The wind properties of O-type stars at sub-SMC metallicity

Context. Powerful radiation-driven winds heavily influence the evolution and end-of-life products of massive stars. Feedback processes from these winds strongly impact the thermal and dynamical properties of the interstellar medium of their host galaxies. The dependence of mass loss on stellar properties is poorly understood, particularly at low metallicity (Z).
Aims. We aim to characterise global, photospheric, and wind properties of hot massive stars in Local Group dwarf galaxies with metal contents below that of the Small Magellanic Cloud and to compare our findings to theories of radiation-driven winds.
Methods. We performed quantitative optical and ultraviolet spectroscopy on a sample of 11 O-type stars in nearby dwarf galaxies with Z < 0.2 Z_⊙. We used the stellar atmosphere code FASTWIND in combination with the genetic algorithm KIWI-GA to determine the stellar and wind parameters. Clumpy structures present in the wind outflow were assumed to be optically thin.
Results. The winds of the sample stars are very weak, with mass loss rates of ∼10^-9-10^-7 M_⊙ yr^-1. Such feeble winds can only be constrained if ultraviolet spectra are available. The modified wind momentum as a function of luminosity (L) for stars in this Z regime is in agreement with extrapolations to lower Z of a recently established empirical relation for this quantity as a function of both L and Z. However, theoretical prescriptions do not match our results nor those of other recent analyses at low luminosity (L ≤ 10^5.2 L_⊙) and low Z. In this regime, they predict winds that are stronger by an order of magnitude or more. Conclusions. For our sample stars at Z ∼ 0.14 Z_⊙ with masses ∼30-50 M_⊙, stellar winds strip only a small amount of mass during the bulk of the main-sequence evolution. However, if the steep dependence of mass loss on luminosity found here also holds for (so far undiscovered) much more massive stars at these metallicities, these more massive stars may suffer (almost) as severely from main-sequence mass stripping as well-known very massive stars in the Large Magellanic Cloud and Milky Way.