Evidence for Dense Gas Heated by the Explosion in Orion KL

Abstract

We mapped the kinetic temperature structure of Orion KL in a ∼20″ (∼8000 au) sized region with para-H 2 CS 7 07  − 6 06 , 7 26  − 6 25 , and 7 25  − 6 24 making use of Atacama Large Millimeter/submillimeter Array Band 6 Science Verification data. The kinetic temperatures obtained with a resolution of 1.″65 × 1.″14 (∼550 au) are deduced by modeling the measured averaged velocity-integrated intensity ratios of para-H 2 CS 7 26  − 6 25 /7 07  − 6 06 and 7 25  − 6 24 /7 07  − 6 06 with a RADEX non-LTE model. The kinetic temperatures of the dense gas, derived from the para-H 2 CS line ratios at a spatial density of 10 7 cm −3 , are high, ranging from 43 to >500 K with an unweighted average of ∼170 K. There is no evidence for internal sources playing an important role in the heating of the various structures identified in previous work, namely the elongated ridge, the northwestern clump, and the eastern region of the compact ridge, while the high temperatures in the western region of the compact ridge may be dominated by internal massive star formation. Significant gradients of kinetic temperature along molecular filaments traced by H 2 CS indicate that the dense gas is heated by the shocks induced by the enigmatic explosive event which occurred several hundred years ago and greatly affected the energetics of the Orion KL region. Thus, with the notable exception of the western region of the compact ridge, the high temperatures of the dense gas in Orion KL are probably caused by shocks from the explosive event, leading to a dominant component of externally heated dense gas.