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Comet observing program : Water in comets: water ice ~50% of bulk composition of cometary nuclei

Observations of comets with Odin satellite. Comet observing program : Water in comets: water ice ~50% of bulk composition of cometary nuclei water vapor: sublimation drives cometary activity close to the sun main constituent (> 80%) of the gaseous coma

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Comet observing program : Water in comets: water ice ~50% of bulk composition of cometary nuclei

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  1. Observations of comets with Odin satellite Comet observing program: Water in comets: water ice ~50% of bulk composition of cometary nuclei water vapor: sublimation drives cometary activity close to the sun main constituent (> 80%) of the gaseous coma water coma: expansion velocity ~ 0.8 km/s adiabatic cooling => cold: ~50 to 100 K => Strong (optically thick), narrow sub-mm lines, not observable from the ground H2O(110-101) line at 556.9 GHz observed from space (main ground vibrational state, fundamental rotational line of water) - first detection: in comet C/1999 H1 (Lee) with SWAS satellite - Spectrally resolved for the first time: comet C/2001 A2 (LINEAR) with Odin (first astronomical target detected by Odin on April 27, 2001) - 2’ spatial resolution maps obtained with Odin on comets since June 20, 2001 - Odin sensitivity: S/N > 7 in one orbit (1.5h) for QH2O(rh==1 AU)=1028 mol./s

  2. Astronomical Observations with the Odin satellite Telescope characteristics: Diameter: 1.1 m, off-axis secondary: HPBW = 127” at 557GHz, Beam_eff. =85% Sky subtraction: - “Dicke switching”: 2 reference beam offset by 44º - position switching (but loss of efficiency) - frequency switching at 119 GHz “Comet tracking” mode for moving targets Solar elongation constraint: 60º -120º Instrument characteristics Heterodyne Receivers: 5 (up-to) receivers running in parallel: 118.75 GHz (O2) (frequency splitter) (Tsys  600 K) 495, 549, 555, 572 GHz, tunable to ±8GHz (Achieved Tsys  3200 K) (polarization splitter and Source/Reference beam alternating) objectives: (CS, CI, O2; 13CO, H216O and H218O(2 Rx); CO, NH3) Spectrometers: 3 backends: Acousto-optical Spectrometer: 1 GHz bandwidth, 1728 Channels Resolution: 1.2 MHz 2 Auto-Correlators: 800-100 MHz bandwidth Resolution: 1.2MHz to 150kHz ( maximum spectral resolution = 80m/s at 557GHz)

  3. Modelling H2O emission in comets • Model exist (Bockelée-Morvan,1987; Chin et al., 2001) • Radiative excitation of vibrational bands by solar radiation • Collisional excitation • Coma in spherical expansion (~ 0.8 km/s) : line shapes. • Coma temperature (~ 60 K) estimated from molecular rotational lines (e.g. methanol) • Line shapes and intensities deduced by numerical integration using a radiative transfer code. • Model recently updated to consider collisional excitation by electrons (Biver, 1997) • Electron density scaled from 1/P Halley.

  4. (Submitted to Icarus)

  5. Activity of 19/P Borrelly monitored from OH (Nançay,1994&2001) and H2O (Odin,2001) observations ODIN

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