The ASAS-SN Catalog of Variable Stars VII: Contact Binaries are Different Above and Below the Kraft Break

TL;DR

This study analyzes over 71,000 contact binary stars, revealing a clear division in their properties above and below the Kraft break, with distinct period-luminosity relations and temperature trends.

Contribution

It provides a detailed characterization of contact binaries, identifying a sharp break in their properties related to the Kraft break, and derives separate period-luminosity relations for different populations.

Findings

Clear period-luminosity break at log(P/d) ≈ -0.30

Distinct temperature-period trends for early and late-type binaries

Derived separate PLRs for different binary populations

Abstract

We characterize ${\sim} 71,200$ W UMa type (EW) contact binaries, including ${\sim} 12,600$ new discoveries, using ASAS-SN $V$-band all-sky light curves along with archival data from Gaia, 2MASS, AllWISE, LAMOST, GALAH, RAVE, and APOGEE. There is a clean break in the EW period-luminosity relation at $\rm \log (\rm P/d){\simeq}-0.30$, separating the longer period early-type EW binaries from the shorter period, late-type systems. The two populations are even more cleanly separated in the space of period and effective temperature, by $\rm T_{eff}=6710\,K-1760\,K\,\log(P/0.5\,d)$. Early-type and late-type EW binaries follow opposite trends in $\rm T_{eff}$ with orbital period. For longer periods, early-type EW binaries are cooler, while late-type systems are hotter. We derive period-luminosity relationships (PLRs) in the $W_{JK}$, $V$, Gaia DR2 $G$, $J$, $H$, $K_s$ and $W_1$ bands for the late-type and early-type EW binaries separated both by period and effective temperature, and by period alone. The dichotomy of contact binaries is almost certainly related to the Kraft break and the related changes in envelope structure, winds and angular momentum loss.