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

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.

References in corpus (4)

• Observing the gas temperature drop in the high-density nucleus of L 1544
• Depletion and low gas temperature in the L183 prestellar core : the N2H+ - N2D+ tool
• The Different Structures of the Two Classes of Starless Cores
• CN in prestellar cores

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