Star Formation History and Chemical Evolution of the Sextans Dwarf
  Spheroidal Galaxy
  We present the star formation history and chemical evolution of the Sextans
dSph dwarf galaxy as a function of galactocentric distance. We derive these
from the $VI$ photometry of stars in the $42' \times 28'$ field using the SMART
model developed by Yuk & Lee (2007, ApJ, 668, 876) and adopting a closed-box
model for chemical evolution. For the adopted age of Sextans 15 Gyr, we find
that $>$84% of the stars formed prior to 11 Gyr ago, significant star formation
extends from 15 to 11 Gyr ago ($\sim$ 65% of the stars formed 13 to 15 Gyr ago
while $\sim$ 25% formed 11 to 13 Gyr ago), detectable star formation continued
to at least 8 Gyr ago, the star formation history is more extended in the
central regions than the outskirts, and the difference in star formation rates
between the central and outer regions is most marked 11 to 13 Gyr ago. Whether
blue straggler stars are interpreted as intermediate age main sequence stars
affects conclusions regarding the star formation history for times 4 to 8 Gyr
ago, but this is at most only a trace population. We find that the metallicity
of the stars increased rapidly up to [Fe/H]=--1.6 in the central region and to
[Fe/H]=--1.8 in the outer region within the first Gyr, and has varied slowly
since then. The abundance ratios of several elements derived in this study are
in good agreement with the observational data based on the high resolution
spectroscopy in the literature. We conclude that the primary driver for the
radial gradient of the stellar population in this galaxy is the star formation
history, which self-consistently drives the chemical enrichment history.
