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Imaging photometry of the optical continuum during Deep Impact
Tanyu Bonev1 and Klaus Jockers Max Planck Institute fЭr Aeronomie

· Feasibility · Brightness and color measurements of a disintegrated comet · Simulations of the impact ejecta dust cloud · Conclusions
1

on leave from I nsti tute of Astronomi e, Bulgari an Aca demy of Sc i ence s


Feasibility or
· What we know about the dust coma of comet 9P/Tempel? and · Is the material added by the impact sufficient to be detected?


Comet 9P/Tempel 1 on July 04, 2005 Heliocentric distance = 1.5 AU Geocentric distance = 0.9 AU Total magnitude = 9.5 (expected)

With following simple assumptions · The brightness comes from solar radiation scattered by dust particles · 1 micron particles dominate the dust coma · Albedo в phase function = 0.04 we derive a total number of dust particles: ~2.5 10
21


The crater
The expected crater is gravity dominated with diameter in the range 60 to 2 40 m and with 4:1 diameter to depth ratio (from M. A'Hearn's et al. poster "Deep Impact ... ") How much material is contained in the crater? Crater volume = R3 ( (1/2) s + (1/6) s3 ); s = depth/R

E.g. a crater with a diameter 100 m has a volume ~ 105 m3 For consistency with the dust coma we will consider particles of size 1 micron. The total number of particles is 8 1021. This is about 3 times more than in the dust coma of the comet, BUT


Not all excavated particles will escape the gravity field of the comet Velocity should be larger than the escape velocity of the comet.
With Comet radius = 3000 m, and Assumed density = 1000 kg m- 3: Ve = 2.2 m s
-1

Ejecta scaling laws use the nondimensional velocity v(g R) - 1/ 2 = 7.75. < 3 % of the excavated material will reach velocities larger than the escape velocity.

Conclusion: The contribution of ejected material should be ~10% of the total dust content in the coma. Detection is feasible.

House n e t a l., 1983 , JGR,Vol.88, Eje cta crate r scali ng la ws ...


Comet C/1999 S4 (LINEAR) - Activity before the disintegration

BRIGHTNESS

COLOR

Bonev et al 2002: I carus 160, 419.


Comet C/1999 S4 (LINEAR) - Chronology of the disintegration

BRIGHTNESS

COLOR

Bonev et al 2002: I carus 160, 419.


Comet C/2000 WM1 Dec 3, 2001 Phase angle: 45°

Bright ness

Color


Ejecta cloud - brightness vs. time after the impact amax B



amin(t)

Qsca(a,) P11(, a) n(a) (a) f(a) da,

f(a)

a


Crater ejecta distribution projected on the ecliptic (FOV = 105 km)
hours after impact

3

6

9

12

24

-2.5 max(Intensity) = max(Intensity_Comet)/200000 -3.5

dn

a da


max(Intensity) = max(Intensity_Comet)/4000

0.0

1.0

2.0

3.0

log(Relative Intensity)


Crater ejecta distribution ­ view from Earth (FOV = 105 km)
hours after i mpa ct

3

6

9

12

24

-2.5 max(Intensity) = max(Intensity_Comet)/200000 -3.5

max(Intensity) = max(Intensity_Comet)/4000 ada 0.0 1.0 2.0 3.0

dn

log(Relative Intensity)


Color vs. time after the impact ama B
x



amin(t)

Qsca(a,) P11(, a) n(a) (a) f(a) da,

f(a)

a

Color

1 ­

2

When do particl es of given siz e leave a 200 km diaphragm centered at the nucleus?
(200 km = 0.3 5 arcse c for 9P/Tempe l 1)

! Particle size, micron

Qsc a(a , ) P11(, a)

B1 ­ B

s ec onds a fte r i mpac t 400 13 00 4000
2


Prospect for long term monitoring
The e ffe c ti ve ac ti ve are a requi red to explai n the obse rved gas producti on of 9P/Tempe l 1 i s 5 .2 km2 (A'Hea rn e t a l. 1995 , I c a rus 118, pp. 223 ) The e xpe cted a rea of the cra ter i s about 0.01 km2.

Two pos si ble outcomes of the i mpac t: 1. The crater turns over to a ne w ac ti ve a rea, i .e. a new fresh source of gases and dus t wi ll be adde d on the nucle us surface of the comet. Cons equence s: · · The total bri ghtne ss should i nc rease by a bout 0.002 magni tude s. Beca use thi s i s a loca li zed source i t should a dd a vari able structure to the persi stent dus t c oma. Ana lysi s of thi s s truc ture c an gi ve i mporta nt c lue s for the rotati on state of the nuc leus.

2.

The c rate r wi ll be se aled by a loc a l c rus t of compress ed and/or me lte d mate ri a l. Conse quenc e: The tota l bri ghtnes s s hould dec reas e s li ghtly. Diffi cult to be de te cte d.


Conclusions
· Detection of the ejecta cloud is feasible. · The rate of the brightness decrease will be indicative for the dust particle size distribution. · Color measurements in the first several hundreds of seconds bear information on the particle size distribution (if small particles dominate the ejected material). · Later measurements of the color (if enough S/N) can be conclusive on material characteristics. · High S/N observations of the comet before the impact are as important as the observations during and after the impact.


Imaging ? photometry of the optical continuum during Deep Impact


Comet C/2000 WM1 (LINEAR), Nov 19, 2001
On the next day Earth crossed the orbital plane of the comet

Phase angle: 12°

BRIGHTNESS

COLOR