Observations and model computations

Spectra of two luminous A-type supergiants in the northeastern arm of M31 were taken in November 2002 with ESI on the Keck II telescope. The Echelle spectra were reduced with the MAKEE package and our own IDL-based routines for order merging and continuum normalisation. Complete wavelength coverage of the spectra between ~3900 and ~9300A was obtained at high signal-to-noise ratio (>150 in the visual) and moderately high resolution (R ~ 8000). The data set is complemented by Keck I HIRES spectra with a more limited wavelength coverage (R ~ 35 000, signal-to-noise ratio ~80) as utilised by Venn et al. (2000).

The model calculations were carried out using a hybrid NLTE approach as discussed in detail by Przybilla et al. (2006). In brief, hydrostatic, plane-parallel and line-blanketed LTE model atmospheres are computed with ATLAS9 (Kurucz 1993) with further modifications (Przybilla et al. 2001). Then, NLTE line formation is performed on the resulting model stratifications. The coupled radiative-transfer and statistical-equilibrium equations are solved and spectrum synthesis with refined line-broadening theories is performed using DETAIL and SURFACE. State-of-the-art NLTE model atoms relying on data from ab initio computations, avoiding rough approximations wherever possible, are utilised for the stellar-parameter and abundance determinations.

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