Blue straggler star populations in globular clusters. I. Dynamical properties of blue straggler stars in NGC 3201, NGC 6218, and ω centauri

Abstract

We present the first dynamical study of blue straggler stars (BSSs) in three Galactic globular clusters, NGC 3201, NGC 5139 (ω Cen), and NGC 6218, based on medium-resolution spectroscopy (R10, 000) obtained with the Inamori-Magellan Areal Camera & Spectrograph mounted at the 6.5 m Baade Magellan telescope. Our BSS candidate selection technique uses HST/ACS and ESO/WFI photometric data out to >4.5 rc . We use radial velocity measurements to discard non-members and achieve a success rate of 93%, which yields a sample of 116 confirmed BSSs. Using the penalized pixel-fitting method (pPXF), we measure the vsin (i) values of the sample BSSs and find their distribution functions peaked at slow velocities with a long tail toward fast velocities in each globular cluster. About 90% of the BSS population in NGC 3201 and NGC 6218 exhibits values in the range 10-50 km s-1, while about 80% of the BSSs in ω Cen show vsin (i) values between 20 and 70 km s-1. We find that the BSSs in NGC 3201 and NGC 6218 that show vsin (i) > 50 km s-1 are all found in the central cluster regions, inside a projected 2rc , of their parent clusters. We find a similar result in ω Cen for BSSs with vsin (i) > 70 km s-1, which are all, except for two, concentrated inside 2rc . In all globular clusters, we find rapidly rotating BSSs that have relatively high differential radial velocities that likely put them on hyperbolic orbits, suggestive of strong dynamical interactions in the past. Based on stellar spin-down and dynamical crossing timescales, we estimate that all the observed rapidly rotating BSSs are likely to form in their central cluster regions no longer than 300 Myr ago and may be subsequently ejected from their host globular clusters. Using dereddened V-I colors of our photometric selection, we show that blue BSSs in ω Cen with (V-I)0 ≲ 0.25 mag show a significantly increased vsin (i) dispersion compared with their red counterparts and all other BSSs in our sample, therefore strongly implying that fast-rotating BSSs in ω Cen are preferentially bluer, i.e., more massive. This may indicate that this particular blue BSS population was formed in a unique formation event and/or through a unique mechanism.© 2014.The American Astronomical Society. All rights reserved.