The homogeneity of chemical abundances in H ii regions of the Magellanic Clouds

We use very deep spectra obtained with the Ultraviolet-Visual Echelle Spectrograph at the Very Large Telescope to derive physical conditions and chemical abundances of four H ii regions of the Large Magellanic Cloud (LMC) and four H ii regions of the Small Magellanic Cloud (SMC). The observations cover the spectral range 3100-10400 angstrom with a spectral resolution of Delta lambda >= lambda/11600, and we measure 95-225 emission lines in each object. We derive ionic and total abundances of O, N, S, Ne, Ar, Cl, and Fe using collisionally excited lines. We find average values of 12 + log (O/H) = 8.37 in the LMC and 8.01 in the SMC, with standard deviations of sigma = 0.03 and 0.02 dex, respectively. The S/O, Ne/O, Ar/O, and Cl/O abundance ratios are very similar in both clouds with sigma = 0.02-0.03 dex, which indicates that the chemical elements are well mixed in the interstellar medium of each galaxy. The LMC is enhanced in N/O by similar to 0.20 dex with respect to the SMC, and the dispersions in N/O, sigma = 0.05 dex in each cloud, are larger than those found for the other elements. The derived standard deviations would be much larger for all the abundance ratios, up to 0.20 dex for N/O, if previous spectra of these objects were used to perform the analysis. Finally, we find a wide range of iron depletions in both clouds with more than 90 per cent of the iron atoms deposited on to dust grains in most objects.

Automated summary

Using deep spectra, we derived the physical conditions and chemical abundances of H ii regions in the Large Magellanic Cloud and Small Magellanic Cloud. The chemical elements are well mixed in both clouds, with slightly higher N/O ratio in the LMC and larger dispersion. Most of the iron atoms are deposited on dust grains in both clouds.