When even moderately hot stars such as our Sun have surface temperatures of around 6,000°C, it's hard to imagine that ice plays an important part in their formation.
But that's exactly what astrophysicists have recently discovered by turning to surface scientists for help.
At the Condensed Matter conference today, part of the Institute of Physics Congress in Brighton, UK, Dr. Mark Collings, a member of Dr. Martin McCoustra's research team from the University of Nottingham, presented the results of surface science experiments that mimic the conditions in the interstellar medium where new stars are born.
The interstellar medium is the region of space between the stars in a galaxy. It contains various different gases, molecules, and particles, as well as clouds of dust. Astrophysicists know that the interactions between the gas and dust are crucially important to the birth of stars. However, the details of these interactions are not well understood.
As a consequence, theories of star formation do not match up with telescope observations. In an effort to resolve this situation, Dr. McCoustra's team has carried out a range of experiments that look at the physical chemistry occurring on the surface of individual grains of dust.
Ice forms on the dust because the temperatures in the interstellar medium are very low (around -260°C). The ice is made mainly from water, but it also contains other molecules that are formed by chemical reactions on the dust and condense out of the clouds, including methanol, carbon monoxide, ammonia and carbon dioxide.
The clouds containing these ice-covered dust grains warm up as they collapse in on themselves under gravity to form stars, so the water, carbon monoxide and other molecules become gas again as the ice evaporates. If these molecules are not released for some reason, the cloud can not collapse enough to form a star, "so it becomes very important to know when molecules will re-appear in the gas phase," explains Dr. McCoustra.
To discover this, McCoustra's team has been using two standard surface science techniques to measure how strongly molecules stick to surfaces under these conditions.
Temperature programmed desorption (TPD) involves placing molecules onto an ice surface, heating the surface slowly, then using an instrument known as a mass spectrometer to detect when the molecules are desorbed (released) from the ice.
With reflection-absorption infrared spectroscopy (RAIRS), infrared light is shone at molecules that have been deposited onto a reflective surface. Analyzing the reflected light signal then reveals what arrangement the molecules have on the surface.
Both techniques are carried out in an ultra-high vacuum chamber that has been cooled to a very low temperature in an attempt to replicate the environment in space.
When the researchers looked at how strongly water molecules were bound to each other, they found that the molecules were released at higher temperatures and different rates from those previously believed.
It had also been assumed that carbon monoxide formed a separate layer of ice on top of the water ice on the dust surface, like the layers of an onion. If so, gas molecules of carbon monoxide would re-appear first when the temperature rose.
"What our experiments show us is that this is clearly not the case," states McCoustra. "When you grow water films at very, very low temperatures they act like a sponge, and carbon monoxide deposited on top of these films can flow into the cavities when warmed very slightly.
"When you warm it up further, some of these cavities close off, trapping carbon monoxide inside the water ice. That carbon monoxide doesn't then re-appear until all the water leaves the surface because it is trapped within it," he explains.
Dr. McCoustra's collaborators at University College London are now starting to incorporate these results into their models of star formation.
These discoveries should also further our understanding of the processes occurring in other astronomical environments, including the clouds of gas and dust surrounding comets. - By Sharon Ann Holgate
Materials Congress 2002
[Contact: Dr. Martin R. S. McCoustra, Sharon Ann Holgate]