A NEW INSIGHT TO THE GALACTIC OVZ STARS

Based on a large sample of spectra from the Galactic O-Star Spectroscopic Survey (GOSSS, Maiz Apellaniz et al. 2011) a systematic study of the Galactic O dwarfs belonging to the luminosity subclass Vz is being performed. Preliminary results suggest a redefinition of the quantitative criterion to assign the “z” qualifier to the spectra. As a high-mass star probably begins to burn H while still accreting material, the Zero-Age MainSequence (ZAMS) phase of the O-type star is very difficult to observe in the UV-optical range. However numerous O stars appear to be very young. The luminosity class Vz was introduced to describe those O-dwarf spectra that show the He ii 4686 absorption stronger than any other He line (e.g. Walborn & Blades 1997). This characteristic is hypothesized to represent an “inverse” behaviour of the Of effect, which is due to the emission filling of the line and correlates with luminosity. Thus, OVz objects are expected to be less luminous and less evolved, i.e., nearer to the ZAMS. Quantitative studies place some objects on and others apart from the ZAMS. Based on the VFTS data (Evans et al. 2011), SabinSanjulian et al. (SS13) analyzed a sample of OVz stars in 30 Doradus, finding that, as a class, they are closer to the ZAMS, although there is a considerable number of cases with ages of 2-4 Myr. They also proposed that the large number of OVz stars in 30 Dor is related to the low metallicity of the LMC. Our sample of study consists of 131 O dwarfs selected from the GOSSS database. SB2 and peculiar spectra have been discarded. 59 of these objects (∼ 45%) are classified as Vz. They show a peculiar spectral-type distribution, with an “excess” of objects at intermediate types, very different from that shown by the “normal” dwarfs (see Fig. 1, top). Both distributions are however very similar to those found in 30 Dor by SS13. As rotational broadening and other effects influence the He i and He ii lines dif-