Spectroscopic Analysis of Very Massive Stars in the Context of Upper End of the IMF and MZR

We perform a spectroscopic investigation of Very Massive Stars (VMSs), exceeding 100 solar masses, within young star clusters (YSCs) situated in both a nearby metal-rich (> 0.5 solar metallicity) star-forming spiral galaxy and metal-poor (< 0.3 solar metallicity) star-bursting dwarfs. Our study utilizes FUV spectra acquired from the Hubble/COS and Hubble/STIS for star clusters in M101, NGC 1140 and NGC 4449. Compact YSCs within M101, a nearby spiral galaxy, are potential birthplaces for VMSs regardless of the clusters' mass, metallicity, and age, within the context of the universality of the upper end of the initial mass function (uIMF) across various environments and aligned with mass-metallicity relation studies. Using SESAMME, a Python-based FUV spectral fitting tool, we analyze VMSs' spectral features and explore the variation of the uIMF in multiple dimensions. Comparing the FUV spectral features of star clusters in dwarf galaxies (NGC 1140, NGC 4449) with those in M101, we find robust emission lines of He II and N IV, alongside conspicuous O V absorption lines attributed to VMSs' stellar winds, implying an exceptionally young upper age limit (2-3 Myr) for VMSs' lifetimes. This confirms universality of the uIMF, providing insights into the formation and evolution of massive stars and their impact on galaxy-wide processes, while discussing their potential as analogs for compact sources observed at high redshift, such as Green Pea galaxies, within pristine environments.