A new sample of massive B-type contact binary candidates from the OGLE survey of the Magellanic Clouds

TL;DR

This study tackles the scarcity of massive B-type contact binaries by mining the OGLE-IV photometric survey in the Magellanic Clouds and combining empirical light-curve criteria with large grids of MESA binary models and PHOEBE light-curve synthesis. It defines an EW-like subsample and a bona fide CB subset, identifying 37 CB candidates (28 LMC, 9 SMC) dominated by B-type systems with periods near $0.6$–$1$ day, consistent with predicted $q\approx1$ populations. The analysis reveals a degeneracy between true contact and near-contact configurations in light curves, underscoring the need for spectroscopic follow-up to measure mass ratios and validate the evolutionary scenario of massive CBs. Overall, the work expands the observed CB census by an order of magnitude and validates population-level predictions, providing a framework to connect detailed binary evolution with large photometric surveys. Spectroscopic RV data from surveys like BLOeM or 4MOST will be crucial to confirm mass ratios and refine merger-rate implications for massive CBs.

Abstract

Massive contact binaries (CBs) are key to understanding close-binary evolution and stellar mergers, yet their study has been limited by the scarcity of observed systems, particularly of B-type binaries expected to dominate this class. We bridge this gap by mining a large sample of massive CB candidates from the OGLE-IV database, increasing their known numbers in the Magellanic Clouds by nearly an order of magnitude. Using main-sequence colour-magnitude limits, an observationally informed period-luminosity-colour relation for CBs, and a high morph-parameter cut ($c\geq0.7$), we identified 68 O- and B-type binaries that exhibit smooth, sinusoidal light curves with nearly equal eclipse depths. We then isolated a bona fide sample of 37 CB candidates (28 in the LMC and 9 in the SMC) that match theoretical colour-magnitude and period distributions derived from an extensive grid of MESA binary models. The bona fide sample, dominated by B-type systems with $P\approx0.6-1$ d, agrees with the predicted population and may contain many $q\approx1$ binaries, as expected from models showing mass equalization preceding temperature equalization during nuclear-timescale contact. Synthetic PHOEBE light curves of contact and near-contact phases of MESA models reveal a degeneracy between these configurations, suggesting possible misidentifications among these systems. Spectroscopic follow-up is required to test these predictions and refine the evolutionary framework of massive CBs.

Figures (8)

• Figure 1: OB binaries with periods $P>3$ days across all morph parameter values, identified as MS systems from the OGLE sample (light grey dots) on the absolute colour ($M_\textrm{V,max}-M_\textrm{I,max}$) -- magnitude ($M_\textrm{V,max}$) space (corresponding to the maximum brightness of the binary). Overlaid are the empirically identified OGLE EW+ subsample from the LMC (yellow circles, as described in Section \ref{['CB_sample']}). Curves indicate the limits of Zero Age and Terminal Age Main Sequence (ZAMS and TAMS) curves computed from non-rotating and rotating ($v_\textrm{i}=330$ km/s) models of single stars with masses of $7-50$$\mathrm{M}_\odot$.
• Figure 2: Characterization of the OGLE binary sample: Black filled circles represent the ratio of LMC to SMC OGLE binaries, red circles are the same ratio but for $Gaia$ MS stars, and blue triangles are the percentage binary fractions (OGLE/$Gaia$) for the LMC (filled triangles) and SMC (open triangles). The x-axis represents the absolute V-band magnitude $M_\textrm{V}$ (or $M_\textrm{V,max}$ in the case of OGLE binaries). Error bars represent Poisson uncertainties $\sqrt{N}$, where $N$ is the sample size. Absolute magnitudes were obtained by adopting distance moduli of 18.48 and 18.96 for the LMC and SMC respectively, with mean extinction corrections of 0.34 and 0.25 magnitudes.
• Figure 3: Exemplary I-band light curves from the OGLE database, classified by this study as EW (top), EB (middle) and ELL (bottom). The OGLE database itself uses a different classification scheme.
• Figure 4: Panels showing the evolution of the exemplary 'System 2' binary model from Paper I, with initial parameters: M$_\mathrm{T,i} = 26\,\mathrm{M_{\odot}}$, P$_\mathrm{i}$ = 1.0 $\mathrm{d}$, q$_\mathrm{i}$ = 0.8. The parameters from top to bottom panels are: the fill-out factor (R/R$_\textrm{RL}$), the temperature and mass ratio (T$_2$/T$_1$; q), orbital period (P), mass of either component and the mass-transfer rate. Also shown are the contact (light-green region) phase and near-contact (pink region) phase, which is further divided into the detached phase (cross hatching) and semi-detached phase (striped hatching). The open circles are the instances at which synthetic PHOEBE light curves are computed (Fig. \ref{['PHOEBE_LC']}).
• Figure 5: Light curves computed for System 2 at the various evolutionary points marked in Fig. \ref{['orbital_fig']}, at inclinations of $i=30^{\circ},60^{\circ}$ and 90$^0$. Top panel: during the contact phase (green region in Fig. \ref{['orbital_fig']}); the cyan light curve here represents the dominant type of CB expected from models, i.e., one which has acquired temperature equalization. Bottom panel: during the near-contact phase CB (pink striped region in Fig. \ref{['orbital_fig']}), which is further split as a semi-detached (red and orange light curves ) and detached system (cyan light curve). Also labelled are the relative eclipse depths, $f_{\Delta A}$, except for ELLs which have $f_{\Delta A}$ of the order of 0.001.