Chemical Abundances of the Galactic H ii Region NGC 3576 Derived from Very Large Telescope Echelle Spectrophotometry

Abstract

We present echelle spectrophotometry of the Galactic H II region NGC 3576. The data have been taken with the VLT UVES echelle spectrograph in the 3100-10400 A range. We have measured the intensities of 458 emission lines, 344 are permitted lines of H0, He0, C+, N0, N+, N++, O0, O+, Ne+, S++, Si0, Si+, Ar0, and Ar+; some of them are produced by recombination and others mainly by fluorescence. Electron temperatures and densities have been determined using different continuum and line intensity ratios. We have derived He+, C++, O+, O++, and Ne++ ionic abundances from pure recombination lines. We have also derived abundances from collisionally excited lines for a large number of ions of different elements. Remarkably consistent estimations of t2 have been obtained by comparing Balmer and Paschen with [O III] temperatures, and O++ and Ne++ ionic abundances obtained from collisionally excited and recombination lines. The chemical composition of NGC 3576 is compared with those of other Galactic H II regions and with the one from the Sun. A first approach to the gas-phase Galactic radial abundance gradient of C as well as of the C/O ratio has been made.