Figure 1 . Spectrum of a bright region near the apex of the HH 7 bowshock
( as published in Pike et al ., ApJ , 822 : 82 , 2016 ). All lines are due to molecular hydrogen and are labeled according by their vibrational and rotational transitions .
The blue trace is the same as the black trace , but is multiplied by a factor of 150 and offset vertically to show the weaker lines , which had not been detected previously in any astronomical object . two temperatures : 1,800 K and 5,000 K . Approximately 98.5 % of the H 2 is at the lower temperature , which corresponds closely to the temperature expected for a C-shock . The higher temperature component , which is only 1.5 % of the hot H 2
, accounts for virtually all of the emission by the most highly excited H 2
. The origin of the 5,000 K component is of intense interest . It seems most likely to be due to H 2 that has reformed on dust grains following destruction by the shock wave .
The formation of H 2 by the collision of two H atoms in the gas phase is an extremely unlikely process . However , hydrogen atoms will stick to a dust particle and can easily hop around on it , find each other , and make H 2
. Their association produces a lot of energy , some of which ejects the newly formed H 2 molecule from the dust particle and some of which leaves the molecule in a highly excited state , from which it can emit spectral lines as it cools . Qualitatively this explains
the observations , but many questions remain , especially regarding how well the relative line strengths match predictions of the “ formation spectrum .”
A Fundamental Question … and the Answer
A basic question about this discovery was whether the high temperature H 2 is unique to HH 7 or is found in other clouds that have been subjected to high velocity shocks . To begin to answer this question , Burton , Pike , and I observed the shocked H 2 in the location where it was initially discovered in 1976 , and where it is brighter than anywhere else : the Orion Molecular Cloud ( OMC-1 ). Using as a guide the exquisite images obtained by John Bally ( University of Colorado ) and collaborators with the multi-conjugate adaptive optics System at Gemini South , we positioned the long slit of the Gemini Near- Infrared Spectrograph ( GNIRS ) on Gemini
10 GeminiFocus April 2017