Non-LTE radio recombination line analysis towards H ii region G34.25+0.14

Abstract

ABSTRACT The electron temperature $T^{*}_\mathrm{ e}$ of H ii regions in local thermodynamic equilibrium (LTE) is usually calculated based on the ratio of radio combination lines (RRLs) and continuum emission. In this work, the LTE $T^{*}_\mathrm{ e}$ in the H ii region G34.25+0.14, which is especially luminous and regarded as a prime target to investigate the physical properties of an H ii region, is obtained by analysing both RRLs and continuum in L band, and employing the correction factor $b_n^{*}$ to correct for non-LTE conditions. The LTE $T^{*}_\mathrm{ e}$ values in L band are calculated to be $2580\, {\rm K}$ for H i/OH/Recombination line survey (THOR) observations and $3690\, {\rm K}$ for Survey of Ionized Gas in our Galaxy, using the Arecibo telescope (SIGGMA), respectively. By employing a non-LTE correction, we derive Te of 8900 $\, {\rm K}$ and the corresponding $b^{*}_n$ values are calculated to be 1 and close to 1 in X band and C band, respectively. However, $b_n^{*}$ at L band (${\sim} 1.4\, {\rm GHz}$) is calculated as 4.2 and 2.4 for THOR and SIGGMA, suggesting that non-LTE corrections are necessary and significantly affect the results. This work demonstrates that the assumption of LTE to calculate $T^{*}_\mathrm{ e}$ is not valid for the observed transitions in L band, and hence, non-LTE corrections are needed. Furthermore, Ne for G34.25+0.14 is calculated from multiple bands independently, consistent with previous work. This study provides a method which could be used for other H ii regions to investigate their degree of departure from LTE.