XIII: On the nature of O Vz stars in 30 Doradus
1 Instituto de Astrofísica de Canarias, 38200 La Laguna, Tenerife, Spain
2 Departamento de Astrofísica, Universidad de La Laguna, 38205 La Laguna, Tenerife, Spain
3 Space Telescope Science Institute, 3700 San Martin Drive, Baltimore MD 21218, USA
4 Universitäts-Sternwarte, Scheinerstrasse 1, 81679 München, Germany
5 Instituto de Astrofísica de Andalucía-CSIC, Glorieta de la Astronomía s/n, 18008 Granada, Spain
6 UK Astronomy Technology Centre, Royal Observatory Edinburgh, Blackford Hill, Edinburgh EH9 3HJ, UK
7 University of Vienna, Department of Astrophysics, Türkenschanzstr. 17, 1180 Vienna, Austria
8 Astronomical Institute Anton Pannekoek, University of Amsterdam, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands
9 Instituut voor Sterrenkunde, Universiteit Leuven, Celestijnenlaan 200 D, 3001 Leuven, Belgium
10 Centro de Astrobiología (CSIC-INTA), Ctra. de Torrejón a Ajalvir km-4, 28850 Torrejón de Ardoz, Madrid, Spain
11 Institute of Astronomy with NAO, Bulgarian Academy of Sciences, PO Box 136, 4700 Smoljan, Bulgaria
12 Armagh Observatory, College Hill, Armagh BT61 9DG, UK
Received: 4 October 2013
Accepted: 4 December 2013
Context. O Vz stars, a subclass of O-type dwarfs characterized by having He ii λ4686 stronger in absorption than any other helium line in their blue-violet spectra, have been suggested to be on or near the zero-age main sequence (ZAMS). If their youth were confirmed, they would be key objects with which to advance our knowledge of the physical properties of massive stars in the early stages of their lives.
Aims. We test the hypothesis of O Vz stars being at a different (younger) evolutionary stage than are normal O-type dwarfs.
Methods. We have performed the first comprehensive quantitative spectroscopic analysis of a statistically meaningful sample of O Vz and O V stars in the same star-forming region, exploiting the large number of O Vz stars identified by the VLT-FLAMES Tarantula Survey in the 30 Doradus region of the Large Magellanic Cloud (LMC). We obtained the stellar and wind parameters of 38 O Vz stars (and a control sample of 46 O V stars) using the FASTWIND stellar atmosphere code and the IACOB-GBAT, a grid-based tool developed for automated quantitative analysis of optical spectra of O stars. In the framework of a differential study, we compared the physical and evolutionary properties of both samples, locating the stars in the log g vs. log Teff, log Q vs. log Teff, and log L/L⊙ vs. log Teff diagrams. We also investigated the predictions of the FASTWIND code regarding the O Vz phenomenon.
Results. We find a differential distribution of objects in terms of effective temperature, with O Vz stars dominant at intermediate values. The O Vz stars in 30 Doradus tend to be younger (i.e., closer to the ZAMS) and less luminous, and they have weaker winds than the O V stars, but we also find examples with ages of 2−4 Myr and with luminosities and winds that are similar to those of normal O dwarfs. Moreover, the O Vz stars do not appear to have higher gravities than the O V stars. In addition to effective temperature and wind strength, our FASTWIND predictions indicate how important it is to take other stellar parameters (gravity and projected rotational velocity) into account for correctly interpreting the O Vz phenomenon.
Conclusions. In general, the O Vz stars appear to be on or very close to the ZAMS, but there are some examples where the Vz classification does not necessarily imply extreme youth. In particular, the presence of O Vz stars in our sample at more evolved phases than expected is likely a consequence of modest O-star winds owing to the low-metallicity environment of the LMC.
Key words: Magellanic Clouds / stars: atmospheres / stars: early-type / stars: fundamental parameters / stars: massive
Based on observations at the European Southern Observatory Very Large Telescope in program 182.D-0222.
Appendices are available in electronic form at http://www.aanda.org
© ESO, 2014