The Nitrogen-to-Oxygen evolution in galaxies: the role of the star   formation rate
  The main objective of the present work is to ckeck if the star formation efficiency plays a relevant role in the evolution of the relative abundance N/O. We analyze the evolution of the nitrogen-to-oxygen ratio as predicted by a set of computed theoretical models of simulated galaxies with different total masses which are evolved assuming different collapse time scales and different star formation efficiencies, thus producing different star formation histories. The stellar yield set we use for all of them (Gavil\'{a}n et al. 2005) have an important contribution of primary nitrogen proceeding from low and intermediate mass stars. It allows to obtain a dispersion of results in the N/O-O/H plane, when star formation efficiencies vary, in general agreement with observations. The model results for the N/O abundance ratio are in good agreement with most observational data trends: the extragalactic H{\sc ii} regions is well reproduced with present time resulting abundances and the low N/O values estimated for high-redshift objects, as well as the higher and constant values of N/O observed for irregular and dwarf galaxies or halo stars, can be simultaneously obtained with our models at the same low oxygen abundances. We conclude that differences in the star formation history of galaxies and regions within them are a key factor to explain the data in the N/O-O/H plane. 
M. Moll\'a, J. M. V\'ilchez, M. Gavil\'an, A. I. D\'iaz