Unveiling the nitrogen-rich massive star in the metal-poor galaxy NGC 4068

Abstract

ABSTRACT We report the identification of the unusual emission-line stellar-like object in the nearby low-metallicity ($Z \sim 0.1 \, \mathrm{Z_{\odot }}$) dwarf galaxy NGC 4068. Our observations performed with long-slit spectrograph and Fabry–Perot interferometer demonstrate high velocity dispersion in H α line, presence of He ii λ4686Å line and peculiarly low [S ii]/[N ii] fluxes ratio for this object. From observational data, we derived that the object represents a single star of high bolometric luminosity (L* ∼ 1.5 × 106 L⊙) surrounded by an expanding nebula with kinematical age of t ∼ 0.5 Myr. The nebula exhibits significant nitrogen overabundance [log (N/O) ∼ −0.05, that is by ∼1.4 dex higher than expected for low-metallicity galaxies]. We suggested that this is a massive blue supergiant (BSG) or Wolf–Rayet (WR) star surrounded by its ejecta interacting with the interstellar medium. We calculated the models of the nebula using cloudy photoionization code, applying cmfgen-modelled BSG and WR stars as ionization sources. We found a best agreement between the modelled and observed spectra for the model assuming ionization by low-metallicity WR star of mass $M_*\approx 80\, \rm M_\odot$, ionizing the nebula through the strong wind and enriching the interstellar medium with nitrogen.