Detailed Investigation of a W UMa Contact Binary with an Ultralow Mass Ratio and a Third-Body as a Potential Merger Candidate

TL;DR

This work targets the extremely low mass-ratio end of W UMa contact binaries by reanalyzing TYC 3801-1529-1 with TESS photometry and BSN-based MCMC, yielding a record-low $q = 0.024_{-0.001}^{+0.002}$ and a near-zero third-light contribution. Photometric modeling (PHOEBE) validated against prior methods, followed by absolute-parameter inference, indicates a brown-dwarf-like secondary and a low-mass stellar tertiary, with orbital parameters implying a dynamically evolving system. The O-C timing analysis reveals a cyclic modulation consistent with a third body of approximately $0.40$–$0.58~M_\odot$ and $P_{ m mod} oughly 8.75$ years, supported by robust periodogram checks against sampling aliasing. Collectively, the system appears dynamically unstable and a merger candidate, analogous in significance to V1309 Sco, and the authors advocate high-resolution spectroscopy to definitively constrain the masses and stability of this intriguing triple-contact system.

Abstract

The lower limit of the mass ratio in contact binaries remains uncertain, with observations suggesting systems exist below theoretical predictions. The stability of such very low mass ratio systems is still debated. Based on our review of systems within the mass ratio cutoff range, we reanalyzed TYC 3801-1529-1 and found it to have the lowest known mass ratio, q=0.024_{-(1)}^{+(2)}, among analyzed contact binaries. The reanalysis of this target was carried out using the BSN application and the MCMC method. We then compared our light curve solution obtained from TESS observations with the results of a previous study. We studied the period variations of this system and identified a cyclic trend over the past six years. After the third-body contribution has been removed, the system's period variations can be described either by a linear trend with a negative slope or by a quadratic trend with a downward curvature. These results indicate that it is still not possible to definitively determine whether the orbital period is increasing or decreasing, underscoring the importance of future observations. By considering the challenges of detecting the faint secondary in extremely low mass ratio systems, we estimated the absolute parameters of the target. Based on our analysis, the secondary component of the binary is likely a brown dwarf, while the third body appears to be a low-mass M-type dwarf. According to our analysis, TYC 3801-1529-1 is dynamically unstable and thus represents a remarkable candidate for a binary merger.

Figures (8)

• Figure 1: Observed TESS light curve together with synthetic model.
• Figure 2: The corner plots of TYC 3801-1529-1 were determined by MCMC modeling.
• Figure 3: 3D views of the stars in the system at orbital phases 0, 0.25, 0.5, and 0.75, respectively.
• Figure 4: Time of primary minimum is shown on the left, time of secondary minimum on the right, as observed in this study through the $V$ filter.
• Figure 5: The O-C diagram after removing the linear trend, along with its residuals. Points represent observed minima from TESS (blue), ZTF (green), the literature (orange), and the minima derived in this study (gray). The red line shows the best-fit sinusoidal model.