Abstract
The ratio of the populations in the upper, N u and lower, N l levels of the ground state of HI is given by the Boltzmann relation, where the statistical weights in these levels are 3 and 1, respectively: \(N_{\mathrm {u}}/N_{\mathrm {l}}=3\,{\exp }(-0.0682/T_{\mathrm {s}})\). Assume that the spin temperature, T s, equals the kinetic temperature, T K. Calculate the population ratio for a temperature of 100 K. Repeat for a temperature of 3 K (the lowest temperature possible under local thermodynamic equilibrium), for 104 K (the warm interstellar medium) and 106 K. Compare the differences in populations.
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Wilson, T.L., Hüttemeister, S. (2018). Line Radiation of Neutral Hydrogen. In: Tools of Radio Astronomy - Problems and Solutions. Astronomy and Astrophysics Library. Springer, Cham. https://doi.org/10.1007/978-3-319-90820-5_13
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DOI: https://doi.org/10.1007/978-3-319-90820-5_13
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