The chemical abundance in M31 from H II regions

We have obtained multislit spectroscopic observations from 3700 to 9200 angstrom with Low Resolution Imaging Spectrometer at the Keck I telescope for 31 HII regions in the disc of the Andromeda galaxy (M31), spanning a range in galactocentric distance from 3.9 to 16.1 kpc. In nine HII regions we measure one or several auroral lines ([O III] lambda 4363, [N II] lambda 5755, [S III] lambda 6312, [O II] lambda 7325) from which we determine the electron temperature (T-e) of the gas and derive chemical abundances using the direct T-e-based method. We analyse, 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 H II regions in M31 available in the literature, yielding a sample of 85 H II regions spanning distances from 3.9 to 24.7 kpc (0.19-1.2 R-25) from the galaxy centre. 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(-1), 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)(circle dot) = 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 similar to 0.3 dex below the stellar ones. This discrepancy is discussed in terms of oxygen depletion on to dust grains and possible biases affecting T-e-based oxygen abundances at high metallicity.