Spectroscopy of blue stragglers and turnoff stars in M67 (NGC 2682)

We have analyzed high-resolution spectra of relatively cool blue stragglers and main-sequence turnoff stars in the old open cluster M67 (NGC 2682). We attempt to identify blue stragglers whose spectra are least contaminated by binary effects (contamination by a binary companion or absorption by circumstellar material). These best stragglers have metallicities ([Fe/H] = -0.05) and abundance ratios not significantly different from those of the turnoff stars. Based on arguments from hydrodynamical models of stellar collisions, we assert that the current upper limits for the lithium abundances of all blue stragglers observed in M67 (by us and others) are consistent with a formation process without mixing, assuming pre-main-sequence and main-sequence depletion patterns observed for M67 main-sequence stars. We discuss composition signatures that could more definitively distinguish between blue straggler formation mechanisms in open cluster stars. We confirm the spectroscopic detection of a binary companion to the straggler S1082. From our spectra, we measure a projected rotational speed of 90 +/- 20 km s(-1) for the secondary and find that its radial velocity varies with a peak-to-peak amplitude of greater than or similar to 25 km s(-1). Because the radial velocities do not vary with a period corresponding to the partial eclipses in the system, We believe this system is currently undergoing mass transfer. In addition we present evidence that S984 is a true blue straggler (not an unresolved pair). If this can be proved, our detection of lithium may indicate a collisional origin.