Determination of the primordial helium abundance based on NGC 346 an HII region of the Small Magellanic Cloud

Abstract

To place meaningful constraints on Big Bang Nucleosynthesis models, the primordial helium abundance determination is crucial. Low-metallicity HII regions have been used to estimate it since their statistical uncertainties are relatively small. We present a new determination of the primordial helium abundance, based on long slit spectra of the HII region NGC 346 in the small Magellanic cloud. We obtained spectra using three $409'' \times 0.51''$ slits divided in 97 subsets. They cover the range $λ\lambda3600-7400$ of the electromagnetic spectrum. We used PyNeb and standard reduction procedures to determine the physical conditions and chemical composition. We found that for NGC 346: $X=0.7465$, $Y=0.2505$ and $Z=0.0030$. By assuming $ΔY / ΔO = 3.3\pm0.7$ we found that the primordial helium abundance is $Y_{\rm P}= 0.2451 \pm 0.0026$ (1$σ$). Our $Y_{\rm P}$ value is in agreement with the value of neutrino families, $N_ν$, and with the neutron half-life time, $τ_{n}$, obtained in the laboratory.