Mass-to-light ratios for globular clusters. II. The low-concentration clusters NGC 288, NGC 5466, and M55 (NGC 6809)

Carlton Pryor, Robert D. Mcclure, J. M. Fletcher, James E. Hesser

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We have measured radial velocities, accurate to ∼1 km s-1, for 20-30 stars in each of the three low-concentration globular clusters NGC 288, NGC 5466, and M55. These data are sufficient to calculate velocity dispersions and script M sign/L's for these clusters, which we use to investigate their stellar populations. We have fit single- and multicomponent King models to our velocity data and to surface-brightness profiles taken from the literature. NGC 288 has a projected central script M sign/L, (script M sign/Lv)0, of 2.1 ± 0.5 (in units of script M sign/Lv⊙), NGC 5466 has (script M sign/Lv)0 = 1.0 ± 0.4, and M55 has (script M sign/Lv)0 = 1.3 ± 0.4. Our results are: (1) The (script M sign/L)0'S of NGC 288, NGC 5466, and M55 do not differ significantly from each other or from those of the cluster population as a whole. (2) The (script M sign/L)0 of NGC 288 is lower than would be expected for a cluster that had a power law initial mass function as shallow as the observed present-day main-sequence mass function. (3) Models with initial mass functions consisting of two power law segments fit NGC 288 best when 50%-70% of the cluster mass is in stellar remnants, which would imply that 1.0-2.0 times the present cluster mass was lost due to stellar evolution alone in forming these remnants. (4) Because of the small amount of mass segregation expected in low-concentration clusters, we are able to place the first strong limits on the number of low-mass stars in a globular cluster, and thus on the total cluster masses and global script M sign/L's. Fitting King models with different lower cutoffs for the power law mass functions produces a 97.5% confidence upper limit on the global script M sign/Lv of 2.5 for NGC 5466 and 3.4 for M55. The corresponding lower limits to the mass-function cutoff are ∼0.2script M sign and ∼0.1script M sign, respectively, with the best match to the dynamical script M sign/L occurring for a cutoff above ∼0.4script M sign and at ∼0.3script M sign, respectively.

Original languageEnglish (US)
Pages (from-to)1026-1042
Number of pages17
JournalAstronomical Journal
Issue number3
StatePublished - Sep 1991

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science


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