Newly Identified Star Clusters in M33. I. Integrated Photometry and Color-Magnitude Diagrams

We present integrated photometry and color-magnitude diagrams (CMDs) for 24 star clusters in M33, of which 12 were previously uncataloged. These results are based on Advanced Camera for Surveys observations from the Hubble Space Telescope of two fields in M33. Our integrated V magnitudes and V - I colors for the previously identified objects are in good agreement with previously published photometry. We are able to estimate ages for 21 of these clusters using features in the CMDs, including isochrone fitting to the main-sequence turnoffs for 17 of the clusters. Comparisons of these ages with the clusters' integrated colors and magnitudes suggest that simple stellar population models perform reasonably well in predicting these properties.

[1]  Doug Geisler,et al.  RR Lyrae Variables in M33. I. Evidence for a Field Halo Population , 2006, astro-ph/0605580.

[2]  G. Carraro,et al.  Star cluster detection with WFC/ACS in M 33, 33 new clusters and 51 candidates , 2005 .

[3]  M. Bellazzini,et al.  The distance of M 33 and the stellar population in its outskirts , 2004, astro-ph/0405465.

[4]  Xu Zhou,et al.  Age and Mass Estimates for 41 Star Clusters in M33 , 2004 .

[5]  J. Huchra,et al.  Structural Parameters and Dynamical Masses for Globular Clusters in M33 , 2002, astro-ph/0208169.

[6]  L. Girardi,et al.  Theoretical isochrones in several photometric systems I. Johnson-Cousins-Glass, HST/WFPC2, HST/NICMOS, Washington, and ESO Imaging Survey filter sets , 2002, astro-ph/0205080.

[7]  Xu Zhou,et al.  Estimating the Metallicity of Old Star Clusters in M33 , 2002 .

[8]  Xu Zhou,et al.  Spectral Energy Distributions and Age Estimates of 78 Star Clusters in M33 , 2002, astro-ph/0202363.

[9]  H. Ford,et al.  Kinematics of Star Clusters in M33: Distinct Populations , 2002 .

[10]  Xu Zhou,et al.  Spectral Energy Distributions, Ages, and Metallicities of Star Clusters in M33 , 2001 .

[11]  U. Florida,et al.  Determination of the Distance to M33 Based on the Tip of the Red Giant Branch and the Red Clump , 2001, astro-ph/0110006.

[12]  H. Ford,et al.  Star clusters in M 33 - IV. A new survey from deep HST images , 2001 .

[13]  N. Benı́tez,et al.  The Photometric Performance and Calibration of the Hubble Space Telescope Advanced Camera for Surveys , 2005, astro-ph/0507614.

[14]  L. Girardi,et al.  Evolutionary tracks and isochrones for low- and intermediate-mass stars: From 0.15 to 7 , and from to 0.03 , 1999, astro-ph/9910164.

[15]  H. Ford,et al.  Star Clusters in M33. I. Detection, Morphology, and Photometry , 1999 .

[16]  P. Harding,et al.  Accepted by The Astrophysical Journal Letters HST WFPC2 Color-Magnitude Diagrams of Halo Globular Clusters in M33: Implications for the Early Formation History of the Local Group 1 , 1998 .

[17]  Xiaohui Fan,et al.  Deep wide-field spectrophotometry of the open cluster M67 , 1996, astro-ph/9604178.

[18]  Stefano Casertano,et al.  THE PERFORMANCE AND CALIBRATION OF WFPC2 ON THE HUBBLE SPACE TELESCOPE , 1995 .

[19]  Peter B. Stetson,et al.  THE CENTER OF THE CORE-CUSP GLOBULAR CLUSTER M15: CFHT AND HST OBSERVATIONS, ALLFRAME REDUCTIONS , 1994 .

[20]  N. Caldwell,et al.  Kinematics of Star Clusters in M33 , 1991 .

[21]  R. Schommer,et al.  BVI photometry of star clusters in M33 , 1988 .

[22]  P. Stetson.  DAOPHOT: A COMPUTER PROGRAM FOR CROWDED-FIELD STELLAR PHOTOMETRY , 1987 .

[23]  R. Schommer,et al.  The cluster system of M33. , 1982 .

[24]  A. Riess,et al.  Time Dependence of ACS WFC CTE Corrections for Photometry and Future Predictions , 2004 .

[25]  W. A. Hiltner.  Colors and Magnitudes of Clusters in M31 and M33. , 1960 .