The K2 M67 Study: An Evolved Blue Straggler in M67 from K2 Mission Asteroseismology

Yellow straggler stars (YSSs) fall above the subgiant branch in optical color-magnitude diagrams, between the blue stragglers and the red giants. YSSs may represent a population of evolved blue stragglers, but none have the direct and precise mass and radius measurements needed to determine their evolutionary states and formation histories. Here we report the first asteroseismic mass and radius measurements of such a star, the yellow straggler S1237 in the open cluster M67. We apply asteroseismic scaling relations to a frequency analysis of the Kepler K2 light curve and find a mass of 2.9 $\pm$ 0.2 M$_{\odot}$ and a radius of 9.2 $\pm$ 0.2 R$_{\odot}$. This is more than twice the mass of the main- sequence turnoff in M67, suggesting S1237 is indeed an evolved blue straggler. S1237 is the primary in a spectroscopic binary. We update the binary orbital solution and use spectral energy distribution (SED) fitting to constrain the color-magnitude diagram (CMD) location of the secondary star. We find that the secondary is likely an upper main-sequence star near the turnoff, but a slightly hotter blue straggler companion is also possible. We then compare the asteroseismic mass of the primary to its mass from CMD fitting, finding the photometry implies a mass and radius more than 2$\sigma$ below the asteroseismic measurement. Finally, we consider formation mechanisms for this star and suggest that S1237 may have formed from dynamical encounters resulting in stellar collisions or a binary merger.