Dust in Comet 103P/Hartley 2 Coma During EPOXI Mission Q ⇑ E

Icarus 222 (2013) 774–785

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Icarus

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Dust in Comet 103P/Hartley 2 coma during EPOXI mission q ⇑ E. Hadamcik a, , A.K. Sen b, A.C. Levasseur-Regourd c, R. Gupta d, J. Lasue e, R. Botet f a UPMC (U. P. & M. Curie, Sorbonne Universités), LATMOS/CNRS, 11 Bld d’Alembert, 78280 Guyancourt, France b Assam Univ., Silchar 788 001, India c UPMC (U. P. & M. Curie, Sorbonne Universités), LATMOS/CNRS, 4 Place Jussieu, 75005 Paris, France d IUCAA, Post Bag 4, Ganeshkhind, 411007 Pune, India e Univ. de Toulouse, UPS-OMP, IRAP, BP 44346, 31028 Toulouse, France f Univ. Paris-Sud, LPS/CNRS, Bat. 510, 91405 Orsay, France article info abstract

Article history: The Deep Impact spacecraft flew by Comet 103P/Hartley 2 on 4 November 2010 (EPOXI mission). In situ Available online 16 July 2012 observations are complemented by a systematic ground- and space-based observation campaign. In the present work, imaging polarimetry is used to emphasize different dust regions in the coma and follow Keywords: their evolution over a period including the EPOXI fly-by. Comets, Dust On the intensity images, the coma is asymmetric with an important tailward feature. Jets in the sun- Comets, Coma ward direction are observed to present an extension that depends on the nucleus phase. The azimuthal Polarimetry integrated intensity presents a nominal radial decrease (1 in log–log scale) for optocentric distances larger than a few hundred kilometers. Through cometary continuum narrow band filters, the aperture polarization decreases with the optocentric distance. On the polarization maps, the short sunward jets are more polarized than other parts of the coma. Intensity variations may be induced by large slow particles in the inner coma and possibly by their fragmentation into smaller particles, under ice sublimation processes. The decrease of linear polarization with increasing optocentric distance is correlated with intensity variations and may be induced by the same physical process. The optical behavior of 103P/Hartley 2 is finally compared to those of other Jupiter-family comets such as Comet 9P/Tempel 1 impacted by the Deep Impact projectile in 2005 and Comet 67P/Churyumov– Gerasimenko, target of the Rosetta mission (2014–2015), since the polarimetric properties of both comets have been monitored remotely in the recent past. Ó 2012 Elsevier Inc. All rights reserved.

1. Introduction gas jets emerging from the surface (Farnham et al., 2011). The small nucleus is very active with an outgassing dominated by

Comet 103P/Hartley 2 was discovered in 1976. Its orbit inclina- CO2, which drags off large pure ice particles from the sub-surface tion is relatively small (about 14°). It had two close approaches (Sunshine et al., 2011) and darker particles. The nucleus is bi-lobed with Jupiter in 1947 and 1971, which decreased its orbital period with a ‘waist’ reminiscent in shape of Comet 19P/Borrelly (Boice to a value of 6.47 years. The nucleus is small but very active. It et al., 2002). The relative abundances of volatiles ejected by the was observed at each perihelion passage since its discovery. For three parts of the nucleus are different (A’Hearn et al., 2011), show- the last passage (2010), the closest approach to Earth on October ing a stronger water signal near the waist of the body and more

20 was at 0.12 AU, making it easily observable. CO2 ejections and organics from the smaller lobe from which clus- The Deep Impact spacecraft ﬂew by Comet 103P/Hartley 2, on 4 tered jets are issued (Feaga et al., 2011). The dust jets are corre-

November 2010 as part of the extended mission EPOXI (A’Hearn lated with the CO2 emissions. The large particles (chunks) that et al., 2011). Amazing images of ice particles moving in the neigh- are found all around the nucleus and move slowly are possibly ﬂuf- borhood of the nucleus were obtained with well-localized dust and fy aggregates of micrometer-sized constituent grains consisting of water ice and dark refractory materials. From EPOXI observations, it is not expected that the water ice particles survive further than 3 q Based on observations made in part at Observatoire de Haute Provence (CNRS), the innermost coma of the comet (<10 km; Lisse et al., 2011). France and in part at IUCAA Girawali Observatory, India. An important Earth- and space-based campaign using different ⇑ Corresponding author. techniques, including the study of light scattered by the ejected E-mail addresses: [email protected] (E. Hadamcik), asokesen@ particles and its linear polarization has been developed in support yahoo.com (A.K. Sen), [email protected] (A.C. Levasseur-Regourd), rag@iucaa. ernet.in (R. Gupta), [email protected] (J. Lasue), [email protected] (R. of the EPOXI mission (Meech et al., 2011). An unusual CO2-driven Botet). activity was observed near perihelion, which seemed to persist

