Chemical composition of B-type supergiants in the OB 8, OB 10, OB 48, OB 78 associations of M 31

¹ The Department of Pure and Applied Physics, The Queen's University of Belfast, Belfast BT7 1NN, Northern Ireland
² The Isaac Newton Group of Telescopes, Apartado de Correos 321, 38700, Santa Cruz de La Palma, Canary Islands, Spain
³ Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge, CB3 0HA, USA
⁴ Institutio de Astrofísica de Canarias, 38200 La Laguna, Tenerife, Spain

Corresponding author: C. Trundle, ct@ing.iac.es

Received: 23 April 2002
Accepted: 15 July 2002

Abstract

Absolute and differential chemical abundances are presented for the largest group of massive stars in M 31 studied to date. These results were derived from intermediate resolution spectra of seven B-type supergiants, lying within four OB associations covering a galactocentric distance of 5-12  kpc. The results are mainly based on an LTE analysis, and we additionally present a full non-LTE, unified model atmosphere analysis of one star (OB 78-277) to demonstrate the reliability of the differential LTE technique. A comparison of the stellar oxygen abundance with that of previous nebular results shows that there is an offset of between ~ dex between the two methods which is critically dependent on the empirical calibration adopted for the R₂₃ parameter with [O/H]. However within the typical errors of the stellar and nebular analyses (and given the strength of dependence of the nebular results on the calibration used) the oxygen abundances determined in each method are fairly consistent. We determine the radial oxygen abundance gradient from these stars, and do not detect any systematic gradient across this galactocentric range. We find that the inner regions of M 31 are not, as previously thought, very “metal rich”. Our abundances of C, N, O, Mg, Si, Al, S and Fe in the M 31 supergiants are very similar to those of massive stars in the solar neighbourhood.