Herschel observations of gas and dust in comet C/2006 W3 (Christensen)
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Transcript of Herschel observations of gas and dust in comet C/2006 W3 (Christensen)
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Herschel observations of gas and dust in cometC/2006 W3 (Christensen) at 5 AU from the Sun
HssO Paris meeting, 2014
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C/2006 W3 (Christensen) observations
I Searched for emission in the H2O and NH3 ground-staterotational transitions, JKaKc (110–101) at 557 GHz andJK (10–00) at 572 GHz simultaneously with HIFI
I Photometric observations of the dust coma in the 70 µm and160 µm channels were acquired with PACS
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Herschel C/2006 W3 (Christensen) observations
I long-period comet that was discovered in November 2006 at adistance of 8.6 AU from the Sun.
I It passed perihelion on 6 July 2009 at a heliocentric distanceof 3.13 AU.
Date OD Inst. ObsID Exp. Angle Scan size Speed rh ∆ φ(yyyy-mm-dd.ddd) (min) (◦) (′ × ′) (”/s) (AU) (AU) (◦)
2009-11-01.836 171 PACS 1 342 186 621h 9.4 135 9.9 × 7.4 10 3.33 3.56 16.31
2009-11-01.843 171 PACS 1 342 186 622h 9.4 45 9.9 × 7.4 10 3.33 3.56 16.312010-08-26.532 469 PACS 1 342 203 478 20.6 45 3.0 × 1.7 20 4.96 4.54 11.222010-08-26.547 469 PACS 1 342 203 479 20.6 135 3.0 × 1.7 20 4.96 4.54 11.222010-09-01.529 475 HIFI 1 342 204 014 48.0 5.00 4.68 11.45
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HIFI observations
556.92556.93556.94556.95
νLSB [GHz]
−10 −5 0 5 10
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v [km s−1]
Tm
B[m
K]
556.92556.93556.94556.95
νLSB [GHz]
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−40
−20
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v [km s−1]T
mB
[mK
]
A tentative detection of the ortho-H2O ground-state transition isobserved in the HIFI spectra with 4-σ significance
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Production rates
Molecule Spec. σTmB
∫TmB dv ∆v Q
(mK) (mK km s−1) (m s−1) (molec. s−1)
H2OWBS 1.5 11.4 ± 3.5 −71 ± 40 1.7 ± 0.5 × 1027
HRS 6.3 16.2 ± 4.6 −77 ± 67 2.3 ± 0.7 × 1027
NH3WBS 1.5 <13 <1.5 × 1027
HRS 6.3 <17 <1.9 × 1027
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Outgassing evolution
3 3.5 4 4.5 5 5.51027
1027.5
1028
1028.5
rh [AU]
Q[m
olec
.s−
1 ]
Water production rate measured by HIFI and the production rateat the subsolar point on the nucleus shown by the solid curve agree
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H2O and NH3 production rates
I The H2O line is only marginally detected
I derived H2O production rate and line shape and velocity shiftare consistent with the emission feature having a cometaryorigin
I We derive a water production rate of(2.0 ± 0.5) × 1027 molec. s−1 using a spherically symmetricradiative transfer model
I we aimed to detect the ground-state rotational transitions ofortho-NH3 in the USB of HIFI’s band 1b
I a 3-σ upper limit for the ammonia production rate of< 1.5 × 1027 molec. s−1 is derived, corresponding to a mixingratio QNH3
/QH2O< 0.75
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Dust emission
�
−20020
−20
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20
∆α2000 [′′]∆δ 2
000
[′′]
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−20020
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0
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∆α2000 [′′]
∆δ 2
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[′′]
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−20020
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[′′]
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−20020
−20
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∆α2000 [′′]
∆δ 2
000
[′′]
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Dust production rates
I To determine the dust production rate Qdust, we comparedthe flux densities measured on the brightest pixels of the blueand red PACS maps with those expected from a model of dustthermal emission
I The model used for this study is the same as that applied tothe PACS data obtained in 2009 at 3.35 AU from the Sun(Bockelee-Morvan et al. 2010)
I Absorption cross-sections calculated with the Mie theory wereused to compute the temperature of the grains solving theequation of radiative equilibrium
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Radial profiles
We fit the radial dependence of the surface brightness with radiallysymmetric profiles for the blue and red bands.
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Results
I The blueshift of the water line detected by HIFI suggestspreferential emission from the subsolar point.
I Also possible that water sublimation occurs in smallice-bearing grains that are emitted from an active region onthe nucleus surface at a speed of ∼ 0.2 km s−1.
I Dust production rates derived in August 2010 are roughly oneorder of magnitude lower than in September 2009
I Dust-to-gas production rate ratio remained approximatelyconstant during the period when the activity becameincreasingly dominated by CO outgassing.
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References
Bockelee-Morvan, D., Hartogh, P., Crovisier, J., et al. 2010, A&A,518, L149