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.
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.
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
hydrogenformation 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=64 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=64 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.