The Effect of Differential Limb Magnification on Abundance Analysis of
  Microlensed Dwarf Stars
  Finite source effects can be important in observations of gravitational
microlensing of stars. Near caustic crossings, for example, some parts of the
source star will be more highly magnified than other parts. The spectrum of the
star is then no longer the same as when it is unmagnified, and measurements of
the atmospheric parameters and abundances will be affected. The accuracy of
abundances measured from spectra taken during microlensing events has become
important recently because of the use of highly magnified dwarf stars to probe
abundance ratios and the abundance distribution in the Galactic bulge. In this
paper, we investigate the effect of finite source effects on spectra by using
magnification profiles motivated by two events to synthesize spectra for dwarfs
between 5000K to 6200K at solar metallicity. We adopt the usual techniques for
analyzing the microlensed dwarfs, namely, spectroscopic determination of
temperature, gravity, and microturbulent velocity, relying on equivalent
widths. We find that ignoring the finite source effects for the more extreme
case results in errors in Teff < 45K, in log g of <0.1 dex and in
microturbulent velocity of <0.1 km/s. In total, changes in equivalent widths
lead to small changes in atmospheric parameters and changes in abundances of
<0.06 dex, with changes in [FeI/H] of <0.03 dex. For the case with a larger
source-lens separation, the error in [FeI/H] is <0.01 dex. This latter case
represents the maximum effect seen in events whose lightcurves are consistent
with a point-source lens, which includes the majority of microlensed bulge
dwarfs published so far.
