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The stellar populations of the globular cluster M55 Mandushev, Georgi


New broad-band, ground-based photometry in four filters (UBVI) for two fields in the sparse, metal-poor Galactic globular cluster M55 (NGC 6809) is presented and analyzed. New values are derived for the reddening (EB-V = 0.13 ± 0.02 and Ev-I = 0.17 ± 0.02), distance modulus ((m - M)v = 14.02 ± 0.08) and age (14 ± 1.2 Gyr) of M55. The main-sequence luminosity function of M55 is found to be different from the luminosity functions of the metal-poor clusters M15, M30 and M92 and this difference is interpreted as a deficiency of low-mass stars by about 50% compared to the other three clusters. The mass function of M55 for masses below O.4M. is found to be fairly flat and consequently low-mass stars do not dominate the cluster mass. The red giant branch of M55 has been observed from nearly its tip to the subgiant branch. In all passbands the observed luminosity of the red giant clump is lower than the predictions of theoretical models. The ratios of the number of stars on the red giant branch, the horizontal branch and the asymptotic giant branch are found to be in a good agreement with theoretical models. Neither the V-band nor the I-band luminosity functions for the evolved populations in M55 show any significant deviation from the theoretical luminosity functions. In particular, no evidence is found for a deficiency of main-sequence stars compared to the number of stars on the subgiant and giant branches. M55 is the only well-studied, metal-poor cluster for which no discrepancy between observations and canonical luminosity functions is found. A large sample of blue stragglers in the core of M55 is identified and analyzed. It is concluded that the blue stragglers in M55 are born with helium-enriched cores but not envelopes, thus resembling stars that have already evolved away from the main sequence. It is also suggested that the observed blue straggler sequence represents the equivalent of a core helium-enriched main sequence where the blue stragglers spend most of their lives. The observations agree qualitatively with unmixed collisional or merger models, although the former are less likely in the low-density environment of M55. [Scientific formulae used in this abstract could not be reproduced.]

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