Main-Sequence Stars and the Star Formation History of the Outer Disk in the Large Magellanic Cloud
Krist, J.; Gallagher, J. S.; Stappers, B.; Clarke, J. T.; Griffiths, R. E.; Ballester, G. E.; Hester, J. J.; Casertano, S.; Scowen, P. A.; Westphal, J. A.; Trauger, J.; Crisp, D.; Mould, J. R.; Watson, A.; Burrows, C. J.; Holtzman, J.; de Feijter, E.; Hoessel, J.; Matthews, L. D.; Stapelfeld, K. R.
Abstract
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.