Main-Sequence Stars and the Star Formation History of the Outer Disk in the Large Magellanic Cloud
Using the Wide Field Planetary Camera 2 on the Hubble Space Telescope, we have obtained a deep color-magnitude diagram in V- and I-band equivalents for more than 2000 stars in a patch of the outer disk of the Large Magellanic Cloud LMC). Aperture photometry is feasible from these data with good signal-to-noise ratio for stars with V ≤ 25, which allows us for the first time to construct a color magnitude diagram for LMC disk stars on the lower main sequence, extending beyond the oldest main sequence turnoff point. We analyze the structure of the main-sequence band and overall morphology of the color-magnitude diagram to obtain a star formation history for the region. A comparison between the distribution of stars across the main-sequence band for M_v ≤ 4 and a stellar population model constrains historical star formation rates within the past 3 Gyr. The stellar populations in this region sample the outer LMC disk for stars with ages of 1 Gyr or older that have had time to spatially mix. The structure of the main-sequence band requires that star formation occurred at a roughly constant rate during most of the past ≈ 3 Gyr. However, the distribution of subgiant stars indicate that a pronounced peak in the star formation rate likely occurred about 2 Gyr ago, prior to which the star formation rate had not been enhanced for several Gyr. Studies over timescales of more than 3 Gyr require a separation of the effects of star formation history and the chemical evolution on the LMC color-magnitude diagrams, which is difficult to achieve without additional constraints. If lower main-sequence stars in the LMC have moderate metallicities, then the age for most LMC disk stars is less than about 8 Gyr.
Additional Information© 1996 American Astronomical Society. Provided by the NASA Astrophysics Data System. Received 1995 September 28; accepted 1996 February 29. We thank Paul Hodge, Tammy Smecker-Hane, Linda Sparke, and Nick Suntzeff for discussions and comments on this project; Andrew Cole for his assistance in preparing this paper; and Rosemary Wyse and an anonymous referee for comments which have improved this manuscript. We also thank Mike Bessell and Mike Dopita for providing results in advance of publication. This work has been carried out as part of the WFPC2 Investigation Team GTO scientific research program and was supported by NASA contract NAS7-1260 to the Jet Propulsion Laboratory.
Published - 1996ApJ___466__732G.pdf