Submillimeter CO Line Emission from Orion

Abstract

Images of an 8 square minute region around the Orion KL source have been made in the J=7-6 (806 GHz) and J=4-3 (461 GHz) lines of CO with angular resolutions of 13" and 18". These data were taken employing on-the-fly mapping and position switching techniques. Our J=7-6 data set is the largest image of Orion with the highest sensitivity and resolution obtained so far in this line. Most of the extended emission arises from a Photon Dominated Region (PDR), but 8% is associated with the Orion ridge. For the prominent Orion KL outflow, we produced ratios of the integrated intensities of our J=7-6 and 4-3 data to the J=2-1 line of CO. Large Velocity Gradient (LVG) models fit the outflow ratios better than PDR models. The LVG models give H_2 densities of ~10^5 per ccm. The CO outflow is probably heated by shocks. In the Orion S outflow, the CO line intensities are lower than for Orion KL. The 4-3/2-1 line ratio is 1.3 for the blue shifted wing and 0.8 for the red shifted wing. Emission in the jet feature extending 2' to the SW of Orion S was detected in the J=4-3 but not the J=7-6 line; the average 4-3/2-1 line ratio is ~1. The line ratios in the Orion S outflow and jet features are consistent with both PDR and LVG models. Comparisons of the intensities of the J=7-6 and J=4-3 lines from the Orion Bar with PDR models show that the ratios exceed predictions by a factor of 2. Either clumping or additional heating by mechanisms such as shocks, may be the cause of this discrepancy.Comment: 30 pages, Latex file, 7 Postscript Figures, Accepted for publication by the Astrophysical Journa

First NH3detection of the Orion Bar

“…Several velocity components are present in the data. Specifically, the observed line profile from the Orion Bar can be decomposed into two components, which are in agreement with observations in high-J CO lines by Wilson et al (2001). Using the source model for the Orion Bar by these authors, our Odin observation implies a total ammonia abundance of NH 3 /H 2 = 5 × 10 −9 .…”

“…For the source model of Wilson et al (2001), we find the abundance of ortho-ammonia relative to H 2 in the Orion Bar to be X o (NH 3 ) = 2.5 × 10 −9 , where the NH 3 (1, 0) → (0, 0) line radiation temperature is T R = 1.7 K, corrected for partial beam filling (∼ 0.2). This result for X o remains unaltered, if the kinetic temperature were higher, e.g., T k = 200 K. The model is also in agreement with the upper limits on the inversion lines of Ho et al (1979) and Wiseman & Ho (1998), provided the ammonia ortho-to-para ratio is not largely different from unity.…”

“…found in the papers of Wilson et al (2001) and of Wiseman & Ho (1998). The latter authors presented VLA-maps in two inversion lines of para-ammonia, showing the emission to arise from finger-like structures.…”

“…For the interpretation of this observation, we will have to rely on source models based primarily on other data. The observed width of the Bar ranges from about 10 to 60 for a variety of molecules (e.g., Tielens et al 1993;Wilson et al 2001). Based on observations in high−J CO lines, estimates of the physical parameters of the Orion Bar have been given by Wilson et al (2001).…”

“…The observed width of the Bar ranges from about 10 to 60 for a variety of molecules (e.g., Tielens et al 1993;Wilson et al 2001). Based on observations in high−J CO lines, estimates of the physical parameters of the Orion Bar have been given by Wilson et al (2001). These authors argue that the geometry is that of a 30 wide rod in the plane of the sky and we assume that their model is applicable also to NH 3 .…”