.01. Right panels: The corresponding period spacing patterns of $l=1$ and $l=2$ g modes.
• Figure 3: Stellar structure and oscillations of a $3.0~{\rm M_{\odot}}$ star from the single-star model (red) and a mass-donor model (black). Left panels: The profiles of the buoyancy frequency $N$ (solid lines: left y-axis) and the hydrogen mass fraction $X$ (dashed lines: right y-axis). Right panels: The corresponding period spacing patterns of $l=1$ and $l=2$ g modes
• Figure 4: Upper panels: Brunt–Väisälä (buoyancy) frequency profiles (orange) in two accretor models: A3-1 (left) and A3-3 (right), the later corresponds to the one with additional Brunt bumps. The normalized Fourier spectra of the period spacings of $l=1$ g modes is shown as the blue and gray lines (super Nyquist part). The vertical red dashed lines indicate the locations of the Nyquist frequency (0.5). The real peaks corresponding to the period-spacing variation are labeled by the orange squares. Note that in the Fourier spectra of A3-3 model, the dominant peak at $0.53$ is above the Nyquist frequency, and its reflection at $0.47$ is a fictitious peak. Lower panels: The corresponding period spacings of $l=1$ g modes as a function of radial order.
• Figure 5: Stellar structure and oscillations of a $2.0~\rm M_{\odot}$ star from the single-star model (red) and a mass-accretor model (black). Left panels: The profiles of the buoyancy frequency $N$ (solid lines: left y-axis) and the hydrogen mass fraction $X$ (dashed lines: right y-axis). Right panels: The corresponding period spacing patterns of $l=1$ and $l=2$ g modes