4D Grid-fitting of UV-optical spectra of massive stars. I. Numerical technique and its associated uncertainties

TL;DR

This paper introduces a 4D spectral grid-fitting technique for massive stars, quantifies its numerical uncertainties, and examines how physical factors affect the accuracy of derived stellar parameters.

Contribution

It presents a novel 4D spectral grid-fitting method for Wolf-Rayet stars and Blue Supergiants, analyzing its precision and the impact of physical variations on parameter accuracy.

Findings

Numerical precision of stellar parameters is within 0.05 dex.

Unaccounted factors like metal abundance and wind laws significantly affect accuracy.

Including ultraviolet spectra improves parameter determination, especially for weak winds.

Abstract

The best way to check the validity of our theories (models) is by direct comparison with the experiment (observations). In this study, we address the numerical inaccuracies intrinsic to the process of comparing theory and observations. To achieve this goal, we built 4D spectra grids for Wolf-Rayet stars (WC and WN spectral classes) and Blue Supergiants (BSGs) characterized by low metallicity similar to that of the Small Magellanic Cloud (SMC). Through rigorous testing on designated `test' models, we demonstrated that the numerical precision of derived stellar parameters (effective temperature, mass-loss rate, luminosity, and wind velocity) is not exceeding 0.05 dex. Moreover, the mean absolute deviation of the numerically derived stellar parameters is consistently below this threshold for objects with both weak (SMC grid) and strong winds (WC and WN grids), even in the presence of Gaussian noise. Furthermore, we explored the influence of unaccounted factors, including variations in the metal abundances, wind acceleration laws, and clumping, on the precision of the derived parameters. We found that the first two factors have the strongest influence on the numerical accuracy of the derived stellar parameters. Variations in abundances predominantly influenced the mass-loss rate for weak-wind scenarios, while effective temperature and luminosity remained robust. We found that the wind acceleration law influence the numerical uncertainty of the derived wind parameters mostly for models with weak winds. Interestingly, different degrees of clumping demonstrated good precision for spectra with strong winds, contrasting with a decrease in the precision for weak-wind cases. We found also that the accuracy of our approach depends on spectral range and the inclusion of ultraviolet spectral range improves the precision of derived parameters, especially for object with weak winds.