1. OSIRIS Dust Group Studies Overview

2. Satellite Search During Approach Phase
Aims:
Discover and study large objects in the vicinity of CG’s nucleus (and their orbits).
Provide security statements for s/c final orbit insertion.
Images:
20 July 2014, 5800 km from CG, α =7°.
NAC Orange F22  RFoV ̴ 110 km (3D: 37% Hill sphere).
6 RUNS, each composed each of 3 short-exp (0.14s) and 3 long-exp (1.36s) consecutive images, spanning from 08:35 to 19:35.

3. Satellite: The Clones Approach
Montecarlo simulation with 50,000 clone satellites randomly located inside the Hill sphere (RH ~ 650 km) and velocity with modulus < escape velocity (1 m/s – Sierks et al Science 2015).
Using long-exp images we measure their displacement on the (x,y) CCD frame:
Within a single frame  no tracks
Within a single run to see if we can median combining
Between different runs to measure possible displacement of the satellite (our search radius)

4. Satellite: The Limiting Flux
AIM: measuring the limiting flux we consider as object detection
First, SExtractor for sources with flux >3σ (blackbox) within an aperture of 2 px radius (> PSF)
Then, usual aperture photometry to measure the sky level of the sources detected by SExtractor ( local background measure)
Our limiting flux = 3*standard deviation of all sky measures (largest threshold among runs 1-5): 8.9E-8 [W/m2*nm*sr] (long-exp).
Rather than the approach we followed at the time of Lutetia

5. Satellite: The Images Co-registration
Aim: removing stellar background and find satellite candidates
Method: co-registration of the median combined images of different runs using the 100 brightest SExtractor sources  stars are defined in co-registred images as sources having same (x,y) ccd position within 5 px tolerance (coming from geometric distortion correction plus central pixel identification error)
Rather than the approach we followed at the time of Lutetia

6. Satellite: Long-exp Results
Satellite would appear as a moving track of consecutive dots (1 dot each run) in the co-registred image
Rather than the approach we followed at the time of Lutetia
A,B = CCD Defects
C = zig-zag motion in X larger than PSF (4 px)  no possible satellite movement compatible with clones study  not a real source (just coincidence)

7. Satellite: Short-exp Results
Similar methodology as long-exp this time including clones acceleration closer than 6 km from the nucleus (anyway effect < 1px due to short exp time)
Major Problem was the presence of the background pattern  first we used Sextractor with fluxes > 4.5σ (best solution after trial-and-error procedure)  3*standard deviation of detected sources sky level  limiting flux =
3.0E-6 [W/m2*nm*sr]
Rather than the approach we followed at the time of Lutetia
A,B = sample CCD defects
C = nucleus
D= ghost

8. Satellite: From Flim to Dlim
From Flim (NAC F22) to Kron-Cousins Rlim using OSIRIS standard calibration fields as in Mottola et al. A&A 2014:
8.9E-8  R = 14.63
3.0E-6  R = 10.81
From Rlim to Johnson Vlim with V-R color of CG’s nucleus (V-R=0.54 from Tubiana et al. 2011):  V=15.17, V=11.35
From Vlim to Hlim with photometric model by Bowell et al inserting real observation geometry s/c-100,000 Montecarlo satellite clones randomly located inside the Hill sphere and G= -0.13±0.01 from Fornasier at al. 2015
From Hlim to Dlim with (Chesley et al. 2002):
where geometric V-band albedo pV=0.061±0.001
(Fornasier et al. 2015)

9. Satellite: Results and Discussion
No objects larger than 6 km within 20 km from the nucleus and larger than 1 m between [20-110] km (87% Hill sphere at perihelion) at the time of observations.
Results consistent with Rotundi et al (Aug 4: 350 orbiting grains within 130 km with size [4cm-crudely 2m]) and Davidsson et al 2015 (Sep 10: orbiting grains with size [ ]m).
No objects larger than few m are (were) orbiting the comet at least from last perihelion passage (or they broke).
Outburst in late April 2014 ejected between E+3 and E+5 kg but was unable to eject large blocks and placing them on orbit (or they broke).
Results underline that even if satellites of few meter size are formed during evolution of the comet its survival would have been jeopardized by many adverse events (e.g. close encounter with Jupiter, changes of pole direction due to strong outgassing, sublimation processes and erosion of the satellite itself)

