WIYN Open Cluster Study. LXXXVII. Hubble Space Telescope Ultraviolet Detection of Hot White Dwarf Companions to Blue Lurkers in M67

Abstract

We present the results of our Hubble Space Telescope far-ultraviolet survey of the blue lurkers (BLs) in M67. We find evidence for two white dwarf (WD) companions among the BLs that are indicative of mass transfer from an evolved companion, one in WOCS 14020 and the other in WOCS 3001. The cooling ages of the WDs suggest that mass transfer in these systems occurred ∼300–540 Myr and ∼600–900 Myr ago, respectively. The rotation periods and cooling ages of the BLs are consistent with spin-up and subsequent single-star spin-down models, and binary evolution models yield plausible evolutionary pathways to both BLs via highly nonconservative mass transfer. We conclude that the BLs are lower-luminosity analogs to the classical blue stragglers.

Wind Roche-lobe Overflow in Low-mass Binaries: Exploring the Origin of Rapidly Rotating Blue Lurkers

“…Moreover, these 11 BLs have a high binary fraction of (at least) 73%, and 45% of them have an orbital period less than 3000 days. Nine et al (2023) confirmed the presence of WD companions in at least two of these BL systems based on observed excess in Hubble Space Telescope/Advanced Camera for Surveys farultraviolet photometry, indicating BLs form via MT. Therefore, BLs are believed to be the low-mass, low-luminosity counterparts of BSSs. In old open clusters (typically >1 Gyr), the majority of binaries containing a BSS or a BL have a wide orbit (with periods on the order of 1000 days; Leiner et al 2019;Leiner & Geller 2021).…”

“…Among these BLs, as shown in Table 3, five have P orb < 3000 days, and the remaining five have P orb > 3000 days. Table 3 does not include WOCS 2068 in Leiner et al (2019) and Nine et al (2023), as this system has two MS stars that may not form through MT.…”

M67 Blue Stragglers with High-resolution Infrared Spectroscopy

“…The detection of S1031ʼs high rotation rate gives further evidence of its relation to the blue stragglers (Leiner et al 2019). Nine et al (2023) reported evidence for a 10,300-10,500 K white dwarf companion of 0.36-0.43 M e to S1031 from their HST UV observations. Their best-fit temperatures corresponded to a cooling age of ∼600-900 Myr.…”

“…Leiner et al (2019) found no evidence of a white dwarf in this binary system, and noted that it is unlikely for a white dwarf to have been ejected. Nine et al (2023) found an indication of an MS stellar companion in the spectral energy distribution (SED) of S277 from their Hubble Space Telescope (HST) far-ultraviolet (FUV) survey of M67 blue lurkers. A mass function too large to correspond with a white dwarf companion was reported.…”

“…Their best-fit temperatures corresponded to a cooling age of ∼600-900 Myr. Based on the orbital properties of the system, Nine et al (2023) suggested the companion is an He white dwarf that is the result of Case B mass transfer, though FUV spectroscopy is needed for confirmation. This star has not previously been analyzed for composition.…”