Molecular oxygen has long been considered as being potentially a substantial and abundant component of molecular clouds. Nonetheless, research carried out over the last twenty years had failed to detect it. Astronomers have just detected the fundamental line of O2 at millimeter wavelengths using the Odin satellite. The abundance of O2 inferred is a thousand times less than earlier expectations.
Oxygen is the most abundant element in the Universe after hydrogen and helium. It is twice as abundant as carbon. It would therefore be expected that species such as O (atomic oxygen), OH (the hydroxyl radical), H2O (water) and O2 (diatomic oxygen or molecular oxygen)should be abundant in interstellar gas. Although the first three species have been detected in the interstellar medium, the O2 molecule had never been seen until now.
Since our atmosphere is full of molecular oxygen, ground-based telescopes are totally blind in the range of frequencies corresponding to O2 lines. One effective way of detecting them is to use a satellite, and this is one of the aims of the ODIN satellite built by Sweden, France, Finland and Canada. Odin, which carries a 1.1 meter sub-millimeter telescope, is equipped in particular with a receiver dedicated solely to looking for molecular oxygen at its fundamental emission line of 119 GHz (in principle the most intense line in environments at temperatures of less than 100 K). Odin has at last managed to detect this line in the direction of a large nearby molecular cloud, Rho Oph A,in the constellation of the Serpent.
This first detection seems to show that the abundance of O2 is 1000 times less than expected, but this is only an initial estimate. Since the angular resolution of the satellite is fairly low (around ten times less than that of a human eye), it is not exactly known exactly whereabouts the emission source is in the direction being targeted. Observations at higher resolution will be carried out by the next satellite, Herschel, which should be launched in 2008.
The international team includes researchers from the Laboratory for the Study of Radiation and Matter in Astrophysics (Laboratoire d'Etude du Rayonnement et de la Matière en Astrophysique, (CNRS)), from the Space and Astrophysics Instrumentation Research Laboratory (Laboratoire d'Etudes Spatiales et d'Instrumentation en Astrophysique, (CNRS)) at the Observatoire de Paris and from the Space Radiation Research Center (Centre d'Etude Spatiale des Rayonnements, (CNRS))
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