Importance of WN stars on the ionisation and chemical evolution of galaxies

Massive stars are primary sources of chemical yields and mechanical and ionization luminosity budgets. Therefore, understanding massive stars' evolution is crucial to have a complete understanding of the chemical and ionization evolution of galaxies. Moreover, with the advent of instruments like MUSE, JWST (upcoming), HST, etc., we have (/are going to have) a comprehensive picture of the impact of dust-masked massive stars in the evolution of galaxies for the wide range of wavebands starting from IR to optical to UV. The evolutionary paths taken by very massive stars, M > 60MSun, remain substantially uncertain: they begin their lives as main sequence O stars, but, depending on their masses, rotation rates, and metallicities, can then pass through a wide range of evolutionary states, yielding an equally broad set of possible surface compositions and spectral classifications. The surface enrichment of He and N is quite common in rotating WNL stars, but the WNL-like surface elemental abundances in slow rotators, as observed by Herrero et al. 2000, Vink et al. 2017, etc., puzzled astronomers for almost two decades. Meynet & Maeder (2000) hypothesized that an exotic scenario of stellar spin-down needs to be invoked in order to explain the origin of these unusually high surface enriched slow rotators. Contrary to this hypothesis, I discovered that these nonrotating metal-rich stars reveal the products of nucleosynthesis on their surfaces because even modest amounts of mass loss expose their “fossil”-convective cores: regions that are no longer convective, but which were part of the convective core at an early stage in the star's evolution. This mechanism provides a natural explanation for the origin of metal-rich ([Fe/H] ≥ -1.0) slowly-rotating WNL stars without any need for exotic spin-downs. These stars have a huge impact on determining the chemical evolution of galaxies at high redshift. I will also discuss the impact of these stars on determining the ionization budgets in different wavebands.