Photometric Analysis of 30 Contact Binaries in M31

TL;DR

This study analyzes 30 contact binaries in the Andromeda galaxy (M31) using PHOEBE with Bayesian optimization and MCMC to derive their absolute and orbital parameters. It identifies 10 systems with a prominent O'Connell effect explained by a dark spot on the primary, 11 extremely low mass-ratio systems (ELMRCBs) as merger candidates, and two massive binaries that may evolve into compact-object binaries, including potential gravitational-wave progenitors. Comparative diagrams reveal that, at fixed mass, M31 targets have larger radii and higher luminosities, and lower orbital angular momenta than Milky Way counterparts, suggesting faster evolution likely driven by higher metallicity in M31. The results underscore environmental effects on binary evolution and highlight targets for future high-precision photometric and spectroscopic follow-up to confirm these progenitor scenarios.

Abstract

M31, as the largest galaxy in the Local Group, is of significant importance for the study of stellar formation and evolution. Based on the data of 5,859 targets observed in M31 by Gu et al (2024), we selected 30 contact binaries by visual inspection for further study. Using the PHOEBE software and employing Bayesian optimization and Markov Chain Monte Carlo sampling, we determined the physical parameters of these 30 systems. The results show that 10 systems exhibit the O'Connell effect, which is well explained by introducing a dark spot on the primary star. 11 systems have mass ratios below 0.15, classifying them as extremely low mass ratio contact binaries, making them promising candidates for binary mergers. Six systems have primary star temperatures exceeding 10,000 K, classifying them as early-type contact binaries. The absolute physical parameters reveal that two contact binary systems contain massive stellar components, which may eventually evolve into compact binary star systems. To compare the effects of different galactic environments on the evolution of contact binaries, we constructed evolutionary diagrams for these 30 targets and for contact binaries in the Milky Way. The results show that, at the same mass, our targets have larger radii, higher luminosities, and lower orbital angular momenta than contact binaries in the Milky Way, indicating that they are at more advanced evolutionary stages. This may be attributed to the higher metallicity in M31 compared to the Milky Way.

Figures (10)

• Figure 1: Probability distributions of $q$, $T_2$, $i$, $f$, $L_{1g}/L_{Tg}$ and $L_{1r}/L_{Tr}$ determined by the MCMC modeling of M31_CB_14.
• Figure 2: Probability distributions of $q$, $T_2$, $i$, $f$, $L_{1g}/L_{Tg}$, $L_{1r}/L_{Tr}$, $L_{3g}/L_{Tg}$, $L_{3r}/L_{Tr}$, $\lambda$, $r_s$ and $T_s$ determined by the MCMC modeling of M31_CB_16.
• Figure 3: The fitting results and residuals of M31_CB_14 and M31_CB_16. In each panel, the upper shows the fitting results, with blue dots for g-band data and orange dots for r-band data, and the black curve indicates the fitting curve. The bottom of the panel shows the O-C residuals of the two bands respectively. The observational errors of M31_CB_16 are smaller than the size of the symbols.
• Figure 4: The M–R, M–L and T-L distributions for contact binaries. The colored dotted lines indicate the evolutionary tracks for solar chemical compositions. The solid and dashed lines indicate the ZAMS and the TAMS, respectively. The gray filled and open circles represent the primary and secondary components of the contact binaries in the Milky Way, while the red filled and open circles represent those of the 30 contact binaries in M31.
• Figure 5: The relation between orbital angular momentum and total mass for contact binaries. The dashed line indicates the boundary line between the detached and contact binaries. The gray filled and open circles represent the primary and secondary components of the contact binaries in the Milky Way, while the red filled and open circles represent those of the 30 contact binaries in M31.