THE PHYSICAL STRUCTURE OF ORION-KL ON 2500-AU SCALES USING THE K-DOUBLET TRANSITIONS OF FORMALDEHYDE

The physical characteristics of a molecular cloud which relate most closely to star formation within it occur on the finest spatial scales. Generally, several transitions of a specific molecule must be mapped to securely determine molecular cloud physics. Toward this goal, interferometric observations of the 1(10) --> 1(11) and 5(14) --> 5(15) transitions of H2CO have been made toward the Orion-KL molecular cloud. With synthesized beamwidths of 5.1'' (2400 AU) and 7.6'' (3500 AU), respectively, we identify emission from the '' hot core,'' ''compact ridge,'' and ''northern cloud'' regions. We also detect 1(10) <-- 1(11) H2CO absorption toward the ''Orion-S'' region. These 1(10) --> 1(11) and 5(14 --> 5(15) emission measurements have been combined with 6'' resolution measurements of the 2(11) --> 2(12) transition of H2CO (Mangum et al. 1990) in a spherical large velocity gradient model of the H2CO excitation to derive the H-2 density and H2CO column density in the hot core, compact ridge, and northern cloud. Typical peak densities lie in the range (3-8) x 10(5) cm-3, with H2CO column densities in the range 10(16)-10(17) cm-2. Highest spatial densities but lowest column densities occur in the northern cloud, while lowest spatial densities but highest column densities characterize the compact ridge. A critical discussion of H2CO as a spatial density probe demonstrates that the K-doublet transitions provide an excellent spatial density probe, attaining substantial optical depths only at the highest column densities.