Stellar wind properties of the nearly complete sample of O stars in the low metallicity young star cluster NGC 346 in the SMC galaxy

Context. Massive stars are among the main cosmic engines driving the evolution of star-forming galaxies. Their powerful ionising radiation and stellar winds inject a large amount of energy in the interstellar medium. Furthermore, mass-loss (M) through radiatively driven winds plays a key role in the evolution of massive stars. Even so, the wind mass-loss prescriptions used in stellar evolution models, population synthesis, and stellar feedback models often disagree with mass-loss rates empirically measured from the UV spectra of low metallicity massive stars. Aims. The most massive young star cluster in the low metallicity Small Magellanic Cloud galaxy is NGC 346. This cluster contains more than half of all O stars discovered in this galaxy so far. A similar age, metallicity (Z), and extinction, the O stars in the NGC 346 cluster are uniquely suited for a comparative study of stellar winds in O stars of different subtypes. We aim to use a sample of O stars within NGC 346 to study stellar winds at low metallicity. Methods. We mapped the central 1' of NGC 346 with the long-slit UV observations performed by the Space Telescope Imaging Spectrograph (STIS) on board of the Hubble Space Telescope and complemented these new datasets with archival observations. Multi-epoch observations allowed for the detection of wind variability. The UV dataset was supplemented by optical spectroscopy and photometry. The resulting spectra were analysed using a non-local thermal equilibrium model atmosphere code (PoWR) to determine wind parameters and ionising fluxes. Results. The effective mapping technique allowed us to obtain a mosaic of almost the full extent of the cluster and resolve stars in its core. Among hundreds of extracted stellar spectra, 21 belong to O stars. Nine of them are classified as O stars for the first time. We analyse, in detail, the UV spectra of 19 O stars (with a further two needing to be analysed in a later paper due to the complexity of the wind lines as a result of multiplicity). This more than triples the number of O stars in the core of NGC 346 with constrained wind properties. We show that the most commonly used theoretical mass-loss recipes for O stars over-predict mass-loss rates. We find that the empirical scaling between mass-loss rates ((M)over dot) and luminosity (L), (M)over dot proportional to L-2.(4), is steeper than theoretically expected by the most commonly used recipes. In agreement with the most recent theoretical predictions, we find within (M)over dot proportional to Z(alpha) that alpha is dependent upon L. Only the most luminous stars dominate the ionisation feedback, while the weak stellar winds of O stars in NGC 346 and the lack of previous supernova explosions in this cluster restrict the kinetic energy input.

Automated summary

The study finds that the commonly used theoretical mass-loss predictions for O stars tend to overestimate the mass-loss rates. The relationship between mass-loss rates and luminosity in O stars at low metallicity is steeper than expected by the theoretical models. In NGC 346, only the most luminous stars dominate the ionisation feedback, and the weak stellar winds and absence of previous supernova explosions limit the kinetic energy input.