A chemical study of M67 candidate blue stragglers and evolved blue stragglers observed with APOGEE DR14

Within the variety of objects populating stellar clusters, blue straggler stars (BSSs) are among the most puzzling ones. BSSs are commonly found in globular clusters, but they are also known to populate open clusters of the Milky Way. Two main theoretical scenarios (collisions and mass transfer) have been suggested to explain their formation, although finding observational evidence in support of either scenario represents a challenging task. Among the APOGEE observations of the old open cluster M67, we found 8 BSS candidates known from the literature and two known evolved BSSs. We carried out a chemical analysis of 3 BSS candidates and of the 2 evolved BSSs out of the sample and found that the BSS candidates have surface abundances similar to those of stars on the main-sequence turn-off of M67. Especially the absence of any anomaly in their carbon abundances seems to support a collisional formation scenario for these stars. Furthermore, we note that the abundances of the evolved BSSs S1040 and S1237 are consistent with the abundances of the red clump stars of M67. In particular, they show a depletion in carbon by $\sim0.25$ dex, which could be either interpreted as the signature of mass transfer or as the product of stellar evolutionary processes. Finally, we summarise the properties of the individual BSS stars observed by APOGEE, as derived from their APOGEE spectra and/or from information available in the literature.
16 pages, 11 figures, accepted by MNRAS