Structural Parameters of M31 Globular Clusters from WFPC2 Imaging

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Session 76 -- Spirals I
Display presentation, Wednesday, 11, 1995, 9:20am - 6:30pm

[76.06] Structural Parameters of M31 Globular Clusters from WFPC2 Imaging

Carl J. Grillmair, S.M. Faber (Lick Observatory), E.A. Ajhar, T.R. Lauer, C.R. Lynds, E.J. O'Neil (NOAO), W.A. Baum (University of Washington), J.A. Holtzman (Lowell Observatory), and the Members of the Wide Field/Planetary Camera Investigation Definition Team

We analyze post-refurbishment Hubble Space Telescope images of four globular clusters in M31. The ability to resolve stars in these images to below the horizontal branch permits us to use star counts to significantly extend surface brightness profiles determined using aperture photometry. By selecting stars on the basis of magnitude and color, and by taking account of the gradient in the surface density of field stars due to the halo of M31, we are able to extend the profiles to over 5 orders of magnitude in surface density. As in the majority of Galactic globular clusters, the resulting profiles resemble King models, though the uncertainties in the fitted tidal radii are large. Core radii are measured taking account of the WFPC2 Point-Spread-Function, and the values we find are consistent with the range of core radii measured for globular clusters in our own Galaxy. Two-dimensional surface brightness profiles indicate significant ellipticity in the isophotes of two of our clusters.

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