2014A&A...564A..39S

OVz stars, a subclass of O-type dwarfs characterized by having HeIIλ4686 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. We test the hypothesis of OVz stars being at a different (younger) evolutionary stage than are normal O-type dwarfs.We have performed the first comprehensive quantitative spectroscopic analysis of a statistically meaningful sample of OVz and OV stars in the same star-forming region, exploiting the large number of OVz stars identified by the VLT-FLAMES Tarantula Survey in the 30Doradus region of the Large Magellanic Cloud (LMC). We obtained the stellar and wind parameters of 38 OVz stars (and a control sample of 46 OV 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 logg vs. logT_eff, logQ vs. logT_eff, and logL/L_☉ vs. logT_eff diagrams. We also investigated the predictions of the FASTWIND code regarding the OVz phenomenon.We find a differential distribution of objects in terms of effective temperature, with OVz stars dominant at intermediate values. The OVz stars in 30Doradus tend to be younger (i.e., closer to the ZAMS) and less luminous, and they have weaker winds than the OV stars, but we also find examples with ages of 2-4Myr and with luminosities and winds that are similar to those of normal O dwarfs. Moreover, the OVz stars do not appear to have higher gravities than the OV 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 OVz phenomenon. In general, the OVz 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 OVz 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.