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Herschel observations of gas and dust in comet C/2006 W3 (Christensen) at 5 AU from the Sun HssO Paris meeting, 2014

Herschel observations of gas and dust in comet C/2006 W3 (Christensen)

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Page 1: Herschel observations of gas and dust in comet C/2006 W3 (Christensen)

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

HssO Paris meeting, 2014

Page 2: Herschel observations of gas and dust in comet C/2006 W3 (Christensen)

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

Page 3: Herschel observations of gas and dust in comet C/2006 W3 (Christensen)

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

Page 4: Herschel observations of gas and dust in comet C/2006 W3 (Christensen)

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

Page 5: Herschel observations of gas and dust in comet C/2006 W3 (Christensen)

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

Page 6: Herschel observations of gas and dust in comet C/2006 W3 (Christensen)

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

Page 7: Herschel observations of gas and dust in comet C/2006 W3 (Christensen)

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

Page 8: Herschel observations of gas and dust in comet C/2006 W3 (Christensen)

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

[′′]

Page 9: Herschel observations of gas and dust in comet C/2006 W3 (Christensen)

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

Page 10: Herschel observations of gas and dust in comet C/2006 W3 (Christensen)

Radial profiles

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

Page 11: Herschel observations of gas and dust in comet C/2006 W3 (Christensen)

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.

Page 12: Herschel observations of gas and dust in comet C/2006 W3 (Christensen)

References

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