New self-consistent wind parameters to fit optical spectra of O-type stars observed with the HERMES spectrograph

Abstract

Aims. We performed a spectral fitting for a set of O-type stars based on self-consistent wind solutions, which provide mass-loss rate and velocity profiles directly derived from the initial stellar parameters. The great advantage of this self-consistent spectral fitting is therefore the reduction of the number of free parameters to be tuned. Methods. Self-consistent values for the line-force parameters (k, α, δ)sc and subsequently for the mass-loss rate, Msc, and terminal velocity, v∞,sc, are provided by the m-CAK prescription introduced in Paper I, which is updated in this work with improvements such as a temperature structure T(r) for the wind that are self-consistently evaluated from the line-acceleration. Synthetic spectra were calculated using the radiative transfer code FASTWIND, replacing the classical β-law for our new calculated velocity profiles v(r) and therefore making clumping the only free parameter for the stellar wind. Results. We found that self-consistent m-CAK solutions provide values for theoretical mass-loss rates of the order of the most recent predictions of other studies. From here, we generate synthetic spectra with self-consistent hydrodynamics to fit and obtain a new set of stellar and wind parameters for our sample of O-type stars (HD 192639, 9 Sge, HD 57682, HD 218915, HD 195592, and HD 210809), whose spectra were taken with the high-resolution echelle spectrograph HERMES (R = 85 000). We find a satisfactory global fit for our observations, with a good accuracy for photospheric He I and He II lines and a quite acceptable fit for H lines. Although this self-consistent spectral analysis is currently constrained in the optical wavelength range alone, this is an important step towards the determination of stellar and wind parameters without using a β-law. Based on the variance of the line-force parameters, we establish that our method is valid for O-type stars with Teff a ¥ 30 kK and log g a ¥ 3.2. Given these results, we expect that the values introduced here are helpful for future studies of the stars constituting this sample, together with the prospect that the m-CAK self-consistent prescription may be extended to numerous studies of massive stars in the future.