2000ApJ...536..335H

We derive the radio spectral index of Barnard's loop (BL) from large-scale radio surveys at four frequencies and find it to be a thermal source. We use the radio data together with Hα data to determine the electron temperature in BL, the λ Ori H II region, and a high-latitude filament; all of these regions are somewhat cooler than typical H II regions. We perform least-squares fits of the DIRBE diffuse IR intensities to the 21 cm line and radio continuum intensities. After the resolution of a ``geometrical conundrum'', this allows us to derive the electron density n_e; we find n_e~2.0 cm^–3 and pressure P/k~24,000 cm^–3 K. Grains within BL are warmer than in H I regions. Trapped Lyα accounts for the extra heating that is required. This is a general effect that needs to be accounted for in all analyses that examine IR emission from H⁺ regions. Very small grains that emit 60 µm radiation are enhanced in BL relative to H I by a factor of 2-3, while polycyclic aromatic hydrocarbons that emit 12 µm are probably deficient by a factor of ∼2.