Solving a 40 year cosmic mystery about interstellar gas only to uncover an even deeper one.
Author: Michael Burton, Director of the Armagh Observatory and Planetarium
Hydrogen molecules have been found in clouds of shocked gas in interstellar space that are at temperatures of around 5,000 degrees. This is much higher than it was expected that such molecules could exist at. The discovery closes one mystery about how interstellar shock waves work but opens another one about how the gas can get so hot and yet survive? We explain the story of discovery below.
Interstellar Molecular Clouds
Spread out between the stars of our Galaxy lies the interstellar medium. This contains vast clouds of gas and dust that encompass the immensity of space. The coldest and densest of these clouds are made of molecules. They are dominated by the simplest molecule of all, the hydrogen molecule. This is made up of two hydrogen atoms that are chemically bonded, sharing their electrons in a covalent bond.
Interstellar molecular clouds are huge regions of space that are comprised of largely of such hydrogen molecules. They form some of the most massive structures in our Galaxy, the Giant Molecular Clouds. These weigh in at several hundred thousand times the mass of our local star, the Sun. Stars form within these clouds from the collapse of the densest and coldest pockets of the gas under the weight of their own gravity. The clouds form the crucible in which the evolution of our Galaxy plays out, driven by the formation of stars.
Despite their great abundance the hydrogen molecules in these molecular clouds are very hard to observe directly. Temperatures can be as low as just 10 degrees above absolute zero, that’s about minus 260 degrees Centigrade. Molecules can be seen if they change state, falling from an excited energy level to a less excited one. However, at just 10 degrees above absolute zero, even the lowest excited energy levels of hydrogen molecules are not populated. That means that there are no emission lines produced by these molecules for astronomers to observe.
The presence of the clouds can still be inferred, however, as dust, which is also embedded within them, blocks light from getting out of the clouds. The location of molecular clouds is then marked by dark lanes running across fields of stars in the night sky. In our Galaxy they are prominent as the dark lanes seen along the shiny band light that is the Milky Way.
Shock Waves and Hot Molecular Gas
When stars form within molecular clouds the gas can be heated up so that the hydrogen molecules become visible. This typically occurs when winds or jets generated by the baby stars ram into the molecular gas, shock heating it to temperatures of around 2,000 degrees. When this happens the energy levels of the hydrogen molecules are excited and emission is produced at infrared wavelengths. With infrared telescopes astronomers can then chart the location of interstellar shock waves by mapping out their bright molecular hydrogen lines.
In the 1980’s I was a PhD student at the University of Edinburgh in Scotland. I was being supervised by Peter Brand and Tom Geballe. We were using the UK Infrared Telescope (UKIRT) in Hawaii to probe the mysteries of star formation by measuring the infrared emission from these shocked hydrogen molecules.
We were probing the jets and shells around young stars, trying to understand how they were produced and why? While most of the shocked gas we found seemed to be at around 2,000 degrees, when we looked for lines from higher energy levels of the molecule we were surprised to find emission that was brighter than expected for gas at this temperature. However, the measurements were difficult to make. The lines were still very faint. While we saw hints of very hot gas it wasn’t clear where it was coming from. We had done as much as was possible with the 4-metre diameter telescopes of that era to try and understand what was going on.
Giant Telescopes and Ultra-hot Molecular Gas
Several decades later and a new generation of telescopes had appeared. With mirrors over 8-metres in diameter, such as the Gemini telescope in Hawaii, and equipped with efficient new instruments, they were able to measure far fainter emission than we were able to see with the UKIRT. We were able to take up the challenge of trying to understand interstellar molecular cloud shock waves once again.
We found we were now able measure weak, high excitation lines of the hydrogen molecule that were one thousand times fainter than its brightest line, a remarkably high dynamic range with which to probe the state of the gas. When we did so in a source known as Herbig Haro 7 (or HH7) we were very surprised to detect a plethora of lines arising from extremely highly excited levels of the molecule. They ranged all the way in energy up to the dissociation limit of the molecule, i.e. the energy needed to rip the molecule apart.
We were even more surprised when we realised that this emission was coming from molecules which were at temperatures of around 5,000 degrees. The 2,000 degree gas was still there, but this was an additional component of the gas that was much hotter. We were also able to determine that about one percent of the shocked gas was in this ultra-hot component.
What produces this gas is unclear. Shock waves that are energetic enough to heat the gas to such temperatures should also break up the gas, dissociating the molecules into their constituent hydrogen atoms. We have speculated that we might be seeing emission from newly formed hydrogen molecules, produced following the earlier dissociation of the molecules and then recombining. The energy released in this process could possibly briefly heat the gas to make it this hot? But this is only speculation, we have no firm evidence for this.
However, the mystery deepened still further a few years later. While we had detected many, many new lines of molecular hydrogen using the Gemini telescope there were two extremely faint lines that we were unable identify in our spectrum. We remarked on this in our paper but said no more. Two colleagues from Meudon in France, Evelyne Roueff and Herve Abgrall, then saw our spectrum and thought they knew what the unidentified lines might be.
They had been undertaking complex calculations of the structure of the hydrogen molecule and had predicted the existence of new “quasi-stable” states. These were levels that were so high in energy that they were actually above the molecule’s dissociation level. If a molecule were to be placed in such a state it would break up, separating into two hydrogen atoms. Yet this would not happen instantaneously, the molecule could remain in this quasi-stable state for a short period of time. Might we have seen emission from when it was?
We looked more closely at these two unidentified lines in our Gemini spectrum and confirmed they had the right wavelength for such quasi-stable lines. By carefully measuring their fluxes we found they were consistent with the same 5,000 degree gas we had seen earlier, but now also coming from levels lying above the hydrogen molecule’s dissociation energy.
The Mystery Continues
We had seen “forbidden” emission, arising from a state that under classical conditions should not exist. We have deepened the mystery of what is causing the hydrogen molecules to get so hot and then survive? Is such behaviour an integral element of molecular shock waves or are there special factors at work that have made this possible in the source we studied, Herbig Haro 7. Does the ultra-hot 5,000 degree component have exactly the same distribution there as the hot 2000 degree, or is it coming from particular parts within it? These are all questions for another day, for another telescope.
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Continue here to Part 2 to learn about some of the scientific papers behind this work.