PASADENA, Calif.-A rare type of ammonia that includes three atoms of
deuterium has been found in a molecular cloud about 1,000 light-years
from Earth. The comparative ease of detecting the molecules means
there are more of them than previously thought.
In a study appearing in the May 20 issue of the Astrophysical Journal
Letters, an international team of astronomers reports on the contents
of a molecular cloud in the direction of the constellation Perseus.
The observations were done with the Caltech Submillimeter Observatory
atop Mauna Kea in Hawaii.
The molecule in question is called “triply deuterated ammonia,”
meaning that each molecule is composed of a nitrogen atom and three
deuterium atoms (heavy hydrogen), rather than the usual single
nitrogen atom and three hydrogen atoms found in the typical bottle of
household ammonia. While not unknown on Earth, the molecules, until
recently, were thought by experts to be quite rare-so rare, in fact,
that the substance was considered too sparse to even be detectable
from Earth.
But now that scientists have detected triply deuterated ammonia in
the interstellar medium, they’re still wondering why they were able
to do so at all, says Tom Phillips, a physics professor at the
California Institute of Technology, director of the Caltech
Submillimeter Observatory, and leader of the Caltech team. No other
molecules containing three deuterium atoms have ever been detected in
interstellar space.
“From simple statistics alone, the chances for all three hydrogen
atoms in an ammonia molecule to be replaced by the very rare
deuterium atoms are one in a million billion,” Phillips explains.
“This is like buying a $1 state lottery ticket two weeks in a row and
winning a $30 million jackpot both weeks. Astronomical odds indeed!”
As for the reasons the molecules would exist in the first place, says
Dariusz Lis, a senior research associate in physics at Caltech and
lead author of the paper, the frigid conditions of the dense
interstellar medium allow the deuterium replacement of the hydrogen
atoms to take place. At higher temperatures, there would be a
back-and-forth exchange of the deuterium atoms between the ammonia
molecules and the hydrogen molecules also present in the interstellar
medium. But at the frosty 10-to-20 degrees above absolute zero that
prevails in the clouds, the deuterium atoms prefer to settle into the
ammonia molecules and stay there.
The study is important because it furthers the understanding of the
chemistry of the cold, dense interstellar medium and the way
molecules transfer from grains of dust to the gas phase, Phillips
explains. The researchers think the triply deuterated ammonia was
probably kicked off the dust grains by the energy of a young star
forming nearby, thus returning to the gas state, where it could be
detected by the Caltech Submillimeter Observatory.
The study was made possible because of the special capabilities of
the Caltech Submillimeter Observatory, a 10.4-meter telescope
constructed and operated by Caltech with funding from the National
Science Foundation. The telescope is fitted with the world’s most
sensitive submillimeter detectors, making it ideal for seeking out
the diffused gases and molecules crucial to understanding star
formation.
In addition to the Caltech observers, the team also included
international members from France led by Evelyne Roueff and Maryvonne
Gerin from the Observatoire de Paris, funded by the French CNRS, and
astronomers from the Max-Planck-Institut fuer Radioastronomie in
Germany.
The main Web site for the Caltech Submillimeter Observatory is at
http://www.submm.caltech.edu/cso.
