Bringing space down to Earth to explain how stars form
In a laboratory in Nottingham, scientists are now creating the uniquely harsh conditions encountered in interstellar space. In an environment where the pressure is only one ten billion billionth (one part in 10 to the power 13) of atmospheric pressure, and the temperature a mere 10 degrees above absolute zero, Dr Martin McCoustra and his colleagues are able to mimic the surfaces of the ice-coated dust grains in interstellar clouds and to study the complex chemical and physical processes that take place there. On Tuesday 9 April Dr McCoustra will tell the National Astronomy Meeting in Bristol why his team`s novel experiments are having far-reaching consequences for understanding the way stars and planets form.
Stars and their planetary systems form in cold, dark clouds of gas and dust that occupy the vast regions between the stars in our own and other galaxies. Radio, millimetre-wave and infrared observations have revealed that they are a `soup` of over 110 different chemicals, some with molecules made of 10 or more atoms. It is clear that complex chemistry is taking place and the key to the chemical origins of life itself might lie in these clouds. For certain, the chemistry controls the way regions in interstellar clouds collapse under their own weight to create the precursors of stars.
Interstellar dust grains play a crucial role in the creation of the rich variety of interstellar molecules and in the process of star formation, even though they account for only 1% of the mass of a typical cloud. About the same size as a fine particles of cigarette smoke, grains are made of silicate minerals and carbon-based materials coated with ice – principally water ice and frozen carbon monoxide.
But what`s the link between the grains and the star formation process? Dr McCoustra explains, “When a star is trying to form, it gets hotter and hotter as more gas collapses in on itself. There comes a point when the gas is so hot it could expand out again as fast as it falls in. But if some of the heat is radiated away, collapse can continue and a star can actually form. It`s hot gas molecules that act as the radiators and the icy mantles of the interstellar grains are the main reservoirs where the gas molecules can come from. We need to understand exactly how the ice gets onto the grains and evaporates from it again so that astrophysicists can accurately simulate the process of star formation with computer models.”
In recent experiments, the Nottingham team have studied how, under interstellar conditions, water ice is released from grains, and the interaction between carbon monoxide and the surface of frozen water. “We have shown that the crude picture of each substance evaporating separately at some theoretical temperature is wrong,” says Dr McCoustra. “Our measurements indicate that a higher temperature than expected is needed to evaporate water ice, and the combined water/carbon monoxide (CO) system is much more complex. The water ice acts like a sponge, trapping the CO in pores. This trapped CO is not released as a gas until all the water has evaporated.”
“Until recently, processes of this kind have been very poorly understood,” adds Dr McCoustra, “but now we are seeing a revolution in what we can achieve. Ultrahigh vacuum and surface science techniques in the laboratory have given us the tools we need to probe the workings of an interstellar cloud. Our unique surface astrophysics experiment is contributing to a fundamental appreciation of the interactions between the gas and dust grains that pervade much of the space between the stars.”
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