The metal abundance of circumnuclear star-forming regions in early-type spirals. Spectrophotometric observations

We have obtained long-slit observations in the optical and near-infrared of 12 circumnuclear HII regions [circumnuclear star-forming regions (CNSFR)] in the early-type spiral galaxies NGC2903, 3351 and 3504 with the aim of deriving their chemical abundances. Only for one of the regions, the [SIII] λ6312Å was detected providing, together with the nebular [SIII] lines at λλ9069, 9532Å, a value of the electron temperature of . A semi-empirical method for the derivation of abundances in the high metallicity regime is presented.

We obtain abundances which are comparable to those found in high metallicity disc HII regions from direct measurements of electron temperatures and consistent with solar values within the errors. The region with the highest oxygen abundance is R3+R4 in NGC3504, 12 + log(O/H) = 8.85, about 1.5 solar if the solar oxygen abundance is set at the value derived by Asplund, Grevesse & Sauval, 12 + log(O/H)_solar = 8.66 +/- 0.05. Region R7 in NGC3351 has the lowest oxygen abundance of the sample, about 0.6 times solar. In all the observed CNSFR the O/H abundance is dominated by the O⁺/H⁺ contribution, as is also the case for high metallicity disc HII regions. For our observed regions, however, also the S⁺/S²⁺ ratio is larger than one, contrary to what is found in high metallicity disc HII regions for which, in general, the sulphur abundances are dominated by S²⁺/H⁺.

The derived N/O ratios are in average larger than those found in high metallicity disc HII regions and they do not seem to follow the trend of N/O versus O/H which marks the secondary behaviour of nitrogen. On the other hand, the S/O ratios span a very narrow range between 0.6 and 0.8 of the solar value.

As compared to high metallicity disc HII regions, CNSFR show values of the O₂₃ and the N2 parameters whose distributions are shifted to lower and higher values, respectively, hence, even though their derived oxygen and sulphur abundances are similar, higher values would in principle be obtained for the CNSFR if pure empirical methods were used to estimate abundances. CNSFR also show lower ionization parameters than their disc counterparts, as derived from the [SII]/[SIII]. Their ionization structure also seems to be different with CNSFR showing radiation field properties more similar to HII galaxies than to disc high metallicity HII regions.