Comet observing program:Water in comets:

water ice ~50% of bulk composition of cometary nucleiwater vapor: sublimation drives cometary activity close to the sun

main constituent (> 80%) of the gaseous comawater 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

Observations of comets with 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 -120ºInstrument 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 H2

18O (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)

Astronomical Observations with the Odin satellite

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.

(Submitted to Icarus)

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

ODIN