Dynamical modeling of the DI dust ejecta cloud

Tanyu Bonev

(Institute of Astronomy and National Astronomical Observatory, Bulgaria)

and the ESO DI observing team

Publication: Deep Impact as a World Observatory Event: Synergies in Space, Time, and Wavelength, ESO Astrophysics Symposia. ISBN 978-3-540-76958-3. Springer Berlin Heidelberg, 2009, p. 177

The ESO DI observing team

Outline

1. Images of the ejecta cloud.2. Initial guess for the velocity size dependence.3. The location of the impact site.4. Monte Carlo model.5. Derivation of the particles size distribution and of the total mass in the ejecta plume.6. Comparison with photometry from the first hours.7. Conclusions

4.972 (+17.8) 5.995 (+42.4)

6.993 (+66.3) 7.955 (+89.4)

Post-impact minus

pre-impact images

FORS2@VLT R-band

Scale: 0.25 arcsec/pxcorresponds to:

162 km/px – 05.07.

167 km/px – 08.07.

Velocities and accelerations: initial guess

v = a tv = √(2 a d)

ס

• d: apex distance

• a: radiation pressure acceleration

•v: initial velocity

Constraints on the impact site location

Position angle of theejecta plume motion.

Cometary equator.Rotation axis orientation:R.A. = 293.8 degDEC = 72.6 deg

latitude of impact (cometocentriccoordinates)

M. A’Hearn, priv. communication(Thomas et al., Icarus)

Selection of the “best” impact site location

The model

1 million dust particles are emitted for a period of 20 minutes starting at the moment of the impact. These particles are distributed in time, space and particle sizes as follows:

• 100 emission events • 200 emission directions randomly distributed in a cone with full opening angle of 180 degree.• particles of radii in the range from 0.1 to 100 micrometer are used, distributed logarithmically in 51 bins.

In a process of trial and error the position of the source (impact site)and the velocity law are adjusted.

Final task: to find the particle size distribution and the related quantities.

Solution by linear regression:

B(x,y) = ∑ Ki * Si(x,y))

i = 0, 50

Si(x,y) is the scattering area produced by the particles of one particular size, i.Initially, S is calculated with anadopted particle size distribution.The solution, Ki, represents the final PSD.

The shown particle distributions are for the first post-impact observation.

Observation Model solution

Simulation of the ejecta plumeobserved 18 hours after the impact

Parameters found from the modeling(the results are derived from the fit of the model to the first post-impact observation, +18 hours)

4600 ton water were created by the impact (Kueppers et al. 2005, Nature, Vol. 437). Dust to water ratio approximately 3.

Comparison of our results with photometryfrom the first post-impact hours

CFTH + Megacam data,Jana Pittichova et al., 2005,ACM’2005.

Brightness decrease with the the velocitylaw used in the Monte Carlo model, calculatedfor 4 different particle size distributions.The data are normalized to their maxima and scaled to the Megacam measurements.

Comparison of our results with photometryfrom the first post-impact hours

Brightness decrease with the velocitylaw used in the Monte Carlo model, calculatedfor 4 different particle size distributions.The data are normalized to their maxima.

Light curve of the cometary dustobtained with OSIRIS.Kueppers et al. 2005, Nature, Vol. 437

1. The ejected dust plume is described by a dynamical model2. The total amount of dust derived is ≈ 12000 ton.3. The particle size distribution can be described by a power

law with index -3 +/- 0.24. The velocities used in the model and their size distribution

indicate acceleration of the particles by the gas in the coma.5. The results derived from dynamical modeling of the ejecta

cloud days after the impact are consistent with the photometry from the first hours.C

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Conclusions

1. The ejected dust plume is described by a dynamical model2. The total amount of dust derived is ≈ 12000 ton.3. The particle size distribution can be described by a power

law with index -3 +/- 0.24. The velocities used in the model and their size distribution

indicate acceleration of the particles by the gas in the coma.5. The results derived from dynamical modeling of the ejecta

cloud days after the impact are consistent with the photometry from the first hours.