The young stellar population of IC 1613 III. New O-type stars unveiled by GTC-OSIRIS ,,

TLDR: In this article, a list of very likely O-type stars in the IC1613 irregular galaxy was built using the reddening-free Q-parameter, evolutionary masses and GALEX photometry.

Abstract: Context. Very low metallicity massive stars are key to understand the reionization epoch. Radiation-driven winds are one of the main agents of the evolution of massive stars, and consequently an important ingredient of our models of the early-Universe. Recent findings hint that the winds of massive stars with poorer meta llicity than the SMC may be stronger than predicted by theory. Besides calling the paradigm of radiation driven winds into question, this result would impact the calculated ionizing radiati on and mechanical feedback of massive stars, and the role these objects play at different stages of the Universe. Aims. The field needs a systematic study of the winds of a large sampl e of very metal poor massive stars. The sampling of spectral types is particularly poor in the very early types. This pape r’s goal is to increase the list of known O-type stars in the dw arf irregular galaxy IC1613, whose metallicity is smaller than the SMC’s b y roughly a factor 2. Methods. Using the reddening-free Q-parameter, evolutionary masses and GALEX photometry, we built a list of very likely O-type stars. We obtained low-resolution (R∼1000) GTC-OSIRIS spectra for a fraction of them and performed spectral classification, the only way to unequivocally confirm candidate OB-stars. Results. We have discovered 8 new O-type stars in IC1613, increasing the list of 7 known O-type stars in this galaxy by a factor of 2. The best quality spectra were analyzed with the model atmosphere code FASTWIND to derive stellar parameters. We present the first spectral type ‐ e ffective temperature scale for O-stars beyond the SMC. Conclusions. The target selection method is successful. From the pre-selected list of 13 OB star candidates, we have found 8 new O-stars, 4 early-B stars and provided a similar type for a for merly known early-O star. Further tests are needed but the presented procedure can eventually make preliminar low resolution spectroscopy to confirm candidates unnecessary. The derived e ffective temperature calibration for IC1613 is about 1000K hotter than the scale at the SMC. The analysis of an increased list of O-type stars will be crucial for the studies of the winds and feedback of massive stars at all ages of the Universe.