Carbon and oxygen in H ii regions of the Magellanic Clouds: abundance discrepancy and chemical evolution

Abstract

We present C and O abundances in the Magellanic Clouds derived from deep spectra of H ii regions. The data have been taken with the Ultraviolet-Visual Echelle Spectrograph at the 8.2-m VLT. The sample comprises 5 H ii regions in the Large Magellanic Cloud (LMC) and 4 in the Small Magellanic Cloud (SMC). We measure pure recombination lines (RLs) of C ii and O ii in all the objects, permitting to derive the abundance discrepancy factors (ADFs) for O 2+ , as well as their O/H, C/H and C/O ratios. We compare the ADFs with those of other H ii regions in different galaxies. The results suggest a possible metallicity dependence of the ADF for the low-metallicity objects; but more uncertain for high-metallicity objects. We compare nebular and Btype stellar abundances and we find that the stellar abundances agree better with the nebular ones derived from collisionally excited lines (CELs). Comparing these results with other galaxies we observe that stellar abundances seem to agree better with the nebular ones derived from CELs in low-metallicity environments and from RLs in high-metallicity environments. The C/H, O/H and C/O ratios show almost flat radial gradients, in contrast with the spiral galaxies where such gradients are negative. We explore the chemical evolution analysing C/O vs. O/H and comparing with the results of H ii regions in other galaxies. The LMC seems to show a similar chemical evolution to the external zones of small spiral galaxies and the SMC behaves as a typical star-forming dwarf galaxy.

A Characteristic Mass Scale in the Mass–Metallicity Relation of Galaxies

“…We also plot the results of Bresolin et al (2016) who report both RL and direct method abundances for six HII regions of different metallicities. These datapoints show a behavior that is consitent with the metallicity trends reported in Toribio San Cipriano et al (2017).…”

“…For the same regions RL abundances of oxygen typically appear between 0.15 and 0.45 dex higher than direct method abundances, and the logarithmic difference between the two is typically referred as the "abundance scale discrepancy factor" (ADF) 12 . For a recent compilation of ADF measurements see Figure 4 12 Except for rare exceptions in which O + recombination lines have been in Toribio San Cipriano et al (2017). This study also finds a metallicity dependence of the ADF, with the smallest offsets (∼ 0.2 dex) seen around 12 + log (O/H) RL 8.5 and an increasing ADF towards lower and higher abundances, reaching values of ∼ 0.4 dex near 12 + log (O/H) RL 8.2 and 8.8.…”

“…showed that IZI abundances computed using the Levesque et al (2010) models used here, agree fairly well with RL abundances, showing a median offset of 0.07 dex and a scatter of 0.1 dex. In Figure 8 we overplot the RL vs. direct method ADF values calculated by Toribio San Cipriano et al (2017) after applying a 0.07 dex offset to bring their RL abundances into a common scale with IZI (filled orange circles). We also plot the results of Bresolin et al (2016) who report both RL and direct method abundances for six HII regions of different metallicities.…”

How common is LBV S Doradus variability at low metallicity?

“…This range can be constrained using the observed metal lines and assuming that AzV261 has SMC metal abundances similar to those found in H ii regions (see, for example, Kurt & Dufour 1998;Garnett 1999;Carlos Reyes et al 2015;Toribio Article number, page 7 of 49 A&A proofs: manuscript no. output San Cipriano et al 2017) and for other hot stars (see, for example, Venn 1999; Rolleston et al 2003;Hunter et al 2007;Trundle et al 2007), as summarized in Dufton et al (2005).…”

Extragalactic archaeology with the C, N, and O chemical abundances

“…Pilyugin et al 2010;Sánchez-Menguiano et al 2016;Belfiore et al 2017;Toribio San Cipriano et al 2017;Amorín et al 2017). This blossoming of observational data will open a new window of interest for galactic astroarchaeology studies.…”