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The double distance and metallicity frontier marked by the SMC has been finally broken with the aid of powerful multi-object spectrographs installed at 8-10m class telescopes. VLT, GTC and Keck have enabled studies of massive stars in dwarf irregular galaxies of the Local Group with poorer metal-content than the SMC. The community is working to test the predictions of evolutionary models in the low-metallicity regime, set the new standard for the metal-poor high-redshift Universe, and test the extrapolation of the physics of massive stars to environments of decreasing metallicity. In this paper, we review current knowledge on this topic.
In this poster we present the results of our analyses of three early massive stars in IC 1613, whose spectra have been observed with VIMOS and analyzed with CMFGEN and FASTWIND. One of the targets resulted a possible LBV and the other two are Of stars with unexpectedly strong winds. The Of stars seem to be strongly contaminated by CNO products. Our preliminary results may represent a challenge for the theory of stellar atmospheres, but they still have to be confirmed by the analysis of more objects and a more complete coverage of the parameter space.
The Tarantula Survey is an ESO Large Programme which has obtained multi-epoch spectroscopy of over 1,000 massive stars in the 30 Doradus region of the Large Magellanic Cloud. The assembled consortium will exploit these data to address a range of fundamental questions in both stellar and cluster evolution.
The Supernova Working Group was re-established at the IAU XXV General Assembly in Sydney, 21 July 2003, sponsored by Commissions 28 (Galaxies) and 47 (Cosmology). Here we report on some of its activities since 2005.
A calibration of the O spectral types and luminosity classes has been plotted onto evolutionary tracks and isochrones for both nonrotating models and models with an initial rotational velocity of 300 km sec–1. Differences in the physical interpretation of the spectral types between the two cases have been investigated.
We present the analysis of UV and optical spectra of M 33 early B-type supergiants. Stellar parameters, metal abundances (Si, Mg, O and N) and mass loss rates are derived from anlyses of optical spectra by means of non-LTE unblanketed unified model atmospheres (fastwind code, Santolaya-Rey, Puls & Herrero 1997). These analyses use wind terminal velocities previously derived from the analysis of UV P-Cygni profiles. Stellar radial gradients of Si, O and Mg are derived for the M 33 disk. Differential abundances of those elements with respect to Galactic counterparts are also presented, along with nitrogen. Finally, the wind momenta of M 33 stars is derived and compared with the one found for Galactic B-type supergiants.
We present the spatial distribution of chemical species in M 31, as derived from intermediate resolution spectra of seven B-type supergiants, lying within four OB associations, covering a galactocentric distance of 5 – 12 kpc. We do not detect any systematic oxygen 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.
The relative abundances of elements in galaxies can provide valuable information on the stellar and chemical evolution of a galaxy. While nebulae can provide abundances for a variety of light elements, stars are the only way to directly determine the abundances of iron-group and s-process and r-process elements in a galaxy. The new 8m and 10m class telescopes and high-efficiency spectrographs now make high-quality spectral observations of bright supergiants possible in dwarf galaxies in the Local Group. We have been concentrating on elemental abundances in the metal-poor dwarf irregular galaxies, NGC 6822, WLM, Sextants A, and GR 8. Comparing abundance ratios to those predicted from their star formation histories, determined from color-magnitude diagrams, and comparing those ratios between these galaxies can give us new insights into the evolution of these dwarf irregular galaxies. Iron-group abundances also allow us to examine the metallicities of the stars in these galaxies directly, which affects their inferred mass loss rates and predicted stellar evolution properties.
We have begun a census of various stellar groups in Local Group Galaxies, using the wide field camera on the Isaac Newton Telescope on La Palma. Here we present a preliminary color-magnitude diagram for the dwarf irregular galaxy IC-10. At present time, metallicity of IC-10 is measured to be Z = 0.005 (Garnett 1990). Comparison with recent literature values of reddening and distance suggest that IC-10's distance is ~ 1 Mpc. Our comprehensive wide-field survey encompasses both broad (g′, r′, i′) and narrow-band (O iii, He ii, Hα, S ii, Strömgren y) observations to look for emission-line objects, including Wolf-Rayet stars and Luminous Blue Variables. The analysis also yields the coordinates of massive stars to an accuracy sufficient for follow-up multi-object spectroscopic observations.
We have investigated the properties of main-sequence O-type stars in the SMC. Mass-loss rates, luminosities and Teff are much smaller for these stars than for Galactic ones, resulting in a steeper wind-momentum relation.
We discuss abundances for eight early B-type giant/supergiant stars in the SMC cluster NGC 330. All are nitrogen rich with an abundance approximately 1.3 dex higher than an SMC main-sequence field. Given the number of B-type stars with low rotational projected velocities in NGC 330 (all our targets have v sin i < 50 kms–1), we suggest that it is unlikely that the stars in our sample are seen almost pole-on, but rather that they are intrinsically slow rotators. Comparing these results with the predictions of stellar evolution models including the effects of rotationally induced mixing, we conclude that while the abundance patterns may indeed be reproduced, those models with initially large rotational velocities do not reproduce the observed range of effective temperatures of our sample, nor the currently observed rotational velocities. Binary models may be able to produce stars in the observed temperature range and provide a promising alternative to single star models for explaining the observations. We also discuss the clear need for stellar evolution calculations employing the correct chemical mix of carbon, nitrogen and oxygen for the SMC.
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