10. Photometry of dust grains of comet 67P and connection with nucleus regions G.Cremonese, E.Simioni, R.Ragazzoni, I.Bertini, F.La Forgia, S.Fornasier, N.Oklay, M.Pajola, M.Lazzarin, and the Osiris team
We have analyzed the images of the NAC obtained on 10 September 2014
The data set is composed of 6 pairs of images obtained with two different filters, F24 and F28 (480 and 744 nm)
We have identified 77 dust tracks all over the 6 pairs
There are no grains identified in more than one pair
The S/N of the tracks most likely was very close to 3
We have analyzed each track measuring FWHM, length, integrated flux, and color
We are comparing the dust color grains with the color of the nucleus regions in order to trace the origin of the single grains

11. Automatic detection of dust grain tracks
Laplace filtered image
Boolean mask
Dilatation and erosion process
Down limit on eccentricity of closed isocurves
Synthetic image
Synthetic image in red and track estimation in blue

12. Error in parameters estimation
Rotation of the image function without any interpolation permits to limit in both the direction the domain to be considered.
Squared error minimization permits trapezoidal approximation.
Method was tested by synthetic image with track injection.
Error in trapezoidal base
Data samples
Error in parameters estimation
Fwhm
Length
Flux
2.1 %
5.0%
7.2%
Error in centroid definition

13. First comparison with nucleus images
We used the NAC images obtained on Aug 1, 2014 from 11:50 UT to 20:44 UT. The phase angle of the observations spans from 9.0° to 11.4°, the scale between 13.4 m/px and 15.1 m/px (phase angle of grains observation was 90°). We first aligned the entire data cube, we then corrected the illumination conditions through incidence angle maps, by using the shape model produced inside the OSIRIS team. We selected 38 images located on both 67P lobes and neck. Then we have another set of nucleus images obtained at the same phase angle as the grains, but showing much smaller regions of the nucleus. We are still working on them to derive the reddening.

14. Histogram of the nucleus regions observed on 1 August

15. Histogram of the dust grains reddening

16. Main assumptions
We are using the Hapke parameters reported in the Fornasier et al. draft for disk integrated
The Hapke parameters will be used to extrapolate the images of the nucleus at the phase angle of the dust grain images, 90 degrees. This is a work in progress.
Next step shoud be to normalize the reddening to the green filter interpolating between F24 and F28. In so doing we can refer to the Fornasier et al. data

17. Other Studies on Grains
Grain rotations (Marco et al. ) – NAC 2014 Oct
OSIRIS observations of rotating grains compared to models put constrains on the shape and density of the detected grains.
Results: oblate (a/b=0.5) fluffly grains few mm in size are the most likely candidates for the rotating grains.

18. Other Studies on Grains
Grain orbits (Bjorn et al., submitted to A&A)
Particle tracks (Jessica et al.)
WAC 2014 Oct 12
Bright tracks followed by a trail system of dots and curly tails.
2..10 x brighter than nucleus surface (at phase angle deg)
Minimum projected speed 10 m/s or lifetime <20s
Observed at least 5x, in three different cameras
Apparent temporal clustering in late August and mid-October

19. Particle tracks – possible causes
Motion-induced smearing of a bright, fast grain (reflected sunlight):
Requires albedo up to 20x larger than nucleus.
Difficult to explain simultaneous presence of tracks and dots.
Reflected sunlight from an optically thick cloud of small, bright grains (e.g. ice)
Requires a parent particle of mm- to cm size.
Expect alignment with local gas flow, but no obvious alignment with radial direction seen.
Meteor (recombination of ionised coma gas)
If all gas in track ionised, recombination can provide sufficient energy for observed flux.
Consistent with high speeds, and with lack of alignment with nucleus direction.
Details of recombination cascade TBD.
Cosmic Ray
Track partly resembles a stopping positive ion (nucleus from CNO group).
Track length consistent with typical energy of such a CRs.

20. Monitoring of Overall Dust Coma
Example: 2014 September 25, 27 km from the comet
We build a synthetic coma from the section we have selected
Azimuthally-averaged radial profiles, ‘Afρ’ vs ρ, total flux vs ρ