Wind properties of Milky Way and SMC massive stars: empirical Z
  dependence from CMFGEN models
  Detailed knowledge about stellar winds and evolution at different
metallicities is crucial for understanding stellar populations and feedback in
the Local Group of galaxies and beyond. Despite efforts in the literature, we
still lack a comprehensive, empirical view of the dependence of wind properties
on metallicity ($Z$). Here, we investigate the winds of O and B stars in the
Milky Way (MW) and Small Magellanic Cloud (SMC). We gathered a sample of 96
stars analyzed by means of the NLTE code CMFGEN. We explored their wind
strengths and terminal velocities to address the $Z$ dependence, over a large
luminosity range. The empirical wind-luminosity relation (WLR) obtained updates
and extends previous results in the literature. It reveals a luminosity and $Z$
dependence, in agreement with the radiatively driven wind theory. For bright
objects ($\log L/L_\odot \gtrsim 5.4$), we infer that $\dot{M} \sim
Z^{0.5-0.8}$. However, this dependence seems to get weaker or vanish at lower
luminosities. The analysis of the terminal velocities suggests a shallow $Z^n$
dependence, with $n \sim 0.1-0.2$, but it should be confirmed with a larger
sample and more accurate $V_{\infty}$ determinations. Recent results on SMC
stars based on the PoWR code support our inferred WLR. On the other hand,
recent bow-shocks measurements stand mostly above our derived WLR. Theoretical
calculations of the WLR are not precise, specially at low $L$, where the
results scatter. Deviations between our results and recent predictions are
identified to be due to the weak wind problem and the extreme terminal
velocities predicted by the models. The Z dependence suggested by our analysis
deserves further investigations, given its astrophysical implications.
