Constraining Mass-transfer Histories of Blue Straggler Stars with COS Spectroscopy of White Dwarf Companions

Recent studies show that the majority of blue straggler stars (BSSs) in old open clusters are formed through mass transfer from an evolved star onto a main-sequence companion, resulting in a BSS and white dwarf (WD) in a binary system. We present constraints on the mass transfer histories of two BSS-WD binaries in the open cluster NGC 188, using WD temperatures and surface gravities measured with Hubble Space Telescope COS far-ultraviolet spectroscopy. Adopting a Gaia-based cluster distance of 1847 ± 107 pc, we determine that one system, WOCS 4540, formed through Case C mass transfer resulting in a CO-core WD with T _eff = 17,000-200 +140 K and a log g = 7.80 -0.06 +0.06, corresponding to a mass of 0.53 -0.03 +0.03 M _o and a cooling age of 105 -5 +6 Myr. The other system, WOCS 5379, formed through Case B mass transfer resulting in a He-core WD with T _eff = 15,500-150 +170 K and a log g = 7.50 -0.05 +0.06, corresponding to a mass of 0.42 -0.02 +0.02 M _o and an age of 250 -20 +20 Myr. The WD parameters are consistent across four different cluster distance assumptions. We determine possible progenitor binary systems with a grid of accretion models using MESA, and investigate whether these systems would lead to stable or unstable mass transfer. WOCS 4540 likely resulted from stable mass transfer during periastron passage in an eccentric binary system, while WOCS 5379 challenges our current understanding of the expected outcomes for mass transfer from red giant branch stars. Both systems are examples of the value in using detailed analyses to fine-tune our physical understanding of binary evolutionary processes.