Herschel observations of gas and dust in cometC/2006 W3 (Christensen) at 5 AU from the Sun

HssO Paris meeting, 2014

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

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

HIFI observations

556.92556.93556.94556.95

νLSB [GHz]

−10 −5 0 5 10

−5

0

5

10

15

v [km s−1]

Tm

B[m

K]

556.92556.93556.94556.95

νLSB [GHz]

−10 −5 0 5 10

−40

−20

0

20

40

60

80

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

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

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

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

Dust emission

�

−20020

−20

0

20

∆α2000 [′′]∆δ 2

000

[′′]

�

−20020

−20

0

20

∆α2000 [′′]

∆δ 2

000

[′′]

�

−20020

−20

0

20

∆α2000 [′′]

∆δ 2

000

[′′]

�

−20020

−20

0

20

∆α2000 [′′]

∆δ 2

000

[′′]

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

Radial profiles

We fit the radial dependence of the surface brightness with radiallysymmetric profiles for the blue and red bands.

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.

References

Bockelee-Morvan, D., Hartogh, P., Crovisier, J., et al. 2010, A&A,518, L149