Some considerations on the origin of nitrogen

The evolution of nitrogen relative to carbon, iron and oxygen in the solar neighbourhood has been computed under the assumption that nitrogen is both a primary and a secondary nucleosynthesis product. In particular, it has been assumed that secondary nitrogen is produced by stars of all masses whereas primary nitrogen is produced by intermediate-mass stars ($$4\lesssim M/M_\odot\lesssim8$$) only, according to the current nucleosynthesis results. However, in order to reproduce the existing data for dwarf stars, nitrogen produced in massive stars ($$M\gt8 M_\odot$$) must be predominantly of primary origin. In this framework, the secondary production of nitrogen cannot be excluded but it must be confined to stars of low and intermediate mass. Finally, nitrogen evolution in the solar neighbourhood has been compared with that in extragalactic H II regions (dwarf irregular galaxies), which are likely to experience a bursting mode of star formation instead of a continuous one. It is reasoned that it is not correct to compare the N/O versus O/H distribution for galactic and extragalactic H II regions with that for dwarf stars in the solar neighbourhood. In fact, the models suggest that the behaviour of N/O with O/H, during galactic evolution, is different from the N/O versus O/H distribution obtained by plotting the present time values of these quantities for different galaxies.