Astronomy and Astrophysics – Astronomy
Scientific paper
Jan 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994mnras.266..497d&link_type=abstract
Monthly Notices of the Royal Astronomical Society, Vol. 266, NO. 2/JAN15, P. 497, 1994
Astronomy and Astrophysics
Astronomy
23
H Ii Regions - Infrared: Stars
Scientific paper
We have measured the line fluxes of He I 2.058 μm and Brγ 2.165 μm 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 ne = 102, 104 and 106 cm-3. From our observations, the He I 2.058 μm/Brγ 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 Te = 5 × 103 K, ne = 104 cm-3 and a Galactic helium abundance of 0.1. Alternatively, for an electron temperature and density of Te = 104 K and ne = 104 cm-3 the He I linewidth must be increased from its Doppler value of 6 km s-1 to about 17 km s-1, e.g. by turbulent motions of the ionized gas, to be consistent with our observations.
We also consider the role of telluric CO2 absorption on the He I recombination line at 2.058 m. Because of the intrinsic narrowness of the He I line (20-40 km 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.
Brand Peter W. J. L.
Doherty R. M.
Doyon Rene
Puxley Phil
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