Presentation #225.04 in the session Fundamental Properties II.
Blue lurkers (BLs) are stars embedded within the main-sequence of clusters that are observed to be anomalously rapidly rotating given their age. They do not have close companions that might have tidally spun them up; rather, they are found in long-period (100 < P < 10,000 days) binaries or are not observed to be velocity variable. It is thought that BLs are low-luminosity analogues to the classical blue straggler stars. Here we present Hubble Space Telescope far-ultraviolet photometry of eight binary BLs in the main sequence of M67 (4 Gyr), taken with the ACS/SBC in the F140LP, F150LP, and F165LP bandpasses. We find two BLs with significant FUV detections, WOCS 3001 and WOCS 14020, suggesting the presence of white dwarf (WD) companions. Both the WD luminosities and the relatively short orbital periods (P ≤ 300 days) indicate helium WDs. Temperatures hotter than 10,000 K suggest cooling ages younger than about 700 Myr. The detection of two hot WDs is consistent with the expected detection rate based on the underlying main-sequence binary population. These detections are evidence of recent red-giant mass transfer in these systems.
The authors acknowledge funding from the National Science Foundation grant AST-1714506. EML is supported by an NSF Astronomy and Astrophysics Postdoctoral Fellowship under award AST-1801937. The observations conducted for this work are associated with program HST-GO-16244.001-A.