The OVz stars in NGC 346: distribution, stellar parameters and insights into low-metallicity effects

TL;DR

This study investigates OVz stars in NGC 346 and 30 Doradus, analyzing their physical properties to understand how low metallicity influences their spectral features and evolutionary status.

Contribution

It provides a detailed spectroscopic analysis of OVz stars in low-metallicity environments, clarifying their characteristics and role in early massive star evolution.

Findings

OVz stars are less luminous and closer to ZAMS.

Low metallicity influences the spectral peculiarities of OVz stars.

The study refines classification criteria for OVz stars in different environments.

Abstract

OVz stars are identified by their optical spectra, in which the He II 4686 absorption line is stronger than any other He line. This spectral characteristic suggests that these stars may be less luminous, less evolved, and located closer to the zero-age main sequence (ZAMS). Although their evolutionary status is still debated, OVz stars are key candidates for understanding the earliest stages of massive star evolution. In this study, we analyze OVz stars in the star-forming regions NGC 346 in the Small Magellanic Cloud (SMC) and 30 Doradus in the Large Magellanic Cloud (LMC), aiming to determine their physical properties and investigate the role of low metallicity in the emergence of the Vz peculiarity. We identified a sample of OVz and OV stars in NGC 346 using spectra obtained with the Magellan Echellette (MagE) spectrograph at Las Campanas Observatory. Spectral classification was based on the equivalent width ratios of key He lines. Quantitative spectroscopic analyses were carried out using the IACOB-BROAD and IACOB-GBAT/FASTWIND tools. For consistency, previously identified OV and OVz stars in 30 Doradus were reclassified following the same criteria.