Physical Conditions, Grain Temperatures, and Enhanced Very Small Grains   in the Barnard Loop
  We derive the radio spectral index of the Barnard 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 HII region, and a high-latitude filament; all of these regions are somewhat cooler than typical HII 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) ~ 24000 cm^{-3} K.   Grains within BL are warmer than in HI regions. Trapped L-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 micron radiation are enhanced in BL relative to HI by a factor of 2-3, while PAH's that emit 12 micron are probably deficient by a factor ~2. 
Carl Heiles, L.M. Haffner, R.J. Reynolds, S.L. Tufte