Hyperfine excitation of N$_2$H$^+$ by H$_2$: Toward a revision of N$_2$H$^+$ abundance in cold molecular clouds

TL;DR

This study provides new hyperfine-resolved collisional rate coefficients for N$_2$H$^+$ with H$_2$, improving the accuracy of molecular emission modeling in cold interstellar clouds and revising previous abundance estimates.

Contribution

We computed hyperfine-structure resolved excitation rates of N$_2$H$^+$ by H$_2$ using a new ab initio potential energy surface, offering more accurate data for astrophysical modeling.

Findings

Significant differences from previous He-based rate coefficients.

Line intensities increase with new H$_2$ rates, affecting emission predictions.

Revised estimates of density, temperature, and abundance in LDN 183.

Abstract

The modelling of emission spectra of molecules seen in interstellar clouds requires the knowledge of collisional rate coefficients. Among the commonly observed species, N$_2$H$^+$ is of particular interest since it was shown to be a good probe of the physical conditions of cold molecular clouds. Thus, we have calculated hyperfine-structure resolved excitation rate coefficients of N$_2$H$^+$(X$^1\Sigma^+$) by H$_2(j=0)$, the most abundant collisional partner in the cold interstellar medium. The calculations are based on a new potential energy surface, obtained from highly correlated {\it ab initio} calculations. State-to-state rate coefficients between the first hyperfine levels were calculated, for temperatures ranging from 5 K to 70 K. By comparison with previously published N$_2$H$^+$-He rate coefficients, we found significant differences which cannot be reproduced by a simple scaling relationship. As a first application, we also performed radiative transfer calculations, for physical conditions typical of cold molecular clouds. We found that the simulated line intensities significantly increase when using the new H$_2$ rate coefficients, by comparison with the predictions based on the He rate coefficients. In particular, we revisited the modelling of the N$_2$H$^+$ emission in the LDN 183 core, using the new collisional data, and found that all three of the density, gas kinetic temperature and N$_2$H$^+$ abundance had to be revised.