The Nessie Nebula is an extremely filamentary and dense Infrared Dark Cloud.

Using the airborne SOFIA observatory, we have imaged [C II] 157.74 and [OI] 63.18 micron line emission from a bright photodissociation region (PDR) associated with the Nessie Bubble, a bright, expanding H II region within the IRDC. A comparison of SOFIA FIR data with ATCA radio data reveals a classic PDR structure, with a uniform progression from ionized gas, to photodissociated gas, and on to molecular gas from the interior to the exterior of the bubble. Toward a FIR-bright protostar in the nebula, both [O I] and [C II] show an absorption feature at a the velocity of an unrelated foreground giant molecular cloud.  Such features are commonly seen toward FIR sources and are puzzling due to their low excitation temperatures ~20 K, inconsistent with the high temperatures expected for photoionized gas (~100 K).

They can be explained, however, as arising from the low density, photodissociated, exterior skins of molecular clouds where the gas densities are so low they lead to subthermal excitation.  The detailed Meudon models confirm that all molecular cloud exteriors exposed to a standard G_0 = 1 interstellar radiation field will produce the observed absorption features.  Finally, the luminous protostar is located precisely where the expanding bubble strikes the Nessie filament. This location suggests that the interaction of older star forming regions with an IRDC filament triggers new star formation.

Astro Colloquium and 'coffee & cookies' department gathering (3:45-4:00pm)

Please join in 538 Davey or click the link to join: https://psu.zoom.us/j/93131536409