WOCS 4540: Detailed Analysis of a Very Long Orbital Period Blue Straggler

WOCS 4540 is the longest orbital period ( P orb = 3030 days) blue straggler (BSS) - white dwarf (WD) pair in the old open cluster NGC 188 . It also contains one of the most luminous BSS in the cluster. Prior Hubble Space Telescope COS spectroscopy measured a WD mass of 0.53 M (cid:12) , indicative of a carbon-oxygen WD and suggesting previous mass transfer from an asymptotic branch giant (AGB). Detailed modeling of the system evolution, including red giant branch phase wind mass transfer, AGB wind Roche-lobe overﬂow and regular Roche-lobe overﬂow, is done with Modules for Experiments in Stellar Astrophysics. The best-ﬁt model produces excellent agreement with a wide array of observational constraints on the BSS, the WD and the binary system. To produce the observed luminosity and eﬀective temperature of the BSS, all three donor mass-transfer mechanisms contribute similarly to build a 1.5 M (cid:12) BSS. The overall mass-transfer eﬃciency is 55%. Regular Roche-lobe overﬂow occurs only during the largest AGB thermal pulse, but yields a very high accretion rate at 75% eﬃciency and brieﬂy (less than 1 Myr) a very high luminosity boost from the accretor.