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

TL;DR

This study investigates whether O Vz stars are genuinely younger than normal O-type stars by analyzing their physical properties in the 30 Doradus region, revealing that O Vz stars are generally near the ZAMS but not always extremely young.

Contribution

It provides a detailed spectroscopic comparison of O Vz and O V stars, showing that O Vz classification does not always indicate extreme youth, especially in low-metallicity environments.

Findings

O Vz stars are mostly near the ZAMS but some are more evolved.

O Vz stars tend to be younger, less luminous, and have weaker winds.

The O Vz phenomenon depends on multiple stellar parameters, not just age.

Abstract

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.