The Excitation of N$_2$H$^+$ in Interstellar Molecular Clouds. I - Models

摘要: We present LVG and non-local radiative transfer calculations involving the rotational hyperfine structure of spectrum N$_2$H$^+$ with collisional rate coefficients recently derived by us. The goal this study is to check validity assumptions made treat physical mechanisms leading observed anomalies. We find that usual hypothesis identical excitation temperatures for all components $J$=1-0 transition not correct within range densities existing in cold dense cores, i.e., a few 10$^4$ $\textless$ n(H$_2$) 10$^6$ cm$^{-3}$. This due different trapping effects components. Moreover, considering typical abundance N$_2$H$^+$, total opacity lines has be taking into account structure. error when only energy can as large 100%. Using models we that, saturation, anomalies appear soon becomes larger than $\simeq$ 20. Radiative scattering less regions enhance these anomalies, particularly, induce differential increase process more effective transitions highest opacities which emerging intensities are also reduced self-absorption effects. These critical HCO$^+$ or HCN, but should taken interpreting spatial extent emission dark clouds.