The reason for the occurrence of W-type contact binaries

TL;DR

The study tackles the longstanding puzzle of W-type contact binaries, where the less massive star appears hotter, by leveraging a comprehensive catalog of 3,580 systems with photometric and spectroscopic parameters. It tests the magnetic-activity hypothesis by deriving absolute stellar parameters via isochrone interpolation (PARSEC and MIST) and correlating W-type occurrence with magnetic proxies such as starspot frequency, Rossby number $R_o$, and relative common-envelope thickness $Th_{CE}/R_1$, using Gaia DR3 and LAMOST parameters. The results reveal a strong link between magnetic activity and W-type frequency, robust to mass-uncertainty checks and consistent with observed A/W-type transitions, while also detailing spot distributions, primary mass–metallicity relations, and a tendency for evolution toward lower mass ratios. Together, these findings advance a magnetic-activity-driven framework for W-type formation, with implications for binary evolution, spot physics, and the interpretation of short-period binaries as distance indicators. The work also provides a large, publicly accessible catalog enabling further statistical exploration of contact-binary properties.

Abstract

For more than half a century, the puzzling W-type phenomenon in contact binaries has challenged astrophysicists. In these systems, the less massive component exhibits a higher surface temperature than its more massive companion, which is a reversal of the typical A-type configuration, where the more massive star is hotter. This counterintuitive temperature inversion defies the basic stellar physics and still lacks a widely accepted explanation. In this study, we assembled a sample of over 3,000 extensively observed contact binaries and derived their complete set of physical parameters. Our statistical analysis revealed a strong positive correlation between the occurrence of W-type contact binaries and the intensity and frequency of magnetic activities. This result strongly supports the hypothesis that magnetic activities are the primary driver of the W-type phenomenon and offers a compelling explanation for the observed transitions between the W-type and A-type.

Figures (10)

• Figure 1: Panels 1-3: The number of W-type ($N_W$), A-type ($N_A$), and their ratio $N_W/N_A$ as a function of the primary star mass ($\log M_1$), common envelope thickness ($Th_{CE}/R_1$), and Rossby Number ($R_o$). Panels 4-6: Same as Panels 1-3, but for the number of contact binaries with starspots ($N_{\text{Spot}}$) and without starspots ($N_{\text{No Spot}}$). Panels 7-9: Comparison of $N_W/N_A$ and $N_{\text{Spot}}/N_{\text{No Spot}}$ from Panels 1-6. The color of the curves corresponds to the color of the Y-axes.
• Figure 2: Distribution of the spots from light curve analysis by 2020PASJ...72..103L. The point size represents the spot's area, and the color, defined by the colorbar, indicates the temperature ratio of the spot to the surrounding photosphere. The symbol shape denotes the binary type. The marginal histograms at the top and right correspond to the distributions along the horizontal and vertical axes, respectively.
• Figure 3: The relationship between primary star mass $M_1$ and metallicity [Fe/H] for all contact binaries (Panel 1), for contact binaries with a temperature difference between the two stars greater than 5% (Panel 2). Different colors represent different binary types (red for A and blue for W), while different shapes indicate different data sources. The same relationship for 500 random stars from LAMOST DR11 (Panel 3) and 300 random stars from Gaia DR3 (Panel 4).
• Figure 4: Distribution of the mass ratio $q$. Data were sourced from Kepler (top left), TESS (top right), CSS (bottom left), and individual studies (bottom right).
• Figure 5: The relationship between mass ratio $q$ and relative common envelope thickness $Th_{CE}/R_1$. The curve represents the boundary for a fill-out factor $f=1$. The data point colors and shapes follow the same scheme as in Figure \ref{['fig:mass_feh_relationship']}.