Chemistry in the dIrr galaxy Leo A

TL;DR

This study determines the chemical composition of two H II regions and a planetary nebula in the low-metallicity dwarf irregular galaxy Leo A using spectroscopic data and photoionisation models.

Contribution

It provides detailed chemical abundance measurements of Leo A's H II regions and planetary nebula, employing both direct and model-based methods for the first time in this galaxy.

Findings

Leo A has very low metallicity with 12+log(O/H)=7.4±0.2.

The N/O and S/O ratios are consistent with low-metallicity environments.

The planetary nebula has similar oxygen abundance but higher N/O ratio compared to H II regions.

Abstract

We present chemical abundance determinations of two H II regions in the dIrr galaxy Leo A, from GTC OSIRIS long-slit spectra. Both H II regions are of low excitation and seem to be ionised by stars later than O8V spectral type. In one of the H II regions we used the direct method: O$^{+2}$ ionic abundance was calculated using an electronic temperature determined from the [O III] $\lambda\lambda$4363/5007 line ratio; ionic abundances of O$^+$, N$^+$, and S$^+$ were calculated using a temperature derived from a parameterised formula. O, N and S total abundances were calculated using Ionisation Correction Factors from the literature for each element. Chemical abundances using strong-line methods were also determined, with similar results. For the second H II region, no electron temperature was determined thus the direct method cannot be used. We computed photoionisation structure models for both H II regions in order to determine their chemical composition from the best-fitted models. It is confirmed that Leo A in a very low metallicity galaxy, with 12+log(O/H)=7.4$\pm$0.2, log(N/O)=$-$1.6, and log(S/O)=$-$1.1. Emission lines of the only PN detected in Leo A were reanalysed and a photoionisation model was computed. This PN shows 12+log(O/H) very similar to the ones of the H II regions and a low N abundance, although its log(N/O) ratio is much larger than the values of the H II regions. Its central star seems to have had an initial mass lower than 2 M$_\odot$.