A new mass estimate method with hydrodynamical atmospheres for very massive WNh stars

TL;DR

This paper introduces a hydrodynamically consistent atmosphere modeling method to estimate the mass of very massive stars, successfully applying it to R136a1 and R144, resulting in a mass estimate of 233 solar masses for R136a1.

Contribution

It presents the first hydrodynamically consistent non-LTE atmosphere models for very massive stars, enabling more accurate mass estimates from spectral data.

Findings

Estimated R136a1 mass of 233 Msun consistent with homogeneous mass relations

Hydrodynamic models fit observed spectra and constrain wind properties

Mass estimate provides a lower limit to initial stellar mass

Abstract

Very massive stars with masses over 100 Msun are key objects in the Universe for our understanding of chemical and energetic feedback in the Universe, but their evolution and fate are almost entirely determined by their wind mass loss. We aim to determine the mass of the most massive star known in the Local Group R136a1. For this we compute the first hydrodynamically consistent non-local thermodynamical equilibrium atmosphere models for both R136a1 (WN5h) as well as the binary system R144 (WN5/6h+WN6/7h) in the Tarantula nebula. Using the Potsdam Wolf-Rayet code, we simultaneously empirically derive and theoretically predict mass-loss rates and wind velocities. By fitting synthetic spectra derived from these models to multi-wavelength observations, we constrain the stellar and wind properties of R144 and R136a1. We first determine the clumping stratification required by our hydro-models to fit the spectra of R144 by using the available dynamical mass estimates for the two components. We then utilise this clumping stratification in hydrodynamic models of R136a1 and estimate a mass of $M_\mathrm{Hydro}$ of 233 Msun. Remarkably, the estimated mass is close to and entirely consistent with chemical homogeneous mass relations. This present-day mass of 233 Msun provides a lower limit to the initial stellar mass, that could be far higher due to previous wind mass loss.