On the molecular hydrogen emission at the Orion Nebula

Abstract

It is proposed that the molecular hydrogen emission observed over a region of about 50 arcsec in diameter toward the Orion Nebula is most likely produced by collisional excitation in a shock-heated region. The preshock densities and shock velocities required to give rise to the observed emission are derived, and vibrational-rotational line emission from molecular hydrogen is modeled along with the cooling of the gas behind the shock. Radiative cooling due to both molecular hydrogen and carbon monoxide is taken into account. Numerical results are presented which indicate that the shock velocity must be less than 24 km/s and that a minimum preshock density of 100,000 per cu cm is required. An explosive event in the Kleinmann-Low region is considered as a possible origin of the shock.