Among the remarkable results obtained, we should mention the detection of water (H2O) and its isotopes in about ten comets and in several galactic sources, the measurement of the H2O content of the Martian atmosphere with exceptional accuracy, the observation of ammonia (NH3) emissions in a very rich carbon-rich star, the spectral survey in Orion and the galactic centre directions, the very low abundance of molecular oxygen (O2) in complete disagreement with theoretical models, and the current observations of spectral structures in the cosmological background.
After four years of operation, Odin had observed the 557-GHz fundamental rotational line of water with high spectral resolution (0.15 kms-¹) in 10 comets. Thanks to its frequency coverage, it also made the first detection of the H218O isotopic species line at 548-GHz in comet 153P/Ikeya-Zhang in April 2002 (Lecacheux et al. 2003, A&A 402, L55). This line was also detected in three new Oort Cloud comets in May 2004 and January 2005 (e.g. Fig. 2). The H216O/H218O ratio found in all comets is similar to that on Earth (499), within error-bars. Due to its optical thickness in cometary comas, the H2O line appears red-shifted as a result of self-absorption in the foreground coma (e.g. Fig. 1). In addition to providing information on line shapes, Odin has been able to make extended maps (at least 3×2.2'-beam wide) of four comets (e.g. Fig. 3). This provides additional constraints on the modelling of water emission and on the importance of collision processes in order to derive accurate water outgassing rates. The ammonia fundamental line at 572.5 GHz was searched for in parallel in three comets observed and tentatively detected in C/2001 Q4 (NEAT).
[From Hjalmarsson et al., Adv. Sp. Res., 2005]
Fig. 3: The result of a 7x7 point grid map at 1' spacing of the 556.9-GHz water line observed in comet C/2001 Q4 (NEAT) on 16 May 2004. Brightness is proportional to the line integrated intensity. The peak intensity is 19 Kkms-¹.
Comets observed by Odin (measurement of water outgassing)
|Comets||Dates of Observations||Rh (AU)||(AU)||Remarks|
|C/2001 A2 (LINEAR)||2001/04/27|
|Deep Space 1 flyby|
|C/2000 WM1 (LINEAR)||2001/12/07|
|153P/2002 C1 (Ikeya-Zhang)||2002/04/22|
Detection of H218O
|C/2002 X5 (Kudo-Fujikawa)||2003/03/03-30||0.99-1.53||0.93-1.54||Monitoring of outgassing|
|29P/Schwassmann-Wachmann 1||2003/06/23-29||5.75||5.31||Upper limit on outgassing rate|
|C/2002 T7 (LINEAR)||2004/01/26,02/01||1.76, 1.67|
|Monitoring of outgassing|
Detection of H218O, search for NH3
|C/2001 Q4 (NEAT)||2004/03/06-04/14|
|Monitoring of outgassing|
Detection of H218O, search for NH3
|C/2003 K4 (LINEAR)||2004/11/27-2005/02/19||1.27-2.23||1.16-2.23||Monitoring of outgassing|
|C/2004 Q2 (Machholz)||2005/01/17-23||1.21||0.39-0.43||Detection of H218O, search for NH3|
rh: heliocentric distance in astronomical units (AU)
: geocentric distance in AU.
Due to solar elongation constraints, Odin could not observe the red planet in August 2003, but did so twice, on 14-18 June and 2-9 November. Thanks to the versatility of its system, it proved possible on both occasions to get a full 4-GHz wide spectrum of the fundamental water line at 557-GHz (Fig.4). The presence of water vapour in Mars' atmosphere has been known for several decades but it had never been observed with both a high spectral resolution (1 MHz) and in a wide band of over 4 GHz allowing us to see the line entirely.
