Ultra-hot Hydrogen Molecules in Interstellar Space – Part 2

Want to know more detail about the research described in this Astronotes article on ultra-hot hydrogen molecules?

Here are five papers spanning more than four decades of research that helped lead to this discovery of ultra-hot molecular hydrogen:

Velocity profiles in the 2.1 micron molecular hydrogen line emission line in the Orion Molecular Cloud

Nadeau & Geballe, Astrophysical Journal, 1979, v230, L169

https://ui.adsabs.harvard.edu/abs/1979ApJ…230L.169N/abstract

Observations of extremely broad molecular hydrogen lines in the Orion star forming cloud, suggesting that there are energetic flows of material coming from young stars that lead to interstellar shock waves that then heats the gas.

Shocked Molecular Hydrogen in the Supernova Remnant IC 443

Burton, Geballe, Brand & Webster, Monthly Notices of the Royal Astronomical Society, 1988, v231, p617

https://ui.adsabs.harvard.edu/abs/1988MNRAS.231..617B/abstract

Reports the unexpected discovery of a giant expanding shell of shocked molecular gas where a supernova remnant is ploughing into a molecular cloud.  The shell was found by mapping the molecular gas with the UKIRT telescope.  Its spectrum displays the characteristics of gas heated to about 2,000 degrees by the shock interaction.

Ratios of molecular hydrogen line intensities in shocked gas: evidence for cooling zones.

Brand, Moorhouse, Burton, Geballe, Bird & Wade, Astrophysical Journal, 1988, v334, p103

https://ui.adsabs.harvard.edu/abs/1988ApJ…334L.103B/abstract

First hint that there is unexpectedly hotter gas present than just the 2,000 degree component seen in shocks.  This is suggested when stronger than expected emission was found with the UKIRT telescope from highly excited lines in the brightest known molecular hydrogen source, the Orion star forming region. 

Highly excited molecular hydrogen in Herbig Haro 7: formation pumping in shocked molecular gas?

Pike, Geballe, Burton & Chrysostomou, Astrophysical Journal, 2016, v822, #82

https://ui.adsabs.harvard.edu/abs/2016ApJ…822…82P/abstract

Discovery of the ultra-hot molecular gas using the Gemini telescope in Hawaii.  In the spectrum of a jet of gas from a relatively nearby star forming region known as Herbig Haro 7 a plethora of highly excited lines were seen.  They came from molecular gas at a remarkably high temperature of 5,000 degrees that comprise about one percent of gas present in the “standard” 2,000 degree component associated with shocks.

Analysis of the first infrared spectrum of quasi-bound H₂ line emission in Herbig-Haro 7

Roueff, Burton, Geballe & Abgrall, Astronomy and Astrophysics, 2023, v669, #L7

https://ui.adsabs.harvard.edu/abs/2023A%26A…669L…7R/abstract

The latest paper.  Further analysis of the spectrum of Herbig Haro 7 identified two very weak lines coming from energy levels that are above the dissociation limit of the hydrogen molecule.  In classical physics such lines should not exist!  These are the new quasi-bound levels that are unstable.  They provide clear evidence that the molecular gas must be dissociating.  Further, they are part of the same ultra-hot 5,000 degree component seen in our previous work.  Their excitation remains a mystery.