BSN: The First Multiband Light Curve Analysis of the W UMa-type Contact Binary System EM Tucanae

TL;DR

The paper analyzes EM Tuc, a W UMa-type contact binary, using ground-based multiband photometry and TESS time-series to derive precise orbital and physical parameters. It employs PHOEBE with MCMC for a detailed light-curve solution, revealing a shallow overcontact configuration with a mass ratio $q \approx 3.987$ and a cool-spot to fit asymmetries, complemented by an O–C analysis showing a period increase at $dP/dt = (1.401 \pm 0.0421)\times10^{-7}$ d yr$^{-1}$ and a mass-transfer rate $\dot{M} = -(4.62 \pm 1.54)\times10^{-8}\,M_\odot$ yr$^{-1}$. Absolute parameters are anchored by Gaia DR3 parallax, yielding $M_h = 0.24 \pm 0.05\,M_\odot$, $M_c = 0.97 \pm 0.19\,M_\odot$, with corresponding radii and luminosities that place the components at different evolutionary stages on $M$–$R$ and $M$–$L$ diagrams. Overall, the study demonstrates active mass exchange in EM Tuc, refines its fundamental parameters, and highlights the utility of combining Gaia-based distances with detailed light-curve analyses for W UMa systems.

Abstract

We present a comprehensive photometric light curve and orbital period analysis of the W UMa-type contact binary EM Tuc. The O-C analysis constructed from all available eclipse timings exhibits a clear upward parabolic trend, indicating a continuous increase in the orbital period at a rate of \(dP/dt = (1.401 \pm 0.042)\times10^{-7}\,\mathrm{d\,yr^{-1}}\). This behavior is consistent with mass transfer from the less massive to the more massive star in the system, and the corresponding mass-transfer rate is estimated to be \(\dot{M} = -(4.62 \pm 1.54)\times10^{-8}\,M_\odot\,\mathrm{yr^{-1}}\). Light curve modeling with the PHysics Of Eclipsing BinariEs (PHOEBE) Python code, refined through MCMC sampling, confirms an overcontact configuration and yields a mass ratio of q=3.987. A cool photospheric starspot on the cooler component is required to reproduce the observed O'Connell asymmetry. Using Gaia DR3 parallax together with the photometric solution, the absolute masses of the components are derived as \(M_h = 0.24 \pm 0.05\,M_\odot\) and \(M_c = 0.97 \pm 0.19\,M_\odot\).

Figures (6)

• Figure 1: The O-C diagram of the EM Tuc contact binary.
• Figure 2: The sum of squared residuals as a function of mass ratio. The black arrow indicates the estimated mass ratio in the $q$-search.
• Figure 3: The corner plots of EM Tuc determined by MCMC modeling. The parameters used in the MCMC procedure, presented in the corner plot, include: $i$, $q$, $T_1$, $T_2$, $f$, $l_1$, and four starspot parameters, respectively.
• Figure 4: Ground- and space-based photometric light curves together with synthetic model.
• Figure 5: 3D view of the target system stars.