The star formation history of the Sculptor Dwarf Irregular Galaxy
  [abridged] We study the resolved stellar populations and derive the SFH of
the SDIG, a gas-rich dwarf galaxy member of the NGC7793 subgroup in the
Sculptor group. We construct a CMD using archival HST observations and examine
its stellar content. We derive its SFH using a maximum-likelihood fit to the
CMD. The CMD shows that SDIG contains stars from 10Myr to several Gyr old, as
revealed from the MS, BL, luminous AGB, and RGB stars. The young stars with
ages less than ~250Myr show a spatial distribution confined to its central
regions, and additionally the young MS stars exhibit an off-center density
peak. The intermediate-age and older stars are more spatially extended. SDIG is
dominated by intermediate-age stars with an average age of 6.4Gyr. The average
metallicity inferred is [M/H]\approx -1.5dex. Its SFH is consistent with a
constant SFR, except for ages younger than ~200Myr. The lifetime average SFR is
1.3x10^{-3} Mo/yr. More recently than 100Myr, there has been a burst of SF at a
rate ~2-3 times higher than the average SFR. The inferred recent SFR from CMD
modelling is higher than inferred from the Ha flux of the galaxy; we interpret
this to mean that the upper end of the IMF is not being fully sampled due to
the low SFR. Additionally, an observed lack of bright blue stars in the CMD
could indicate a downturn in SFR on 10^7-yr timescales. A previous SF
enhancement appears to have occurred between 600-1100Myr ago, with amplitude
similar to the most recent 100Myr. Older bursts of similar peak SFR and
duration would not be resolvable with these data. The observed enhancements in
SF suggest that SDIG is able to sustain a complex SFH without the effect of
interactions with its nearest massive galaxy. Integrating the SFR over the
entire history of SDIG yields a total stellar mass 1.77x10^{7}Mo, and a current
V-band stellar mass-to-light ratio 3.2Mo/Lo.
