The chemical composition of HII regions in the outer Galaxy
J. M. Vilchez, C. Esteban;
MNRAS, 1996, 280, 720
ABSTRACT:New spectroscopic observations, including the optical and near-infrared ranges, have been obtained for a sample
of HII regions located towards the Galactic anticentre.
The sample
includes HII regions with known galactocentric distances, their ionizing
stars having been clearly identified from previous work.
The spectra
correspond to low-to-intermediate excitation nebulae, ionized by one/few late
O/early B star(s).
The global parameters of the regions, as well as their
physical parameters such as electron density and temperature, are
presented.
Abundances have been computed using direct (spectroscopic) determinations of
the electron temperature for some of the objects or alternatively
assuming an adopted electron temperature derived from photoionization model
fitting.
Overall, the data appear to show a substantially flat gradient for the nitrogen
abundance, going up to 18 kpc of galactocentric distance, as well as a mild
gradient, if any, for oxygen.
The data points for sulphur, although less
numerous, give abundances which are consistent with the behaviour observed for
the other elements.
The N/O ratio also presents a very flat gradient
across the outer Galaxy, with a value similar to the one derived from
observations of optical emission lines of HII regions in the solar neighbourhood.
KEYWORDS: ism: abundances, hii regions, galaxy: abundances, galaxies: abundances, galaxies ism, infrared: ism: lines and bands
PERSOKEY:region hii, optical, nir, ,
CODE: vilchez96