The Blue Lurker WOCS 14020: A Long-Period Post-Common-Envelope Binary in M67 Originating from a Merger in a Triple System

Abstract

We present Hubble Space Telescope far-ultraviolet (FUV) spectra of a blue-lurker$-$white-dwarf (BL-WD) binary system in the 4 Gyr open cluster M67. We fit the FUV spectrum of the WD, determining it is a C/O WD with a mass of $0.72_{-0.04}^{+0.05}$ M${}_{\odot }$ and a cooling age of $\sim 400$ Myr. This requires a WD progenitor of $\sim 3$ M${}_{\odot }$, significantly larger than the current cluster turnoff mass of 1.3 M${}_{\odot }$. We suggest the WD progenitor star formed several hundred Myr ago via the merger of two stars near the turnoff of the cluster. In this scenario, the original progenitor system was a hierarchical triple consisting of a close, near-equal-mass inner binary, with a tertiary companion with an orbit of a few thousand days. The WD is descended from the merged inner binary, and the original tertiary is now the observed BL. The likely formation scenario involves a common envelope while the WD progenitor is on the AGB, and thus the observed orbital period of 359 days requires an efficient common envelope ejection. The rapid rotation of the BL indicates it accreted some material during its evolution, perhaps via a wind prior to the common envelope. This system will likely undergo a second common envelope in the future, and thus could result in a short-period double WD binary or merger of a 0.72 M${}_{\odot }$ C/O WD and a 0.38 $M_{\odot }$ Helium WD, making this a potential progenitor of an interesting transient such as a sub-Chandrasekhar Type Ia supernova.

13 pages, 3 figures. Published in ApJL, January 2025

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