SPIRE spectroscopy of the prototypical Orion Bar photodissociation region

摘要

Aims. We present observations of the Orion Bar photodissociation region (PDR) obtained with the SPIRE instrument on-board Herschel. Methods. We obtained SPIRE Fourier-transform spectrometer (FTS) sparse sampled maps of the Orion bar. Results. The FTS wavelength coverage and sensitivity allow us to detect a wealth of rotational lines of CO (and its isotopologues), fine structure lines of C and N~+, and emission lines from radicals and molecules such as CH~+, CH, H_2O or H_2S. For species detected from the ground, our estimates of the column densities agree with previously published values. The comparison between ~(12)CO and ~(13)CO maps shows particularly the effects of optical depth and excitation in the molecular cloud. The distribution of the ~(12)CO and ~(13)CO lines with upper energy levels indicates the presence of warm (～100-150 K) CO. This warm CO component is a significant fraction of the total molecular gas, confirming previous ground based studies.