Nitrogen fractionation in high-mass star-forming cores across the Galaxy

Abstract

The fractionation of nitrogen (N) in star-forming regions is a poorly understood process. To put more stringent observational constraints on the N-fractionation, we have observed with the IRAM-30-m telescope a large sample of 66 cores in massive star-forming regions. We targeted the (1-0) rotational transition of HN<SUP>13</SUP>C, HC<SUP>15</SUP>N, H<SUP>13</SUP>CN and HC<SUP>15</SUP>N, and derived the <SUP>14</SUP>N/<SUP>15</SUP>N ratio for both HCN and HNC. We have completed this sample with that already observed by Colzi et al., and thus analysed a total sample of 87 sources. The <SUP>14</SUP>N/<SUP>15</SUP>N ratios are distributed around the Proto-Solar Nebula value with a lower limit near the TA value (∼272). We have also derived the <SUP>14</SUP>N/<SUP>15</SUP>N ratio as a function of the Galactocentric distance and deduced a linear trend based on unprecedented statistics. The Galactocentric dependences that we have found are consistent, in the slope, with past works but we have found a new local <SUP>14</SUP>N/<SUP>15</SUP>N value of ∼400, i.e. closer to the Prosolar Nebula value. A second analysis was done, and a parabolic Galactocentric trend was found. Comparison with Galactic chemical evolution models shows that the slope until 8 kpc is consistent with the linear analysis, while the flattening trend above 8 kpc is well reproduced by the parabolic analysis.