The VLT-FLAMES Tarantula Survey. XIII: On the nature of O Vz stars in 30 Doradus

C. Sab'in-Sanjuli'an(1,2), S. Sim'on-D'iaz(1,2), A. Herrero(1,2), N. R. Walborn(3), J. Puls(4), J. Ma'i­z Apell'aniz(5), C. J. Evans(6), I. Brott(7), A. de Koter(8,9), M. Garcia(10), N. Markova(11), F. Najarro(10), O. H. Ram'irez-Agudelo(8), H. Sana(3), W. D. Taylor(6) and J. S. Vink(12)

1- Instituto de Astrof'isica de Canarias, E-38200 La Laguna, Tenerife, Spain
2- Departamento de Astrof'isica, Universidad de La Laguna, E-38205 La Laguna, Tenerife, Spain
3- Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
4- Universit"ats-Sternwarte, Scheinerstrasse 1, 81679 Munchen, Germany
5- Instituto de Astrof'isica de Andaluc'i­a-CSIC, Glorieta de la Astronom'ia s/n, E-18008 Granada, Spain
6- UK Astronomy Technology Centre, Royal Observatory Edinburgh, Blackford Hill, Edinburgh, EH9 3HJ, UK
7- University of Vienna, Department of Astrophysics, Turkenschanzstr. 17, 1180, Vienna, Austria
8- Astronomical Institute Anton Pannekoek, University of Amsterdam, Kruislaan 403, 1098 SJ, Amsterdam, The etherlands
9- Instituut voor Sterrenkunde, Universiteit Leuven, Celestijnenlaan 200 D, 3001, Leuven, Belgium
10- Centro de Astrobiolog'ia (CSIC-INTA), Ctra. de Torrej'on a Ajalvir km-4, E-28850 Torrej'on 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, Northern Ireland, UK

AIMS. We test the hypothesis of O Vz stars (characterized by having HeII4686 stronger in absorption than other He lines in their blue-violet spectra) being at a younger evolutionary stage than are normal O-type dwarfs.
METHODS. We have performed a quantitative spectroscopic analysis of a sample of 38 O Vz and 46 O V 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 both samples using the FASTWIND stellar atmosphere code and the IACOB-GBAT grid-based automatic tool. In the framework of a differential study, we compared the physical and evolutionary properties of both samples, regarding Teff, logg, logQ and logL. We also investigated the predictions of the FASTWIND code about 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 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.

Reference: arXiv:1312.3278
Status: Manuscript has been accepted