$^{14}$N/$^{15}$N ratio measurements in prestellar cores with N$_2$H$^+$: new evidence of $^{15}$N-antifractionation

TL;DR

This study measures the $^{14}$N/$^{15}$N ratio in multiple prestellar cores using N$_2$H$^+$, revealing consistent $^{15}$N antifractionation that challenges existing chemical models and suggests the need for network revisions.

Contribution

It extends previous measurements to three additional prestellar cores, demonstrating that $^{15}$N antifractionation in N$_2$H$^+$ is common and not unique to L1544.

Findings

$^{14}$N/$^{15}$N ratios range from 630 to 770

Significant $^{15}$N depletion in prestellar cores

Current chemical models underestimate $^{15}$N depletion

Abstract

Context: The $^{15}$N fractionation has been observed to show large variations among astrophysical sources, depending both on the type of target and on the molecular tracer used. These variations cannot be reproduced by the current chemical models. Aims: Until now, the $^{14}$N/$^{15}$N ratio in N$_2$H$^+$ has been accurately measured in only one prestellar source, L1544, where strong levels of fractionation, with depletion in $^{15}$N, are found ($^{14}$N/$^{15}$N$\: \approx 1000$). In this paper we extend the sample to three more bona fide prestellar cores, in order to understand if the antifractionation in N$_2$H$^+$ is a common feature of this kind of sources. Methods: We observed N$_2$H$^+$,N$^{15}$NH$^+$ and $^{15}$NNH$^+$ in L183, L429 and L694-2 with the IRAM 30m telescope. We modeled the emission with a non-local radiative transfer code in order to obtain accurate estimates of the molecular column densities, including the one for the optically thick N$_2$H$^+$. We used the most recent collisional rate coefficients available, and with these we also re-analysed the L1544 spectra previously published. Results: The obtained isotopic ratios are in the range $630-770$ and significantly differ with the value, predicted by the most recent chemical models, of $\approx 440$, close to the protosolar value. Our prestellar core sample shows high level of depletion of $^{15}$N in diazenylium, as previously found in L1544. A revision of the N chemical networks is needed in order to explain these results.