Understanding molecular hydrogen formation in the universe:from atoms to molecules on interstellar ice

Molecular hydrogen (H2) – the most basic and abundant element in the universe – plays an important role in star formation and the evolution of materials, but its formation process has remained a mystery.

A research team led by Prof. Naoki Watanabe of the Institute of Low Temperature Science developed an experimental devise utilizing an ultra-high vacuum where an interstellar ice analogue (amorphous solid water, ASW) is produced under the low-temperature dark cloud environment. Then they irradiated trace amounts of hydrogen atoms and using a resonance-enhanced multiphoton ionization method, monitored for the first time the process starting with hydrogen atom adsorption until the formation of molecular hydrogen on ASW. As a result, it was shown that production efficiency at extreme low-temperature conditions such as 10K (-263 ºC) is still relatively high and can explain the observed H2 abandance in a dark cloud.

Interstellar ice dust surfaces host not only hydrogen molecule formation but also organic and many other molecule types through their reactions with hydrogen atoms. Data obtained in this research enable a more accurate estimation of the formation efficiency of these molecules. It is also expected that combining this data with past research data at the Institute of Low Temperature Science will make it possible to depict molecular evolution in the universe more clearly.

The results of this study were reported in The Astrophysical Journal , Letters on April 15, 2010. http://iopscience.iop.org/2041-8205/714/2/L233/

Process of molecular hydrogen formation on amorphous solid water (ASW)

Contact information:
Prof. Naoki Watanabe
Institute of Low Temperature Science, Hokkaido University
E-mail: watanabe@lowtem.hokudai.ac.jp