Spectral classification and properties of the O Vz stars in the Galactic   O-Star Spectroscopic Survey (GOSSS)
  On the basis of the Galactic O-Star Spectroscopic Survey (GOSSS), a detailed systematic investigation of the O Vz stars is presented. The currently used spectral classification criteria are rediscussed, and the Vz phenomenon is recalibrated through the addition of a quantitative criterion based on the equivalent widths of the He I 4471, He II 4542, and He II 4686 spectral lines. The GOSSS O Vz and O V populations resulting from the newly adopted spectral classification criteria are comparatively analyzed. The locations of the O Vz stars are probed, showing a concentration of the most extreme cases toward the youngest star forming regions. The occurrence of the Vz spectral peculiarity in a solar-metallicity environment, as predicted by the fastwind code, is also investigated, confirming the importance of taking into account several processes for the correct interpretation of the phenomenon. 
Julia I. Arias, Nolan R. Walborn, Sergio Simon-Diaz, Rodolfo H. Barba,   Jesus Maiz Apellaniz, Carolina Sabin-Sanjulian, Rob...
astro-ph.SR