| 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 H2O content of the martian atmosphere with an exceptional accuracy, the observation of ammonia (NH3) emission in a very rich carbon star, the spectral survey in Orion and the galactic center directions, the very low abundance of molecular oxygen (O2) in complete disagreement with the theoretical models, and the current observations of spectral structures in the cosmological background. |
| Comet observations with Odin |
Fig.1: The 556.9~GHz fundamental rotational water line observed in comet C/2001 Q4 (NEAT) with ODIN and its Accousto Optical Spectrometer. |
After 4 years of operation, Odin has observed the 557 GHz fundamental rotational line of water with high spectral resolution (0.15 kms-¹) in 10 comets to date.
Thanks to its frequency coverage, it has 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 has been recently detected also 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 the Earth (499) within error-bars. Due to its optical thickness in cometary coma, 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 4 comets (e.g. Fig. 3). This provides additional constraints on the modeling of water emission and on the importance of collision processes in order to derive accurate water outgassing rates. 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. 2: The 547.7 GHz H218O line observed in parallel with ODIN and one of its autocorrelator. |
 |
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
2001/06/20-07/09 |
0.94
1.05 |
0.83
0.26 |
First observation
Maps |
| 19P/Borrelly |
2001/09/22-24
2001/11/05 |
1.36
1.48 |
1.47
1.34 |
Deep Space 1 flyby
|
| C/2000 WM1 (LINEAR) |
2001/12/07
2002/03/12 |
1.13
1.16 |
0.33
1.24 |
Map
|
| 153P/2002 C1 (Ikeya-Zhang) |
2002/04/22
2002/04/24-28 |
0.92
1.00 |
0.42
0.41 |
Map
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 |
| 2P/Encke |
2003/11/16,23 |
1.03, 0.91 |
0.26 |
|
| C/2002 T7 (LINEAR) |
2004/01/26,02/01
|
1.76, 1.67
0.94 |
1.86, 1.91
0.33-0.49 |
Monitoring of outgassing
Detection of H218O, search for NH3 |
| C/2001 Q4 (NEAT) |
2004/03/06-04/14
2004/04/26-05/02
2004/04/15-16 |
1.52-1.11
1.01
0.96 |
1.73-0.79
0.45-0.34
0.44 |
Monitoring of outgassing
Detection of H218O, search for NH3
Map |
| 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.
|
| Observations of Mars in 2003 with Odin |
Fig. 4: H2O in november 2003 in Mars' atmosphere observed by Odin and its AOS |
Due to solar elongation constraints, Odin could not observe the red planet in August 2003, but did it twice, on June 14-18 and November 2-9. Thanks to the versatility of its system it has been each time possible 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 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 measures 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 has been 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 10-15 precipitable mm. The CO(5-4) line also provide further constraint on the atmosphere vertical temperature profile.
[From Biver et al., A&A, 2005]
|
Fig. 5: H218O in november in Mars' atmosphere observed by Odin with its autocorrelator 1 |
Fig. 6: CO in november in Mars' atmosphere observed by Odin with its autocorrelator 2 |
| Observations of the interstellar medium with Odin |
 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 formation 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. 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 it's 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 antagonist 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 case of rotation) and magnetic forces contribute also to disturb the gravitationnal collapse. Heating is evacuated by energy radiation through continuum emission spectrum by dust grains, and through discrete spectrum of molecular lines. According to the models, H2O (and also CO, C) are among the key species contributing to most of the gas cooling.
Upper 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 forming, directly opposite strong molecular outflows, associated with the propagation of the shock wave at breathtaking speeds (~ 15 km/s). Lower graph: The measurement of the spectral lineshape of H2O emission with ODIN allows the complex dynamics of the region to be traced and leads 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 2 emission sources: one coming from the water mixed with gas in the region of collapse of the protostellar disk and one coming 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, exploring for the 1st time a large number of molecular transitions, has shown up 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].
|
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Spectral region corresponding to the ortho-H20 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 media water is probably condensed on the interstellar dust grains, forming coats of ice and thus changing the physical and chemical properties of the medium.
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SCIENTIFIC RESULTS
AERONOMY DATA
