DOUBLE BLUE STRAGGLER SEQUENCES IN GLOBULAR CLUSTERS: THE CASE OF NGC 362
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abstract
We used high-quality images acquired with the WFC3 on board the HST to probe
the blue straggler star (BSS) population of the Galactic globular cluster NGC
362. We have found two distinct sequences of BSS: this is the second case,
after M 30, where such a feature has been observed. Indeed the BSS location,
their extension in magnitude and color and their radial distribution within the
cluster nicely resemble those observed in M 30, thus suggesting that the same
interpretative scenario can be applied: the red BSS sub-population is generated
by mass transfer binaries, the blue one by collisions. The discovery of four
new W UMa stars, three of which lying along the red-BSS sequence, further
supports this scenario. We also found that the inner portion of the density
profile deviates from a King model and is well reproduced by either a mild
power-law (\alpha -0.2) or a double King profile. This feature supports the
hypothesis that the cluster is currently undergoing the core collapse phase.
Moreover, the BSS radial distribution shows a central peak and monotonically
decreases outward without any evidence of an external rising branch. This
evidence is a further indication of the advanced dynamical age of NGC 362: in
fact, together with M 30, NGC 362 belongs to the family of dynamically old
clusters (Family III) in the "dynamical clock" classification proposed by
Ferraro et al. (2012). The observational evidence presented here strengthens
the possible connection between the existence of a double BSS sequence and a
quite advanced dynamical status of the parent cluster.
