The ortho and para fractions of molecular hydrogen in protostellar outflows and Herbig-Haro objects

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 2000-2500 K although rotational temperatures vary between 1100-2100 K. 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.