SOFIA/upGREAT far-infrared spectroscopy of bright rimmed pillars in IC 1848

Abstract

Using the upGREAT instrument on SOFIA, we imaged the 158 mu m fine structure line emission in bright-rimmed pillars located at the southern edge of the IC 1848 region, and carried out pointed observations of the 63 and 145 mu m fine structure lines toward selected positions. The observations are used to characterize the morphology, velocity field, and the physical conditions in the G1--G3 filaments. The velocity-resolved spectra show evidence of a velocity shift at the head of the brightest G1 filament, possibly caused by radiation pressure from the impinging UV photons or the rocket effect of the evaporating gas. Archival Herschel PACS and SPIRE observations imply H$_2$ column densities in the range $10^ $ cm$^ $, corresponding to maximum visual extinction $A_V 10$ mag, and average H$_2$ volume density of $ $ in the filaments. The ii emission traces $ 17<!PCT!>$ of the total H$_2$ column density, as derived from dust SED fits. Photon-dominated region models are unable to explain the observed line intensities of the two fine structure lines in IC 1848, with the observed 145 mu m line being too strong compared to the model predictions. The lines in IC 1848 are overall weak and the signal-to-noise ratio is limited. However, our observations suggest that the 63/145 mu m intensity ratio is a sensitive probe of the physical conditions in photon-dominated regions such as IC 1848. These lines are thus excellent targets for future high-altitude balloon instruments, less affected by telluric absorption.