Observations of Spin-down in Post-mass-transfer Stars and the Possibility for Blue Straggler Gyrochronology

Blue stragglers and other mass transfer/collision products are likely born with rapid rotation rates due to angular momentum transfer during mass-transfer, merger, or collisional formation. However, less is known about the angular momentum evolution of these stars as they age. Here we compare rotation rates and post-formation ages of mass-transfer products to models of angular momentum evolution for normal main-sequence (MS) stars and collisionally formed blue stragglers. In our sample, we include both F- and G-type blue stragglers in the cluster NGC 188 and post-mass-transfer GK MS stars in the field, all binaries with white dwarf (WD) companions. We compare ages derived from WD cooling models to photometric rotation periods and/or spectral v sin i measurements. We demonstrate that these systems have rapid rotation rates soon after formation. They then spin down as they age, much like standard solar-type MS stars do. We discuss the physical implications of this result, which suggests that the spin-down of post-mass transfer stars can be described by standard magnetic-braking prescriptions. This opens up the possibility of using gyrochronology as a method to determine the time since formation of blue straggler stars and other post-mass-transfer binaries.