The spectrum is actually the result of five consecutive different tunings each covering 1 GHz. In parallel, other receivers were used to look for O2 at 487-GHz in June (a 0.3% abundance upper limit is inferred) and to measure the line profile of H218O (Fig.5) and CO(5-4) (Fig.6) in November. Although Odin lacks spatial resolution (its beam spans 2.2' while Mars' apparent diameter was around 14''), these results are complementary and in full agreement with the spacecraft observations in Mars orbit. Thanks to the simultaneous observation of strong (H2O at 557 GHz) and weaker (H218O at 548 GHz) water vapour lines with high spectral resolution, it was possible to constrain the average vertical distribution of water vapour in Mars' atmosphere. Surface mixing ratios of 2-3x10-4 for both dates are inferred, corresponding to column densities in the range of 10-15 precipitable mm. The CO(5-4) line also provides a further constraint on the atmosphere's vertical temperature profile.
[From Biver et al., A&A, 2005]
Search for molecular Oxygen in the Universe
Molecular oxygen is the fundamental component of the terrestrial atmosphere needed for life as we know it. Its massive presence in the atmosphere "blinds" us and prevents us from searching for it in the Universe. The best strategy is hence to place a telescope above the atmosphere. This is what we have done with Odin, which is equipped with two receivers sensitive to the presence of molecular oxygen. Searches in numerous different regions of our galaxy have been made, from quiescent cool clouds (in the Taurus and Unicorn complexes) to massive star-forming regions like the large Orion cloud or the Galactic Centre. In spite of very deep searches, representing hundreds of observing hours for each individual direction studied, not even the smallest trace of oxygen has been found. This is surprising, since oxygen is the third most abundant element in the Universe after hydrogen and helium. It has been detected in several forms -carbon monoxide, carbon dioxide (found as a gas in the Earth's atmosphere but observed as solid "dry-ice" in space), water, methanol, ethanol (the alcohol in alcoholic drinks) etc. - but not that which we would expect the most: O2. For the time being, this remains a mystery that we cannot explain. Odin has carried out observations that it will be difficult to better even with the next generation of space telescopes (Herschel). Now theoreticians have to come to terms with this mystery.
Water in a star-forming region
Various physical processes, with antagonistic effects, govern star formation. The dynamical collapse results from the dominant effect of gravity, which leads to the formation of a protostellar core. But the thermal pressure (resulting from the heating produced by gas compression in the collapse), centrifugal force (in the event of rotation) and magnetic forces also disturb the gravitational collapse. Heat is expelled by energy radiation through continuum emission spectrum by dust grains, and through discrete spectrum of molecular lines. According to models, H2O, CO and C are among the key species contributing to most of the gas cooling.
Top graph: IRAS16293 is a binary star system in the process of formation in the complex close to rho-Ophiuchi. Owing to the coupling between rotation and magnetic field, each protostar throws out while it is forming directly opposite strong molecular outflows, associated with the propagation of the shock wave at breathtaking speeds (~ 15 km/s). Bottom graph: The measurement of the spectral lineshape of H2O emission with ODIN allows the complex dynamics of the region to be traced and enables to a better understanding of the physics and chemistry taking place. The spectrum observed with ODIN can be modelled as the result of the superposition of two emission sources, one from the water mixed with gas in the region of collapse of the protostellar disk and one from the water in the outflow. The central dip is caused by self-absorption by gaseous water in the cold cloud surrounding the protostar.
Spectral survey towards Orion
This spectral survey, which explored a large number of molecular transitions for the first time, revealed a variety of molecules present in the star-forming region in Orion.
These measurements have led to a better understanding of the physical conditions of the medium and the wealth of chemical reactions taking place [Hjalmarson et al. 2004, Cospar].
Searching for water in pre-stellar dense and cold clouds
Spectral region corresponding to the ortho-H2O line at 555 GHz looking towards the dark cloud Cha-MMS1. The upper limit of the fractional abundance of water looking towards this object is 1000 times weaker than that observed in star-forming regions, thus confirming the strong under-abundance of gas-phase water in cool dense environments. In these medium water is probably condensed on the interstellar dust grains, forming coats of ice and thus changing the physical and chemical properties of the medium.