Volume 543, July 2012
|Number of page(s)||8|
|Published online||02 July 2012|
A peculiar Of star in the Local Group galaxy IC 1613⋆
1 Instituto de Astrofísica de Canarias, C/ Vía Láctea s/n, 38200 La Laguna, Tenerife, Spain
2 Departamento de Astrofísica, Universidad de La Laguna, Avda. Astrofísico Francisco Sánchez s/n, 38071 La Laguna, Tenerife, Spain
3 Universitätssternwarte München, Scheinerstr. 1, 81679 München, Germany
4 Centro de Astrobiología (CSIC-INTA), Ctra. de Torrejón a Ajalvir km-4, 28850 Torrejón de Ardoz, Madrid, Spain
5 ESA, Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
Received: 2 November 2011
Accepted: 17 April 2012
Context. Results from the theory of radiatively driven winds are nowadays incorporated in stellar evolutionary and population synthesis models, and are used in our interpretation of the observations of the deep Universe. Yet, the theory has been confirmed only until Small Magellanic Cloud metallicities. Observations and analyses of O-stars at lower metallicities are difficult, but much needed to prove the theory.
Aims. We have observed GHV-62024, an O6.5 IIIf star in the low-metallicity galaxy IC 1613 (Z ≈ 0.15 Z⊙) to study its evolution and wind. According to a previous preliminary analysis that was subject to significant restrictions this star could challenge the radiatively driven wind theory at low metallicities. Here we present a complete analysis of this star.
Methods. Our observations were obtained with VIMOS at VLT, at R ≈ 2000 and covered approximately between 4000 and 7000 Å. The observations were analysed using the latest version of the model atmosphere code FASTWIND, which includes the possibility of calculating the N iii spectrum.
Results. We obtain the stellar parameters and conclude that the star follows the average wind momentum–luminosity relationship (WLR) expected for its metallicity, but with a high value for the exponent of the wind velocity law, β. Comparing this with values of other stars in the literature, we suggest that this high value may be reached because GHV-62024 could be a fast rotator seen at a low inclination angle. We also suggest that this could favour the appearance of the spectral “f”-characterictics. While the derived β value does not change by adopting a lower wind terminal velocity, we show that a wrong V∞ has a clear impact on the position of the star in the WLR diagram. The N and He abundances are very high, consistent with strong CNO mixing that could have been caused by the fast rotation, although we cannot discard a different origin with present data. Stellar evolutionary model predictions are consistent with the star being still a fast rotator. We find again the well-known mass-discrepancy for this star.
Conclusions. We conclude that the star follows the WLR expected for its metallicity. The results are consistent with GHV-62024 being a fast rotator seen close to pole-on, strongly contaminated at the surface with CNO products and with a wind structure altered by the fast rotation but without modifying the global WLR. We suggest that this could be a general property of fast rotators.
Key words: galaxies: individual: IC 1613 / stars: early-type / stars: fundamental parameters / stars: mass-loss / stars: rotation / stars: evolution
© ESO, 2012
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