Revisiting the Evolutionary Status of Massive Stars at the central parsec of the Milky Way

TL;DR

The authors reassess the evolution of massive GC stars by using Geneva tracks with updated mass-loss prescriptions for OB winds and RSGs at $Z=0.020$, across $20-60\,M_\odot$. Weaker early winds conserve envelope mass and angular momentum, producing WR stars that are less radially homogeneous and lack hydrogen-free WN phases, in better accord with GC WR abundances. They provide a tabulated set of mean surface abundances for WR subtypes (Ofpe/WN9, WNL, WN/C, WC) and advocate a revised WR subtype ordering for wind collision modelling around Sgr A*, with implications for GC feedback and accretion. While the results improve GC wind characterisation, they highlight the need for higher-$Z$ wind physics and more precise GC WR abundance measurements to fully capture GC environmental conditions.

Abstract

Massive stars and their winds have a large influence in their environment, e.g, determining the accretion rate on to the Galactic Centre (GC) super-massive black hole Sgr A*. The winds of those stars collide and are accreted, at a rate that depends on their chemical composition. Here we aim to revisit the evolutionary status of the evolved massive stars at the GC, by means of new tracks based on updated mass-loss rate recipes for the earlier stages of massive stars. We use the Geneva-evolution-code for initial stellar masses ranging from 20 to 60 $M_\odot$, for metallicity $Z=0.020$. We adopt a new mass-loss rate recipe for the line-driven winds of O-type stars and B-supergiants, plus a new recipe for the dust-driven winds of red supergiants (RSG). Additionally, we set up initial rotation $Ω/Ω_\text{crit}=0.4$, and we adopt the Ledoux criterion for the treatment of convection in inner layers. We found that evolution models adopting new mass-loss rate prescriptions predict that stars will lose less of their outer layers during their initial phases, while a big reduction of mass happens at the RSG phase. As a consequence, the resulting Wolf-Rayet (WR) stars are less radially homogeneous in their inner structure from the core to the surface. Also, these new evolution models predict the absence of hydrogen-free WN stars. These evolutionary predictions agree better with the observed chemical abundances of the WR stars at the GC. We provide a table with the chemical H, He, and CNO abundances calculated for the different subtypes of WR stars. We propose a different re-arrangement of the WR subtypes to be used for the modelling of the collision of their winds. We discuss the potential implications of these changes for the colliding winds generated from the massive stars at the GC, which are accreting onto the supermassive black hole Sgr A*.

Figures (9)

• Figure 1: HR diagram for the new evolution models of this work (solid lines), compared with the old models from yusof22. Empty, dark yellow, and dark cyan circles represent the end of the H-core, the beginning of the He-core, and the end of the He-core burning stages, respectively. The yellow-shaded region corresponds to the zone beyond the Humphreys-Davidson limit, where LBV stars are expected to be found humphreys16.
• Figure 2: Evolution of rotational velocity expressed as absolute magnitude $\varv_\text{rot}$ (left panel) and angular velocity as fraction of the critical velocity $\Omega/\Omega_\text{crit}$ (right panel), as a function of the H-core burning lifetime $\tau_\text{H}$. Solid and dashed lines represent new and old models respectively, same as for Fig. \ref{['HRD_z20']}.
• Figure 3: Evolution of the convective cores, as fractions of the total stellar mass, for our models during the H-core, H-shell and He-core stages. Solid and dashed lines represent new and old models respectively, same as for Fig. \ref{['HRD_z20']}.
• Figure 4: Mass-loss rates after the H-depletion, as a function of the effective temperature, for the evolution tracks from yusof22 and the tracks from this work (solid lines). Solid and dashed lines represent new and old models respectively, same as for Fig. \ref{['HRD_z20']}.
• Figure 5: Evolution of the surface abundances for hydrogen, helium, carbon, nitrogen, and oxygen, for the models reaching the WR phase. Black lines represent the total mass fraction, $M_*/M_\text{zams}$. The nitrogen abundance is multiplied by 10, for illustrative purposes.