later on. An ice halo around the nucleus was inferred by fitting where I\ and I// are the polarized components perpendicular and 103P/Hartley 20 R-band light curve with a model of comet activity parallel to the scattering plane respectively. The third Stokes U V (Meech et al., 2011). The period of rotation of the nucleus increased parameter I equals zero in that case, while I allows to calculate with time from about 17.5 h in October to 18.8 h at the end of the circular polarization, equal to 0 for irregular randomly oriented November (Samarasinha et al., 2011). A period of 18.34 h for the particles. excited state of rotation was determined at encounter from the The polarization of the scattered light by solid particles is a light curve (A’Hearn et al., 2011). dimensionless ratio (Eq. (2)), which makes it unnecessary to nor- Jupiter family comets (hereafter JFCs) have the following ob- malize the fluxes as a function of distance or of the number density served general properties. The dust production rates estimated of particles. by coma observations in the visible is generally small; such comets are qualified as dust poor (low dust-to-gas ratio), requiring the use 2.1. Observatory of Haute-Provence (OHP) instrumentation of narrow band filters to study the dust contribution, while avoid- ing possible contaminations by gas emissions. Hadamcik and The telescope has a diameter of 0.8 m with f ratio of 15. The res- Levasseur-Regourd (2009) reviewed the optical properties of dust olution on the camera is 0.21 arcsec diameter by pixel. A pixel bin- in JFCs, as summarized below. The average geometric albedo of ning of 4 pixels 4 pixels is used to increase the signal-to-noise the nucleus (measured by EPOXI, 0.04) is lower than the one of ratio. The total field of view is 7 arcmin 7 arcmin. The sky back- the ejected dust particles (between 0.04 and 0.09 at zero degree ground is measured at more than 2 arcmin from the optocenter, in phase, for dust particles in different JFCs). The observed light regions without stars. curves are usually similar from one apparition to the next, except Four Polaroid filters are mounted on a rotating wheel. The con- when the nucleus suffers a major outburst or changes its orbit. tinuum ESA narrow band blue (CB) and red (CR) filters, the broad- Fans and jets are observed in the coma. If the physical properties band red Gunn (R) and infrared Gunn (I) filters, installed on a of particles inside such structures are different from the surround- second wheel, avoid or considerably reduce contaminations by ing dust coma, the fans and jets may be detected by color and/or the gaseous species (see Table 1 for their bandwidth). With a linear polarization differences. Further away from the nucleus, star-tracking mode, series of 10–20 short exposure time images the dust particles observed in trails are dark and may reach milli- (typically 30 s depending on the filter and date, up to 240 s for meter sizes. They can survive along the cometary orbit for several the very narrow blue cometary filter) are added to increase the sig- revolutions (Fernandez et al., 2009) but their number density is nal-to-noise ratio. small; they are faint and not observable by polarimetry. Close to the nucleus, the number density of particles in the hundreds of 2.2. Girawali IUCAA Observatory (IGO) instrumentation micrometers to centimeters size range is more important due to their low velocity and their eventual fragmentation. They can then The telescope has a diameter of 2 m with f ratio of 10. The res- be detected by their linear polarization (Hadamcik et al., 2009 and olution on the camera is 0.307 arcsec diameter by pixel. A rotating Section 4.3 for experimental simulations showing the high polari- half-wave plate (HWP) and a Wollaston prism allow to measure zation produced by large particles). Polarization imaging, through the polarization (Sen and Tandon, 1994). The total field of view filters blocking gaseous emission lines, is necessary to correlate for polarization imaging has a 4 arcmin diameter. the intensity and polarization variations of dust through the coma. With the Wollaston, two perpendicularly polarized images are The correlation between intensity, color and polarization gives recorded at the same time on the same CCD frame; their separation indications on the type of physical processes affecting the dust par- on the image is about 1 arcmin. This limits the size of one polarized ticles as they move away from the nucleus, probing for example image to less than 0.5 arcmin. The non-overlapping of the two their possible fragmentation. Fresh material, coming from inside images is controlled by intensity profiles and compared to the va- JFC nuclei, as in the case of an outburst or the Deep Impact collision, lue of the sky background measured at more than 1.5 arcmin from seems to have properties similar to the ones of materials ejected in the optocenters. Specific formulas for the calculation can be found the well-focused jets observed in active comets originating from in Ramaprakash et al. (1998) and Hadamcik et al. (2010). To build the Oort cloud such as Comet C/1995 O1 Hale–Bopp or Comet C/ the polarization maps, it is in the end necessary to separate the two 2001 Q4 NEAT (Hadamcik and Levasseur-Regourd, 2003a; Harker components and re-center them. In this work, the general formulas et al., 2007; Wooden et al., 2004). with four polarized components are considered (Eqs. (3)–(7)). Like at OHP, ESA blue (CB) and red (CR) narrow-band continuum com- 2. Observations and data reduction etary filters and a Bessel red (Rb) broadband filter reduce the gaseous emission contaminations (Table 1). The telescope tracks the This work is part of a French-Indian collaboration. Two tele- comet with a typical exposure time of 900 s per filter. scopes in Cassegrain mode are used. A short description of the instruments and data reduction is given below, more details can 2.3. Observations procedure be found in Hadamcik et al. (2010). The intensity and polarization state of a beam of light scattered During each night, polarized and unpolarized stars are observed by irregular particles can be entirely expressed by the four Stokes to estimate the residual instrumental polarization and to deter- parameters: I, Q, U, V (van de Hulst, 1957). These quantities can be mine the origin of the instrumental reference system h0 (Table calculated from the observed polarized intensities. In the present 2). h0 corresponds to the position angle of the polarization axis of work four polarized images are recorded with their polarization one polarized filter or of the rotating HWP. axis at 45° from each other. If the polarization axis of the images The four images are polarized at 45° from one another. The can be oriented relatively to the scattering plane, the total intensity intensity, the measured polarization degree P, the measured posi- Q tion angle h, the position angle of the polarization plane and finally I and the linear polarization are I and Pr ¼I respectively the values of the polarization Pr and of the position angle hr in the I ¼ I? þ I== ð1Þ coordinate system referring to the scattering plane are calculated by the following expressions: I? I== Pr ¼ ð2Þ I ¼ I0 þ I90 ¼ I45 þ I135 ð3Þ I? þ I== 776 E. Hadamcik et al. / Icarus 222 (2013) 774–785

