Deep CCD Photometry in the Globular Cluster M55: The Mass Function at [Fe/H] = -2

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Session 49 -- Star Clusters in the Milky Way and Other Galaxies
Display presentation, Thursday, June 15, 1995, 9:20am - 4:00pm

[49.05] Deep CCD Photometry in the Globular Cluster M55: The Mass Function at [Fe/H] = -2

K. L. Wu, M. Bolte, P. B. Stetson (UC Santa Cruz, UCO/Lick, DAO)

We have obtained deep B, V, and I CCD data of the globular cluster M55 (NGC 6809) at the Cerro Tololo Inter-American Observatory (CTIO). Preliminary results indicate that the data extend approximately six magnitudes below the main sequence turnoff and allow investigation of the slope of the luminosity and mass functions down to M$_V$ $\sim$ 10.5 and mass $\sim$ 0.22 M$_\odot$.

Our primary goal is to determine the slope of the Population II mass function below 0.5 M$_\odot$. Recent HST results suggest that the mass function for some globular clusters has a turnover point at M$_V$ $\sim$ 10, and hence that faint red stars within them cannot contribute significantly to the mass of the Galaxy's dark matter halo. In contrast, we find that the luminosity and mass functions of M55 are both still rising at our faint limit of M$_V$ $\sim$ 10.5.

Richer et al. (1991) found a strong correlation of the slope of the presently observed mass function of globular clusters with their estimated ``disruption'' time. This correlation was based on only six clusters. As this correlation is a key to understanding the dynamical evolution of globular clusters, we will investigate whether or not this trend extends to M55.

In addition, we have access to a newly improved subdwarf sequence. With the well-determined positions of these subdwarfs on the HR Diagram, we will be able to get a better estimate of the age and metallicity of the cluster.

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