FOR RELEASE: Monday, March 22, 10:30 am

Making the smallest molecule on star dust

Gianfranco Vidali
(315) 443 - 9115, gvidali@syr.edu
Syracuse University
Syracuse, N.Y. 13244-1130

How to contact me while at the APS meeting: voice-mail: (315) 443-9115

Popular Version of Paper EC19.01
Monday, March 22, 10:30 am
APS Centennial Meeting, Atlanta

In 1847 the astronomer Struve observed that the space between stars is not empty, but filled with a thin gas of dust particles smaller than the width of a human hair. Interstellar dust has been considered a nuisance, since it obscures regions of the sky, such as those toward the center of our galaxy, the Milky Way. Little did astronomers know that these tiny specks of dust are responsible for the very existence of the universe as we see it now. (The image at right shows dust and gas in a star-forming region of nearby galaxy 283 first discovered by Caroline Herschel in 1783. Credits: Hubble Heritage Team)

It might not seem so for someone living on Earth, but the most abundant element in the universe is hydrogen. This is the stuff stars are made of. The hydrogen atom is the lightest and simplest atom you can think of. Put two hydrogen atoms together and you have the lightest, most abundant, and simplest molecule.

Molecular hydrogen has two main roles in interstellar space. First, it helps the formation of stars by carrying away some of the heat that is produced when gas rushes towards the core of a future star. Second, molecular hydrogen helps form other, more complex molecules.

How is molecular hydrogen formed in interstellar space? Just about thirty years ago, in a series of landmark papers, Edwin Salpeter and his student David Hollenbach of Cornell University proposed a model. It was known that molecular hydrogen doesn't form by a chance encounter of two hydrogen atoms in the nearly empty space between stars, but a surface is required to make a chemical reaction happen. They said: suppose an atom of hydrogen has landed on the surface of an interstellar dust grain (see cartoon at right. Credits: Visual and Performing Arts Team, Syracuse University). Another hydrogen atom might land on this grain before the first atom has a chance to evaporate. As the two atoms move about, there is a chance they will bump into each other and form a chemical bond. The so-formed molecule, due to the heat released in the reaction, would leave the surface of the dust grain.

Until recently, no experiment was done to check this model. We decided to reproduce in the laboratory, as closely as technically possible, the conditions under which molecular hydrogen is formed in interstellar space. We had to come up with a way to send hydrogen atoms onto a solid target at a very low rate and observe how many of the H atoms would react and form molecular hydrogen, while keeping the target at about 10 degrees above absolute zero and avoiding contamination by residual gases. This feat was accomplished by placing the target in a chamber (see image at left) evacuated to a pressure of one trillionth of atmospheric pressure, and by using beams of hydrogen atoms to fine-tune the energy and the number of atoms sent to the target.

It is only known in vague terms what star dust is made of. We used first a piece of a natural stone, olivine, a silicate, and then one of amorphous carbon, both of them considered as models of star dust. We found that the efficiency of recombination on amorphous carbon was considerably lower than expected based on the model mentioned above, but higher than on olivine. This result helps us narrow the possible types of dust we might expect to find in interstellar space.

The real surprise from analysing our experiment is that the mechanism by which hydrogen atoms move about on dust grains is different from what was previously thought to be. This discovery should impact our understanding of some important chemical reactions occurring on grains, since a large number of molecules detected in space are formed on dust grains and most contain hydrogen. In our model, atoms hop from site to site spurned by heat, in a drunkard's walk, and not by "tunneling'' from site to site, as originally proposed. (Tunneling is a quantum mechanical process by which a particle, under favorable conditions, can move from site to site even though it doesn't have enough energy to overcome an energy barrier.) This latter mechanism doesn't depend on the temperature of the sample, contrary to our experimental evidence.

With the use of surface science and low temperature techniques it is now possible to recreate in the laboratory conditions approaching the ones in astrophysical environments. Our experiment on hydrogen recombination on analogues of interstellar grains has shown that we can get new insights about this important reaction. Other experiments are planned to study reactions of hydrogen in other astrophysically relevant environments and using a larger variety of interstellar dust grain analogues.

Importance of molecular hydrogen in interstellar space

Molecular hydrogen is a molecule whose importance in astrochemistry cannot be underestimated. It assists in the formation of most other molecules found in space and in the formation of stars during the gravitational collapse of a molecular cloud.

How is molecular hydrogen formed in space?

Molecular hydrogen cannot form in space when two atoms meet, since it requires a third body to carry away the energy released in the reaction. It was assumed, but not until recently verified, that molecular hydrogen was formed on tiny specks of dust (less than one hundred thousandth of an inch in size) that float in interstellar space. This dust is formed when a star sheds some of its material, such as carbon and oxygen. These atoms then coalesce and form tiny dust grains.

How can we study in the laboratory the way molecular hydrogen forms in interstellar space?

Until recently, no experiments were done to verify that molecular hydrogen is indeed formed on interstellar dust grains or to understand the physical and chemical processes behind molecular hydrogen formation. It's important to try to replicate the conditions under which molecular hydrogen forms in space. To do that we had to:

• Keep the sample in ultra-high vacuum (less than one trillionth of an atmosphere) to avoid contamination from background gas
• Keep the sample at temperatures typically encountered in space (10 to 15 K, or about 260 degrees centigrade below freezing).
• Split hydrogen molecules into atoms and send the atoms onto the sample with a low energy and low flux.

What we discovered and what we learned

These experiments show the great promise laboratory work holds in fostering our understanding of physical and chemical processes that are inacessible to us since they occur in the interstellar medium.

Important references:

• Pirronello, V., Liu, C., Shen, L. and Vidali, G. 1997 Astrophys. J. 475, L69.
• Pirronello, V., Biham, O., Liu, C., Shen, L. and Vidali, G. 1997 Astrophys. J. 483, L131.
• Vidali, G., Pirronello, V., Liu, C. and Shen, L. 1998 Astrophys. Letters and Comm. (review article) 35, 423.
• Biham, O., Furman, I., Katz, N., Pirronello, V., and Vidali, G., 1998, MNRAS 296, 869.
• Pirronello,V., Liu, C., Roser, J.E., and Vidali, G., 1999, A&A 344 (in press).
• Katz, N., Furman, I., Biham, O., Pirronello, V. and Vidali, G., 1999, Ap.J (submitted).