Hubble Space Telescope Observations of Globular Clusters

Collaborators:
John Bahcall (Institute for Advanced Study)
P. Guhathakruta (University of California at Santa Cruz)
Brian Yanny (Fermi National Accelerator Laboratory)

Over the past few years we have used the Hubble Space Telescope's Planetary Camera (first the first and second versions) to obtain multicolor images of the centers of Galactic globular clusters. Recently we have our analysis of the central region of the centrally concentrated cluster M15.

The data show that the surface density of stars continues to rise as a power law down to the limit of the observations (a radius of 0.3" or 0.017 pc); a flat core with a radius larger than 2" is ruled out at the 95% confidence level. The surface distribution is consistent with the expected profile if the cluster harbors a relatively massive (10^3 M_sun) black hole, but dynamical information is required before the black hole model can be verified. A candidate for the cusp around the black hole, a star called AC 214, has been identified in the HST data; it is very near the center of the cluster, and the high-resolution images show that AC 214 consists of at least three very closely spaced stars.

Globular Cluster Photometry with the Hubble Space Telescope.

I. Description of the method and analysis of the core of 47 Tuc. P. Guhathakurta, B. Yanny, D.P. Schneider, and J.N. Bahcall, AJ, 104, 1790 (1992)

Globular Cluster Photometry with the Hubble Space Telescope.

II. U, V, and I Measurements of M15. B. Yanny, P. Guhathakurta, J.N. Bahcall, and D.P. Schneider, AJ, 107, 1745 (1994)

Globular Cluster Photometry with the Hubble Space Telescope.

III. Blue Stragglers and Variable Stars in the Core of M3. P. Guhathakurta, B. Yanny, J.N. Bahcall, and D.P. Schneider, AJ, 108, 1786 (1994)

WFPC-2 Observations of the Globular Cluster M30.

B. Yanny, P. Guhathakurta, D.P. Schneider, and J.N. Bahcall, ApJL, 435, 59 (1994)

Hubble Space Telescope WFPC2 Observations of the Globular Cluster M15.

V. The Central Density Cusp. P. Guhathakurta, B. Yanny, D.P. Schneider, and J.N. Bahcall, AJ, in press (1995)