Icy insight into star birth

On the inside it is as empty as interstellar space

By Dr David Whitehouse BBC News Online science editor

The harsh conditions encountered in interstellar space are being recreated in the lab to reveal new insights into how stars form.

It is an environment where the pressure is only a miniscule fraction of the Earth's atmospheric pressure, and the temperature is a mere 10 degrees above absolute zero.

When a star is trying to form, it gets hotter and hotter as more gas collapses in on itself

Dr Martin McCoustra, University of Nottingham

Dr Martin McCoustra and colleagues at the University of Nottingham, England, simulate the surfaces of the ice-coated dust grains in interstellar space to study the complex chemical and physical processes that take place on them.

"Until recently, processes of this kind have been very poorly understood," said Dr McCoustra, "but now we are seeing a revolution in what we can achieve.

"Ultra-high 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 reappraisal of the interactions between the gas and dust grains that pervade much of the space between the stars."

Cold, dark, diffuse

Stars and planets form in cold, dark clouds of gas and dust that are spread between the stars. Observations have revealed that the clouds consist of a "soup" of over 110 different chemicals, some with molecules made of 10 or more atoms.

Molecular gas clouds form stars

Even though they account for only about 1% of the mass of a typical interstellar cloud, interstellar dust grains play a crucial role in the creation of this rich variety of molecules and in the process of star formation.

About the same size as fine particles of cigarette smoke, the grains are made of silicate minerals and carbon-based materials coated with ice - principally water ice and frozen carbon monoxide.

Dr McCoustra said: "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 can expand out again as fast as it falls in.

"But if some of the heat is radiated away, the collapse can continue and a star can actually form. It is the 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."

The researchers say they need to understand exactly how the ice gets on to the grains and evaporates from them again so that they can simulate the process of star formation with computers.

More complicated

In recent experiments, members of the Nottingham team have studied how water-ice is released from grains and interacts with carbon monoxide (CO).

"We have shown that the crude picture of each substance evaporating separately at some theoretical temperature is wrong," said Dr McCoustra.

"Our measurements indicate that a higher temperature than expected is needed to evaporate water-ice, and the combined water/carbon monoxide interaction is more complicated that had been thought.

"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."

The research is to be described at the National Astronomy Meeting being held in Bristol.