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

TL;DR

This study introduces a self-consistent wind modeling approach for O-type stars that reduces free parameters and improves the accuracy of stellar and wind parameter determination from optical spectra.

Contribution

The paper develops a self-consistent spectral fitting method using the m-CAK wind solution, enhancing parameter accuracy and reducing free parameters compared to classical models.

Findings

Mass-loss rates align with recent theoretical predictions.

Synthetic spectra fit observed HERMES spectra with high accuracy.

Method provides a pathway for parameter determination without beta-law assumptions.

Abstract

We perform 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. Self-consistent values for the line-force parameters (k,alpha,delta) and subsequently for the mass-loss rate and terminal velocity are provided by the m-CAK prescription introduced in Paper I, updated in this work with improvements such as a temperature structure for the wind, self-consistently evaluated from the line-acceleration. Synthetic spectra are calculated using the radiative transfer code FASTWIND, replacing the classical beta-law for our new calculated velocity profiles. We found that self-consistent m-CAK solutions provide values for theoretical mass-loss rates on 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 whose spectra was taken with the high resolution echelle spectrograph HERMES (R=85000). We find a satisfactory global fit for our observations, with 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 only, this is an important step towards the determination of stellar and wind parameters without using a beta-law. Given these results, we expect that the values introduced here should be helpful for future studies about the stars constituting this sample, together with the prospective that the m-CAK self-consistent prescription be extended to numerous studies about massive stars in future.