How Might We See a Molecule Form in Space?
An infrared image of the star formation region Messier 17. Three infrared wavebands, from 1 to 2.2 microns, have been combined to provide a visual representation of the region as if we had infrared eyesight. The circles show the regions studied with the UNSWIRF camera - the northern and south-western bars of the M17. Original image provided by Michael Merrill from the US National Optical Astronomy Observatories.

Virtually all molecules in space are comprised of the simplest kind, the hydrogen molecule. It is over 10,000 times more abundant than the next most common molecule, carbon monoxide. It might therefore be thought that the study of molecular clouds, the sites for star formation within the Galaxy, would be dominated by various types of measurement of molecular hydrogen line emission. However this is not true. The lowest lying energy levels of the hydrogen molecule are several hundred degrees above the ground state, and therefore are not excited in the cold conditions which are prevalent within molecular clouds.

Hydrogen molecules emit when they are hot, through infrared lines from excited vibrational and rotational states. Thus, when molecular hydrogen is observed in space it points to an energetic event associated with star formation. Typically this indicates either a shock wave or intense ultra-violet radiation. The former occurs when winds or outflows from young stars collide with molecular clouds, heating them to 1-2,000 degrees; the latter when the energetic radiation field from massive stars shines on a molecular cloud, fluorescing the gas.

There is, however, potentially a third emission mechanism to be seen from molecular hydrogen–formation pumping. Hydrogen molecules form on surfaces of dust grains. They release their binding energy on formation, which both ejects them from the dust grain and places them into an excited rotational / vibrational state. From this state they will decay, thus giving rise to a “formation spectrum”. The question is what will this spectrum look like, and could it be observed on top of the strong shocked or fluorescent spectrum which will also arise from the same regions? Predictions for the formation spectrum have had it appearing in low-J, high-v levels, or low-v, high-J levels, amongst other possibilities. Observations have, till now, shed no light on the matter.

We believe that we have now seen the first clear signature of molecular hydrogen formation, through the emission from the v=6–4 O(3) line of the molecule, which occurs at a wavelength of 1.74µm. Using the UNSW Infrared Fabry-Perot (UNSWIRF) on the Anglo Australian Telescope, we used a specially designed filter to pick up several diagnostic emission lines near 1.7µm. We studied the massive star formation region Messier 17 (the Omega Nebula). This is a strong source of fluorescent line emission, arising from the molecular cloud which surrounds a cluster of young stars. Fluorescence is evident in the emission from the v=1 and 2 levels of the hydrogen molecule. However the v=6–4 O(3) line, arising from the v=6, J=1 level of H2, was found to have a quite different spatial distribution to the lower-v lines, a result we attribute to the formation of the molecule in this, or in a nearby, excited state. This is the first clear detection of the emission from the formation of molecular hydrogen in the interstellar medium, a fundamental process which leads to the building of giant molecular clouds, and then to the stars which form in them.

Michael Burton

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