Physical Conditions, Grain Temperatures, and Enhanced Very Small Grains in Barnard’s Loop

Abstract

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 ne; we find ne ≈ 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.