Stellar parameters and elemental abundances

The atmospheric parameters are derived spectroscopically from multiple indicators, following the methodology described by Przybilla et al. (2006). The effective temperature Teii and surface gravity log g are constrained by several NLTE ionization equilibria (C i/ii, N i/ii, Mg i/ii) and from modelling of the Stark-broadened

M31 41-3712

M31 41-3654

He C N 0 NeMgAI Si P S CaSc Ti V CrMnFe Ni Sr Bo He C N 0 NeMgAI Si P S CaSc Ti V CrMnFe Ni SrBa

Figure 35.1. Preliminary results from the elemental-abundance analysis for our two sample stars, relative to the Solar composition (on a logarithmic scale) (Grevesse & Sauval 1998). Filled symbols denote NLTE, open symbols LTE results. The symbol size codes the number of spectral lines analysed - small, 1-5; medium, 6-10; large, more than 10. Boxes are for neutral and circles for singly ionized species. The error bars represent 1a uncertainties from the line-to-line scatter. The grey shaded areas mark the deduced metallicities of the objects within 1a errors. The NLTE abundance analyses imply a scaled Solar abundance distribution for the M31 objects. An exception is constituted by the light elements which have been affected by mixing with nuclear-processed matter.

profiles of the higher Balmer and Paschen lines. The internal accuracy of the method allows the 1a uncertainties to be reduced to ~1%-2% in Teff and to 0.05-0.10dex in log g. Several He i lines are used to derive the helium abundance. The stellar metallicity relative to the Solar standard [M/H] (logarithmic values) is determined from the heavier metals (O, Mg, S, Ti, Fe) in NLTE. Microturbulent velocities £ are obtained in the usual way by requiring abundances to be independent of line equivalent width - consistency is achieved for all NLTE species. Finally, macroturbulences Z and rotational velocities v sin i are determined from line-profile fits. The results are summarised in Table 35.1, where information on the fundamental stellar parameters luminosity L, evolutionary and spectroscopic mass Me/Ms and radius R is also given. The photometric data of Massey et al. (2006) were adopted.

Elemental abundances were determined for several chemical species, with many of the astrophysically most interesting in NLTE and the remainder in LTE; see Figure 35.1. The two M31 supergiants are more metal-rich than the Galactic BA-SGs studied using the same method (Przybilla et al. 2006; Firnstein & Przybilla 2006; Schiller & Przybilla 2006), by up to -0.2 dex. One object is found to have supersolar metallicity. The abundance distribution of the heavier elements follows a scaled Solar pattern, whereas the abundances of light elements have been affected by mixing with CN-cycled matter.

Element

Element

Table 35.1. Atmospheric and fundamental stellar parameters with uncertainties f/f/t' sin i

Table 35.1. Atmospheric and fundamental stellar parameters with uncertainties f/f/t' sin i

Object

Teff (K)

logg

He

[M/H]

(kms-1)

log(L/L0)

MJMq

MJM,Z,

R/Ra

41-3654

9200

1.00

0.13

+0.13

8/20/36

5.63

29

24

257

(A2 Iae)

150

0.05

0.02

0.06

1/5/5

0.04

4

4

15

41-3712

8550

1.00

0.13

-0.04

8/18/25

5.45

24

22

243

(A3 Iae)

150

0.05

0.02

0.05

1/5/5

0.04

3

4

14

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