Spectroscopy of giants in LMC clusters. III - Velocities and abundances for NGC 1841 and Reticulum and the properties of the metal-poor clusters
Academic Article
Overview
Identity
Additional Document Info
Other
View All
Overview
abstract
Velocities and metallicities for giant star members of the distant globular clusters Reticulum (C 0435 -590) and NGC 1841 (C 0444-840) based on spectroscopy with the CTIO Argus multi-fiber system show that these two clusters are likely members of the Large Magellanic Cloud (LMC). The velocity, [Fe/H] metallicity, and absolute visual magnitude for Reticulum are 241.51.5 km s-1, -1.71, and -6.00.2, and for NGC 1841, 212.51.8 km s-1, -2.11, and -7.90.2. The error in the metallicities is 0.1 dex and is dominated by the uncertainties in the metallicities of the Galactic globular clusters used in the calibrations. From the velocities of 12 member giants we derive an intrinsic dispersion of 2.1-2.1+1.3 km s-1, a mass of 1.4-1.4+2.4105 script M sign, and a script M sign/LV=7.3-7.3+12.0 for Reticulum. For NGC 1841 (13 members), the dispersion is 3.6-2.1+1.4 km s-1, the mass is 2.8-2.2+2.5105 script M sign, and the script M sign/LV value is 2.4-1.9+2.2. NGC 1841 is apparently in front of the LMC by 5 kpc, based on the mean magnitude of the RR Lyraes. We review the properties of the LMC Population II clusters and field stars. The LMC has 13 known population II clusters. The mean metallicity of the LMC clusters is 0.3 dex lower than the ensemble average for the Galactic globular clusters, continuing the trend seen in other galaxies that the less luminous galaxies have less metal-rich globular cluster systems. However, the mean cluster metallicity, the absolute magnitude distribution of the clusters, and the relative number of RR Lyraes per unit cluster luminosity are very similar to the Galactic globular cluster population outside of the solar circle. The ratio of luminous mass in the clusters to that in the field population II is 0.02 for both the LMC and the Galaxy. We also find that the total number of Population II clusters, the total luminosity in clusters, and the total luminosity in the Population II field stars all scale as the ratio of the total luminosities of the LMC and the Galaxy. We argue that these characteristics are consistent with the Searle & Zinn (1978) and Zinn (1980) scenario that the Galactic halo formed out of subunits as large as the LMC, and not the tidal disruption of globular clusters. The LMC Population II clusters have a total absolute magnitude MV=-10.8 and luminous mass (for script M sign/L=2.0) of 3.6106 script M sign. The absolute magnitude for the field Population II, based on the number of RR Lyraes out to NGC 1841 as estimated by Kinman, et al. (1992), is -15.2, corresponding to a mass of 2.1108 script M sign. The averaged cluster population contains 13.6 RR Lyraes for MV= -7.5 in luminosity.
