Physical conditions, grain temperatures, and enhanced very small grains in Barnard's loop

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 H alpha data to determine the electron temperature in BL, the lambda 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) approximate to 2.0 cm(-3) and pressure P/ k approximate to 24,000 cm(-3) K. Grains within BL are warmer than in H I regions. Trapped Ly alpha 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 mu m radiation are enhanced in BL relative to H I by a factor of 2-3, while polycyclic aromatic hydrocarbons that emit 12 mu m are probably deficient by a factor of similar to 2.