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We have detected extremely extended ($>$1.5$\deg$, or 12 pc) near-infrared H$_2$ line emission from the Orion A giant molecular cloud. We have used a new instrument, the University of Texas Near-Infrared Fabry-Perot spectrometer, which is optimized for the detection of low surface brightness line emission from 1.4 $\mu$m to 2.4 $\mu$m (Luhman et al. 1995, PASP, in press). Our Orion observations show that we can directly trace H$_2$ along the outlying surfaces of interstellar molecular clouds, well away from the star-forming cores. The diffuse, extended H$_2$ emission provides a unique new view of molecular clouds to complement conventional large-scale tracers such as CO and is an excellent probe of cloud energetics.
In Orion, we have mapped the emission in the 1.601 $\mu$m v=6--4 Q(1), 2.121 $\mu$m v=1--0 S(1), and 2.247 $\mu$m v=2--1 S(1) lines of H$_2$. The surface brightness of the extended H$_2$ line emission is 10$^{-6}$ to 10$^{-5}$ ergs s$^{-1}$ cm$^{-2}$ sr$^{-1}$. Based on the distribution and relative strengths of the H$_2$ emission lines, we conclude that H$_2$ excited by ultraviolet radiation dominates the global H$_2$ line emission from the Orion molecular cloud, even though this cloud has a powerful shock-excited H$_2$ source in its core. We are comparing our H$_2$ data and observations of the 63 $\mu$m [O~I] and 158 $\mu$m [C~II] emission to the predictions of theoretical models of photodissociation or photon dominated regions to examine the physical conditions throughout the Orion A cloud.
This work was supported by the David and Lucile Packard Foundation through a fellowship to D.T.J. and by a NASA Graduate Traineeship grant NGT-50998 held by M.L.L.
