Chemical abundances in M31 from HII regions
  We have obtained multi-slit spectroscopic observations from 3700A to 9200A
with LRIS at the Keck I telescope for 31 HII regions in the disk of the
Andromeda galaxy (M31), spanning a range in galactocentric distance from 3.9
kpc to 16.1 kpc. In 9 HII regions we measure one or several auroral lines
([OIII]4363, [NII]5755, [SIII]6312, [OII]7325), from which we determine the
electron temperature (Te) of the gas and derive chemical abundances using the
'direct Te-based method'. We analyze, for the first time in M31, abundance
trends with galactocentric radius from the 'direct' method, and find that the
Ne/O, Ar/O, N/O and S/O abundance ratios are consistent with a constant value
across the M31 disc, while the O/H abundance ratio shows a weak gradient. We
have combined our data with all spectroscopic observations of HII regions in
M31 available in the literature, yielding a sample of 85 HII regions spanning
distances from 3.9 kpc to 24.7 kpc (0.19 - 1.2 R25) from the galaxy center. We
have tested a number of empirical calibrations of strong emission line ratios.
We find that the slope of the oxygen abundance gradient in M31 is
-0.023+/-0.002 dex/kpc, and that the central oxygen abundance is in the range
12+log(O/H) = 8.71 - 8.91 dex (i.e. between 1.05 and 1.66 times the solar
value, for 12+log(O/H)_solar=8.69), depending on the calibration adopted. The
HII region oxygen abundances are compared with the results from other
metallicity indicators (supergiant stars and planetary nebulae). The comparison
shows that HII region O/H abundances are systematically ~0.3 dex below the
stellar ones. This discrepancy is discussed in terms of oxygen depletion onto
dust grains and possible biases affecting Te-based oxygen abundances at high
metallicity.
