Extragalactic Stellar Astronomy

Quantitative spectroscopy of massive, early-type stars is feasible also in nearby galaxies, because of the immense luminosity of OB-type stars and in particular BA-type supergiants. This opens up the possibility to study stellar evolution as a function of metallicity, one of the key quantities of the models.

Local Group. Earlier versions of our model atoms were used by Korn et al. (2002, 2005) in an analysis of high-resolution spectra of unevolved B-stars in the Large Magellanic Cloud (LMC). Pristine present-day abundances of the light elements

5For the most part, relatively steep abundance gradients (see e.g. Fig. 1) are discussed in the literature. Note, however, that the Milky Way has a central bar (e.g. Lopez-Corredoira et al. 2007). Barred spiral galaxies usually show shallow abundance gradients (e.g. Zaritsky et al. 1994), which may be attributed to homogenisation due to large-scale radial gas flows induced by the bar (e.g. Roberts et al. 1979). Apparently, a conundrum needs to be solved.

Figure 19: The Galactic oxygen abundance gradient. Upper panel: detail of Fig. 1. Lower panel: preliminary results from our work on BA-type supergiants (dots) and B-stars (triangles). Individual error bars are indicated (1-a statistical uncertainties), lines mark model predictions of Chiappini et al. (2001), see Fig. 1. A clear trend is found and the scatter in abundance is dramatically reduced when typical systematic errors are eliminated. Excellent agreement with the work of Esteban et al. (2005) on HII regions (boxes) is found after applying a correction of +0.08 dex to account for depletion on dust grains in the nebulae. Data from Przybilla et al. (2006a), Firnstein & Przybilla (in prep.), Przybilla, Nieva & Heber (in prep.).

Figure 19: The Galactic oxygen abundance gradient. Upper panel: detail of Fig. 1. Lower panel: preliminary results from our work on BA-type supergiants (dots) and B-stars (triangles). Individual error bars are indicated (1-a statistical uncertainties), lines mark model predictions of Chiappini et al. (2001), see Fig. 1. A clear trend is found and the scatter in abundance is dramatically reduced when typical systematic errors are eliminated. Excellent agreement with the work of Esteban et al. (2005) on HII regions (boxes) is found after applying a correction of +0.08 dex to account for depletion on dust grains in the nebulae. Data from Przybilla et al. (2006a), Firnstein & Przybilla (in prep.), Przybilla, Nieva & Heber (in prep.).

were obtained from stars for the first time. It was shown that the LMC is indeed nitrogen-poor, as indicated earlier by studies of H ii regions. More work on OB-type dwarfs/giants is required to study both slow and fast rotators in order to derive comprehensive observational constraints on the evolution models near the main sequence. Here, the Flames survey of massive stars (Evans et al. 2005, 2007) provides a unique database for three galaxies: the Milky Way, the Large, and the Small Magellanic Cloud.

First results from an application of our analysis methodology to high-resolution spectra of selected A-SGs in the Magellanic Clouds and in M 31 are shown in Fig. 17, cf. also Venn & Przybilla (2003). An extension of the work to larger samples of supergiants in these and other galaxies of the Local Group will in particular allow the metallicity-dependent mixing efficiency of the stellar evolution models (e.g. Maeder & Meynet 2001) to be verified.

A-type supergiants as tracers for galactochemical evolution were studied at high spectral resolution in several galaxies of the Local Group. These comprise first steps towards a determination of abundance gradients/patterns in spiral galaxies like M 31 (Venn et al. 2000) or in dwarf irregular (dIrr) galaxies like NGC 6822 (Venn et al. 2001). Gas-rich dIrr galaxies are of particular interest, as they are the closest analogues to the basic building blocks in hierarchical galaxy formation scenarios ('near-field cosmology'). Detailed information on abundances for various elements permits deeper insights in the nucleosynthesis histories of dIrr galaxies beyond the Mag-

Figure 20: NGC 3621 at a distance of 6.6 Mpc. The positions of stars (circles) and HII regions (boxes) observed with ForsI on the ESO VLT are marked on a colour image obtained by combining 5 min B, V and I-band frames taken with the same instrument in imaging mode. The field of view is approximately 7' x 7'. Different coloured markers are used for clarity only. From Bresolin et al. (2001).

Figure 20: NGC 3621 at a distance of 6.6 Mpc. The positions of stars (circles) and HII regions (boxes) observed with ForsI on the ESO VLT are marked on a colour image obtained by combining 5 min B, V and I-band frames taken with the same instrument in imaging mode. The field of view is approximately 7' x 7'. Different coloured markers are used for clarity only. From Bresolin et al. (2001).

ellanic Clouds to be obtained (Venn et al. 2003, and references therein). Progress in this branch of extragalactic stellar astronomy is slow, as high-resolution spectroscopy of supergiants at these distances is costly. Several hours of observing time on 8-10m-class telescopes are required per object.

Quantitative studies at intermediate spectral resolution allow only less-comprehensive information to be derived. However, they also have their advantages: large observational samples can be easily accessed using multi-object spectrographs, and fainter targets become observable, facilitating quantitative spectroscopy of supergiants in galaxies beyond the Local Group.

Beyond the Local Group. Observations of blue supergiant candidates in NGC3621 (Fig. 20) at a distance of 6.6Mpc pushed the capabilities of ForsI and

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