H-2 EMISSION AS A TRACER OF MOLECULAR-HYDROGEN - LARGE-SCALE OBSERVATIONS OF ORION

We have detected extremely extended (>1.degrees 5, or 12 pc) near-infr ared H-2, line emission from the Orion A molecular cloud. We have mapp ed emission in the 1.601 mu m v = 6-4 Q(1) and 2.121 mu m v = 1-0 S(1) lines of H-2 along a similar to 2 degrees R.A. cut and from a 6' x 6' region near theta(1) Ori C. 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, lines, we conclude that UV fluorescence is most likely the dominant H-2 emission mechanis m in the outer parts of the Orion cloud. Shock-heated gas does not mak e a major contribution to the H-2 emission in this region. The fluores cent component of the total H-2 v = 1-0 S(1) luminosity from Orion is 30-40 L.. Molecular hydrogen excited by UV radiation from nearby OB st ars contributes 98%-99% of 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. The ability to detect large-scale H-2 directly opens up new possibilities for the study of molecular clouds.