The O Vz stars in NGC 346: Distribution, stellar parameters, and insights into low-metallicity effects

¹ Universidad de La Serena, Departamento de Física y Astronomía, Av. Raúl Bitrán 1305, La Serena, Chile
² Instituto de Astrofísica de Canarias, E-38200 La Laguna, Tenerife, Spain
³ Instituto de Astrofísica de La Plata, CONICET-UNLP, Paseo del Bosque s/n, B1900FWA La Plata, Argentina
⁴ Facultad de Ciencias Astronómicas y Geofísicas, Universidad Nacional de La Plata, Paseo del Bosque s/n, B1900FWA La Plata, Argentina
⁵ Las Campanas Observatory, Carnegie Observatories, Casilla 601, La Serena, Chile

^⋆ Corresponding author: johanna3193@gmail.com

Received: 31 January 2025
Accepted: 23 June 2025

Abstract

Context. O Vz stars are identified by optical spectra where the He II λ4686 absorption line is more intense than any other He line, suggesting they may be less luminous, less evolved, and closer to the zero-age main sequence (ZAMS). Despite ongoing debate, these stars remain candidates for understanding the early evolutionary stages of O-type stars.

Aims. We study O Vz stars in the star-forming regions NGC 346 in the Small Magellanic Cloud and 30 Doradus in the Large Magellanic Cloud by determining their physical parameters and exploring the influence of low metallicity on the development of the Vz spectral peculiarity.

Methods. We identified a sample of O Vz and O V stars in NGC 346 using spectra obtained with the Magellan Echellette spectrograph at Las Campanas Observatory. The stars were classified based on the relationship between the equivalent width of the relevant He lines. Quantitative spectroscopic analysis was performed using the IACOB-BROAD, IACOB-GBAT/FASTWIND tools. The O V and O Vz stars from 30 Doradus, previously identified in an earlier study, were reclassified following the same criteria.

Results. In NGC 346, 8 O Vz stars (29%) and 20 O V stars (71%) were identified. After a revised classification in 30 Doradus, 28 O,Vz stars (33%) and 56 O V stars (67%) were identified. Despite differences in sample sizes and metallicity, the proportion of O Vz stars is similar in both regions. O Vz stars in NGC 346 exhibit lower projected rotational velocities (v sin i < 70 km s⁻¹), higher effective temperatures (37–40 kK) and similar surface gravities (logg = 3.7 − 4.1) compared to O V stars. Contrary to prior assumptions, O Vz stars are not consistently closer to the ZAMS. In the Hertzsprung-Russell diagram, O Vz stars appear in both young and evolved regions, suggesting that the Vz phenomenon is related to stellar wind properties.