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Weekly Science Update

Wednesday, May 21, 2008
A Shocking Discovery
The complex sequence of events that lead to star formation, processes once thought to involve just the simple coalescence of material under the influence of gravity, continues to amaze and surprise astronomers. Newly formed stars show evidence, for example, of rotating (possibly preplanetary) circumstellar disks along with dramatic outflows of material in the form of narrow jets that squirt outward in two opposing directions perpendicular to these disks. The formation of planetary disks has obvious importance for scientists trying to unravel the conditions in the solar system when the earth formed. The outflowing jets are also significant because they disrupt the young star's environment, although it still not clear how severely.

SAO astronomers Achim Tappe, Charlie Lada, and August Muench, together with a colleague, used the Spitzer Space Telescope to study the emission at the tip of the jet HH211, the spot marking where the jet collides with the ambient interstellar medium. Atoms and molecules normally shine in these regions because the jets' motions are fast enough - over twenty kilometers per second, or forty thousand miles per hour - to shock them into bright emission. Scientists use the information to diagnose the nature of the outflow, as well as the chemistry in the gas.

The outflow HH211 contained a dramatic surprise: the first detection ever made of very hot OH molecules in the interstellar medium. Analysis of the data shows that the OH molecules are excited to effective temperatures of about 28,200 kelvin, over ten times hotter than the typical temperature of a few thousand kelvin usually seen in such jets. Not only did the astronomers discover hot OH, they also found evidence for water, and for the deuterated molecule, HD. Not least, they discovered that these very hot species are also situated ahead of the shock as well as along its front edge.

Their analysis, which is continuing while these first results are announced, indicates that a shock moving at about forty kilometers per second has generated strong ultraviolet radiation, and that ultraviolet light dissociates water in the gas into hot OH, both locally as well as ahead of the shock. The results provide important new insights into the dynamic processes and chemistry that can occur when new stars form, as well as raising new questions about why this particular outflow source seems to be so exceptional.


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