Speaker
Description
There is a growing evidence that our Sun was born in a rich cluster that also contained massive stars (Adams, 2010; Taquet et al. 2016). Therefore, the study of the chemical content and chemical processes (such as fractionation) in high-mass star-forming regions is key to understand our chemical heritage. We are thus undertaking a huge observational effort to derive the $^{14}$N/$^{15}$N ratios in a large sample of massive star-forming cores, by means of different molecular tracers. In fact, molecules found in comets and other pristine Solar System bodies are enriched in $^{15}$N, because they show a lower $^{14}$N/$^{15}$N ratio with respect to the value representative of the Proto-Solar Nebula (PSN) (441$\pm$6, Marty et al. 2010), but the reasons of this enrichment cannot be explained by current chemical models. First of all, we have measured the $^{14}$N/$^{15}$N ratio in a sample of 27 high-mass star forming cores, from observations of HCN(1-0) and HNC(1-0) (Colzi et al. 2017), observed with the IRAM-30m Telescope. We have found values of the $^{14}$N/$^{15}$N ratio that are distributed remarkably well around that of the PSN value. Moreover, because these sources were divided into different evolutionary categories (high-mass starless cores, high-mass protostellar objects, and ultracompact H$_{II}$ regions), we have investigated the possible dependence of the isotopic ratio with evolution: we have found no statistically significant variations, suggesting that time does not seem to play a role in the fractionation of nitrogen. Therefore, the cause of $^{15}$N enrichment in pristine Solar System bodies is likely not due to the chemical evolution of the parent core. Afterwards, we have added to this initial sample 60 additional sources observed with the IRAM 30-m as well, for a total of 87 objects. We have computed the $^{14}$N/$^{15}$N ratios from the same molecules and we have found that these ratios are concentrated in the range $\sim$100 and $\sim$800 (Colzi et al., submitted to MNRAS). Thanks to the very large statistics and to the fact that these sources span a wide range of distances from the Galactic center, we have derived a new Galactocentric trend of $^{14}$N/$^{15}$N, for HNC and HCN. Moreover, we have estimated that the $^{14}$N/$^{15}$N ratio in the local interstellar medium is about 400, i.e. very close to the PSN value.
