One of the most important tools to investigate the chemical history of our Galaxy and our own Solar System is to measure the isotopic fractionation of chemical elements. This is the process that distributes the less abundant stable isotopes of an element in different molecules. The isotopic ratios are governed by two main processes: 1. chemical evolution of the whole Galaxy due to stellar nucleosynthesis; and 2. local fractionation effects. For the case of nitrogen (N), the 14N/15N isotopic ratio found for the proto-Solar nebula, 440, is significantly higher than that measured in pristine Solar System materials, like comets (around 140). This suggests a local chemical enrichment of 15N during the Solar System formation. However, the causes of the 15N-enrichment are still uncertain. In this talk I will briefly review the state-of-the-art of the astronomical observations and theoretical chemical models devoted to the study of nitrogen fractionation. I will show the overall behavior of the 14N/15N ratio across the Galaxy. In particular, based on a large survey of star-forming regions, we have confirmed that the 14N/15N ratio increases with the galactocentric distance. This overall trend can be explained by nucleosynthesis Galactic chemical evolution models. Furthermore, I will present the first interferometric maps of N-fractionation of N2H+ towards a star-forming region (see Fig. 1 in attachment). Our results highlight the importance of local effects, and in particular of isotope-selective photodissociation of N2, in determining the 15N-enrichments in star-forming regions.

Nitrogen isotopes in the interstellar medium: A chemical journey across the galaxy

ACS Spring Meeting 2021

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