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Description
The nitrogen isotopic ratio is considered an important diagnostic tool of the star formation process, and N_2_H^+^ is particularly important because it is directly linked to molecular nitrogen N_2_. However, theoretical models still lack to provide an exhaustive explanation for the observed^14^N/^15^N values. Recent theoretical works suggest that the^14^N/^15^N behaviour is dominated by two competing reactions that destroy N_2_H^+^: dissociative recombination and reaction with CO. When CO is depleted from the gas phase, if N_2_H^+^ recombination rate is lower with respect to the N^15^NH^+^ one, the rarer isotopologue is destroyed faster. In prestellar cores, due to a combination of low temperatures and high densities, most CO is frozen in ices onto the dust grains, leading to high levels of depletion. On the contrary, in protostellar cores, where temperature are higher, CO ices evaporate back to the gas phase. This implies that the N_2_H^+^ isotopic ratio in protostellar cores should be lower than the one in prestellar cores, and consistent with the elemental value of ~440. We aim to test this hypothesis, producing the first sample of N_2_H^+^ /N^15^NH^+^ measurements in low mass protostars. We observe the N_2_H^+^ and N^15^NH^+^ lowest rotational transition towards six young stellar objects in Perseus and Taurusmolecular clouds. We model the spectra with a custom python code using a constant T_ex_ approach to fit the observations. We discuss in appendix the validity of this hypothesis. The derived column densities are used to compute the nitrogen isotopic ratios. Our analysis yields an average of^14^N/^15^N|_pro_=420+/-15 in the protostellar sample. This is consistent with the protosolar value of 440, and significantly lower than the average value previously obtained in a sample of prestellar objects. Our results are in agreement with the hypothesis that, when CO is depleted from the gas-phase, dissociative recombinations with free electrons destroy N^15^NH^+^ faster than N_2_H^+^ , leading to high isotopic ratios in prestellar cores, where carbon monoxide is frozen onto dust grains.
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