ELECTRON TEMPERATURES IN ORION A

Abstract

We measured the hydrogen, helium, and carbon 106α radio recombination lines (RRLs), three different β and γ lines, and radio continuum at 4' east, west, and south of the peak of the H ii region NGC 1976 (Messier 42, Orion A) with the NRAO Green Bank Telescope at 5 GHz. The and integrated line intensities for the offset positions are in agreement with local thermodynamic equilibrium, and so the ratio of the integrated intensity from RRL to the thermal continuum allows for a simplified calculation of the electron temperature, Te. For the offset positions, the average value is Te = 7200 ± 100 K. The best value of Te for the peak continuum position, based on previous RRL data, is 8300 ± 200 K. Thus, there is a decrease in Te with angular offset. These data are consistent with the empirical model of Wilson et al., based on RRL data. There is good agreement between the Te values from RRLs and those from optical collisionally excited lines, [O iii], and Balmer decrement data, but the Te value from optical recombination lines is significantly lower. Analysis of this prototype H ii region provides additional inputs for the determination of element abundances and a benchmark for interpretations of more distant H ii regions.