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Results from HST Observations of Six LMC Globular Cluster Fields

Published online by Cambridge University Press:  25 May 2016

K. A. G. Olsen
Affiliation:
Cerro Tololo Inter-American Observatory, Casilla 603, La Serena, Chile
P. W. Hodge
Affiliation:
Astronomy Department, Box 351580, University of Washington, Seattle, WA 98195, USA
M. Mateo
Affiliation:
Department of Astronomy, University of Michigan, 821 Dennison, Building, Ann Arbor, MI 48109, USA
E. W. Olszewski
Affiliation:
Steward Observatory, University of Arizona, Tucson, AZ 85721, USA
R. A. Schommer
Affiliation:
Cerro Tololo Inter-American Observatory, Casilla 603, La Serena, Chile
N. B. Suntzeff
Affiliation:
Cerro Tololo Inter-American Observatory, Casilla 603, La Serena, Chile
A. R. Walker
Affiliation:
Cerro Tololo Inter-American Observatory, Casilla 603, La Serena, Chile

Abstract

We present deep HST color-magnitude diagrams of fields centered on the six old LMC globular clusters NGC 1754, NGC 1835, NGC 1898, NGC 1916, NGC 2005, and NGC 2019. Separate cluster and field star CMDs are shown. The time of formation of the LMC is studied from an analysis of the cluster CMDs. Based on a comparison of the CMDs with sequences of the Milky Way clusters M3, M5, and M55, we suggest that the LMC formed its first stars at the same time as the Milky Way to within 1 Gyr. We find additional evidence that these LMC globular clusters are as old as the oldest Milky Way clusters through a comparison of our data with the horizontal branch evolutionary models of Lee, Demarque, & Zinn (1994).

The evolution of the LMC following its formation is studied through an analysis of the field star CMDs. Through an automated comparison with stellar evolution models, we extract the star formation histories implied by the CMDs. Our best-fit star formation histories imply that the LMC has been actively forming stars over the last 4 Gyr, in agreement with previous field star studies. The four fields that lie in the Bar also contain significant numbers of stars with ages of 4–8 Gyr in the best-fit cases. The most notable disagreement between the best-fit models and observed CMDs is in the color of the red giant branch.

Type
Part 6. Stellar Clusters
Copyright
Copyright © Astronomical Society of the Pacific 1999 

References

Bertelli, G. et al. 1994, A&AS, 106, 275 Google Scholar
Dolphin, A. 1997, New Astronomy, 2, 397 CrossRefGoogle Scholar
Lee, Y.-W., Demarque, P., & Zinn, R. 1994, ApJ, 423, 248 CrossRefGoogle Scholar
VandenBerg, D., Bolte, M., & Stetson, P. 1990, AJ, 100, 445 CrossRefGoogle Scholar
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