1997A&A...327.1206S

Hydrogen molecules exist in two forms, according to the alignment of the nuclear spins. We present an accurate method for extracting the distributions of the two forms from three K-band infrared transitions. Images of the 1-0 S(0), S(1) and S(2) emission lines yield the ortho and para spatial distributions with negligible extinction and model dependence. The ortho-para ratio from individual pointings to various Herbig-Haro outflows are (re-)calculated using our analytical approach. All accurate determinations yield a value between 2.5 and 3.0. The vibrational temperature remains in the range 2000-2500K although rotational temperatures vary between 1100-2100K. Applied to new data for OMC-1, we find no strong evidence for spatial variations in the ortho/para ratio (3.0±0.4) on all scales down to 1". The vibrationally-excited molecules have either (1) not been modified since their formation in a warm state or (2) have reached equilibrium through conversions within a warm state. Collisional excitation to the first vibrational level dominates, with no fluorescent component. We suggest that collisions with atomic hydrogen in the warm shock layers converts the ortho-para ratio to 3 locally. We also find strong evidence for spatial variations in the rotational excitation in OMC-1: the rotational temperature increases from east to west.