Improving the determination of Galactic abundance gradients with very deep spectra of HII regions
Cesar Esteban
Instituto de Astrofisica de Canarias



I will present results from very deep spectroscopy of a sample of about 35 Galactic HII regions taken with GTC and VLT. The objects are located along the Galactic disc, with Galactocentric distances from 4.4 to 17 kpc. The electron temperature can be derived by direct methods for all objects. This is the largest collection of precise determinations of abundances in Galactic HII regions. We obtain - for the first time - the radial abundance gradient of N without assuming an ionization correction factor. The radial distribution of the N/O ratio is rather flat, indicating that standard secondary processes do not form the bulk of N. One of the goals of the study is to make a reassessment of the shape of the radial O abundance gradient using an homogeneous dataset. The data confirm the absence of flattening of the O gradient in the outer Galactic disc but, in contrast, a drop or flattening of the O abundance is found in the inner part of the Milky Way. This result confirms previous ones from abundance distributions based on Cepheids. Finally, the results indicate that the scatter of N and O abundances of HII regions with respect to the gradient fittings is not substantially larger than the observational uncertainties, suggesting that both chemical elements are well mixed in the interstellar gas at a given distance along the Galactic disc.