To their great surprise, astronomers of the Max-Planck-Institute for
Radio Astronomy in Bonn and the California Institute of Technology in
Los Angeles have discovered a new molecule in space: ND3. This is a
special form of ammonia, NH3, where all three hydrogen atoms are
replaced by the heavier and rare isotope deuterium (similar as in heavy
water — D2O). Such molecules which contain deuterium occur mostly in
very cold clouds of gas and dust in our Milky Way. In such clouds,
molecules with one or two deuterium atoms had been found before. The
new molecule, however, is the first one with three deuterium atoms.
The results are published in a letter to Astronomy & Astrophysics
(Vol 388(3), L53: June IV, 2002).
Floris van der Tak and Peter Schilke made their observation at the
Caltech Submillimeter Observatory, a telescope on the summit of Mauna
Kea in Hawaii. This telescope operates at ultrahigh radio frequencies,
where atmospheric water vapour strongly affects the measurements. At
this site at 4.5 km above sea level, the air is exceptionally dry,
making observations at frequencies up to 900 000 MHz (or only 0.33 mm
wavelength) possible.
Atoms and molecules emit light at very specific frequencies, so-called
spectral lines. By analyzing at which frequencies a cloud emits
radiation, astronomers can determine the different molecules in the
cloud. With the v¶P«Wsensitive receivers of the Caltech Submillimeter
Observatory, a spectral line exactly at 309909.4 MHz was found. "This
frequency exactly matches measurements of ND3 in laboratories on Earth.
No other molecule emits at this frequency, and the detailed frequency
profile of the line matches theoretical expectations, so that the
identification is secure", says Floris van der Tak.
The clouds, where the new molecule were detected, are located in the
constellations of Perseus and Ophiuchus, at distances of 500 to 1000
light-years from Earth. Far away from any stars that would warm them
up, these clouds have temperatures of approximately ten degrees above
absolute zero (corresponding to -260 degrees centigrade). The chemical
reactions that lead to the substitution of hydrogen with deuterium
atoms are most efficient at low temperatures. There are two kinds of
such reactions: gas-phase reactions (like the burning of a flame),
and reactions on the surfaces of solid particles (as occur in the
catalysts in modern cars). Astronomers want to understand which kind
of reaction produces molecules in interstellar space. The discovery
of ND3 provides an opportunity to decide between the two routes, as
gas-phase reactions typically replace more hydrogen with deuterium
than surface reactions do.
Van der Tak and Schilke have also observed ordinary ammonia (NH3) in
the same clouds, with the Effelsberg 100-m telescope in Germany. NH3
emits radiation at 24,000 MHz. At this frequency the atmosphere is
transparent, allowing observations from sea level. The ratio of NH3
to ND3 varies in the observed clouds from 1600 to 3400. These values
are by far too low for surface reactions, and suggest that
interstellar ammonia is formed by gas-phase reactions.
"Deuterium enhancement in the interstellar medium has to be very
efficient to produce as much ND3 as we have found", says Peter
Schilke. It is amazing that this molecule has been detected at all
since no-one had previously assumed that three-times deuterated
ammonia could be detected in space.
Besides Floris van der Tak and Peter Schilke the team includes: Darek
Lis and Tom Phillips from the California Institute of Technology,
Holger Müller from the University of Cologne, and Maryvonne Gerin
and Evelyne Roueff from the Observatoire de Paris. The research was
sponsored by the National Science Foundation and by the Deutsche
Forschungsgemeinschaft.
IMAGE CAPTION:
[http://www.mpg.de/news02/news0214_bild1.jpg (690KB)]
The molecular cloud NGC 1333 in the constellation of Perseus. The
photograph in visible light was taken with the 0.9m telescope at Kitt
Peak National Observatory (copyright: John Bally). The position where
ND3 was found is south of the center of the image. Photo: John Bally
Contact:
Dr. Floris van der Tak
Max-Planck-Institute for Radio Astronomy
Telephone: +49-228-525-273
Fax: +49-228-525-229
e-mail: vdtak@mpifr-bonn.mpg.de
Dr. Peter Schilke
Telephone: +49-228-525-392
e-mail: pschilke@mpifr-bonn.mpg.de
Dr. Norbert Junkes
Fax: +49-2257-301-105
e-mail: njunkes@mpifr-bonn.mpg.de
