Blue straggler stars in the globular cluster NGC 5053

Abstract

Twenty-four blue straggler stars have been identified in the low central concentration globular cluster NGC 5053. New deep color-magnitude (C-M) diagrams to 23 mag, constructed from photometry of over 6000 stars on 4-shooter CCD frames, show that they form a well-defined sequence in the C-M diagram, extending up to ~2.2 mag brighter than the main-sequence turnoff point. The 12 most luminous blue stragglers are found to be significantly more centrally concentrated than the cluster subgiants with magnitudes in the same interval (a similar result is known for the blue stragglers in the globular cluster NGC 5466). Furthermore, they are also found to be more centrally concentrated than the 12 lower luminosity blue stragglers. Comparisons of the projected radial distributions of the bright and faint blue stragglers, with the radial distributions that are expected for stars of mass 0.8, 1.6, and 2.4 M_⊙ (calculated using multimass King models) suggests that the brightest blue stragglers have an average mass of〈M〉= 1.3 + 0.3 M_⊙, which is less than or comparable to twice the mean mass of a main-sequence turnoff star, and the lower luminosity blue stragglers have a mean mass similar to that of the main-sequence turnoff stars (i.e., M ~ 0.8 M_⊙)- By fitting theoretical isochrones computed by Bell and VandenBerg to the observed main-sequence turnoff and subgiant branch regions of NGC 5053, a distance modulus of (m — M)_0 = 16.05 ± 0.14 mag, and an age of 18 ± 3 Gyr are derived for NGC 5053. The main-sequence luminosity function shows no sign of " turning over " for stars brighter than M_g ~ 5 mag.

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