To clump or not to clump The impact of wind inhomogeneities on the optical and NIR spectroscopic analysis of massive OB stars

TL;DR

This study compares optical and near-infrared spectroscopic analyses of massive OB stars to assess the impact of wind clumping on derived stellar parameters, finding consistent results with larger uncertainties in the infrared and no clear advantage of complex clumping laws.

Contribution

It demonstrates that optical and infrared spectroscopic analyses yield similar stellar parameters for massive OB stars despite wind clumping, highlighting the limitations in constraining clumping laws.

Findings

Infrared analysis produces similar stellar parameters as optical with larger uncertainties.

Clumping improves fits for certain spectral lines but not universally.

Different lines favor different clumping laws, indicating complex wind structures.

Abstract

Winds of massive stars have density inhomogeneities (clumping) that may affect the formation of spectral lines in different ways, depending on their formation region. Most of previous and current spectroscopic analyses have been performed in the optical or ultraviolet domain. However, massive stars are often hidden behind dense clouds rendering near-infrared observations necessary. Our objective is to investigate whether a spectroscopic analysis using either optical or infrared observations results in the same stellar parameters with comparable accuracy, and whether clumping affects them in different ways. We analyzed optical and near-infrared observations of a set of massive O stars with spectral types O4-O9.5 and all luminosity classes. We obtain similar stellar parameters in the optical and the infrared, although with larger uncertainties in the near-infrared, both with and without clumping, albeit with some individual deviating cases. We find that the inclusion of clumping improves the fit to H$_\alpha$ or HeII 4686 in the optical for supergiants, as well as that of Br$_\gamma$ in the near-infrared, but it sometimes worsens the fit to HeII 2.18$\mu$m. Globally, there are no significant differences when using the clumping laws tested in this work. The infrared can be used for spectroscopic analyses, giving similar parameters as from the optical, though with larger uncertainties. The best fits to different lines are obtained with different (linear) clumping laws, indicating that the wind structure may be more complex than adopted in the present work. No clumping law results in a better global fit, or improves the consistency between optical and infrared stellar parameters. Our work shows that the optical and infrared lines are not sufficient to break the dichotomy between the mass-loss rate and clumping factor.