Spectroscopy of H ii regions at 2 ${rm mu m}$: ultracompact sources

We have measured the line fluxes of He i $2.058 \ {\rm \mu m}$ and Br $\gamma \ 2.165 \ {\rm \mu m}$ from ultra-compact (UC) H ii regions excited by stars of spectral types O4 to B1. The ratio of these two lines is compared with theoretical predictions of the dependence of this ratio on the effective temperature of the exciting star(s) for electron densities of $n_{\rm e}=10^2, 10^4$ and $10^6 \ {\rm cm}^{-3}$. From our observations, the He i $2.058 \ {\rm \mu m}/{\rm Br} \ \gamma$ ratio is approximately constant over this range of spectral types. Our observations are in good agreement with the theoretical model of Doyon, Puxley & Joseph for an electron temperature and density of $T_{\rm e}=5 \times 10^3 \ {\rm K},\ n_{\rm e}=10^4 \ {\rm cm}^{-3}$ and a Galactic helium abundance of 0.1. Alternatively, for an electron temperature and density of $T_{\rm e}=10^4 \ {\rm K}$ and $n_{\rm e}=10^4 \ {\rm cm}^{-3}$ the He i linewidth must be increased from its Doppler value of $6 \ {\rm km} \ {\rm s}^{-1}$ to about $17 \ {\rm km} \ {\rm s}^{-1}$, e.g. by turbulent motions of the ionized gas, to be consistent with our observations. We also consider the role of telluric ${\rm CO}_2$ absorption on the He i recombination line at $2.058 \ {\rm \mu m}$. Because of the intrinsic narrowness of the He i line ($20-40 \ {\rm km} \ {\rm s}^{-1}$) the percentage of He i line flux removed varies as the Earth revolves. For the observations presented here, up to 36 per cent of the He i line is absorbed before it reaches the telescope.