Table 1 Instrumentation at OHP and IUCAA.

Observatory Telescope diameter aperture CCD resolution binning Polarizer Cometary continuum ﬁlters OHP Cassegrain 0.21 arcsec/pixel Rotating ESA narrow band 0.8 m 4 pixels 4 pixels 4 polaroids CB 443 nm, Dk 4nm f/15 CR 684 nm, Dk 9nm Broadband R 650 nm, Dk 90 nm I 810 nm, Dk 150 nm IUCAA Cassegrain 0.307 arcsec/pixel Rotating half-wave plate + Wollaston prism ESA narrow band CB, CR 2m 1 1 pixel Rb 630 nm, Dk 120 nm f/10

Table 2

Polarized and unpolarized standard stars. Observations through red (R) and blue ﬁlters (B). P and PA from the literature (Walborn, 1968; Schmidt et al., 1992). Pobs and PAobs from observations.

Star P (%) PA (°)Pobs (%) PAobs (°) h0 OHP October HD19820 (R) 4.526 ± 0.025 114.45 ± 0.16 4.75 ± 0.1 114 ± 2 0 ± 5 4.71 ± 0.05 115 ± 2 1 ± 5 HD236633(R) 5.376 ± 0.028 93.04 ± 0.15 5.25 ± 0.10 92 ± 1 1 ± 5 HD21447 (B) 0.017 ± 0.03 28.63 0.02 ± 0.05 30 ± 2 1 ± 5 HD21447 (R) 0.01 ± 0.05 HD204827 (R) 4.89 ± 0.02 59.1 ± 0.17 4.96 ± 0.05 58.6 ± 1.0 0 ± 1 IUCAA HD210027 (R) 0.002 ± 0.006 – 0.01 ± 0.01 HD19820 (R) 4.526 ± 0.025 114.45 ± 0.16 4.57 ± 0.02 114 ± 1 0 ± 5 OHP November BD + 59°389 (R) 6.43 ± 0.02 98.1 ± 0.02 6.2 ± 0.15 103 ± 2 5 ± 5 BD + 32°3739 (B) 0.039 ± 0.021 79.38 0.05 ± 0.03 86 ± 10 7 ± 10 HD236633 (R) 5.376 ± 0.028 93.04 ± 0.15 5.51 ± 0.05 93 ± 1 0 ± 5 HD21447 (B) 0.017 ± 0.03 28.63 0.021 ± 0.05 24 ± 5 5±5 Hiltner 960 (R) 5.21 ± 0.029 54.54 ± 0.16 4.9 ± 0.1 49 ± 5 5 ± 5

qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 2 cations (Table 2). h0 is in average equal to 0°. The instrumental ðI0 I90Þ þðI45 I135Þ P ¼ 2 ð4Þ polarization is found to be smaller than 0.1% for both telescopes. I0 þ I90 þ I45 þ I135 The comet was observed during two epochs, one about 3 weeks or as expressed with the Stokes parameter formalism before the EPOXI encounter in October 2010 and one around the qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi encounter in November 2010. The log of the observations is given 2 2 in Table 3 and the precise dates for each night and filter are given Q þ U P ¼ ð5Þ in Table 4. The seeing was between 2 arcsec and 3 arcsec at OHP, I depending on the date and it was less than 1.5 arcsec at IUCAA.

I45 I135 h ¼ 0:5 arctan ð6Þ 2.4. Data reduction I0 I90

Each individual polarized image is corrected for bias and flat- hr ¼ h h0 ð/ 90Þð7Þ field. To control that errors are not included in polarization values owing to the averaging of the intensity on the field, calculations are P ¼ P cosð2h Þð8Þ r r made with and without flat-field correction. The sky background is The polarized intensities are measured in the instrumental ref- estimated in a region outside the coma and free of faint stars (gen- erence system. The polarization P defined by (4) is always positive. erally between the two polarized images of the same plate for

However, the value of Pr as defined by Eq. (1) can be negative or IUCAA observations). The sky background is first subtracted. Then, positive depending on the orientation of the polarization plane fluxes through apertures with diameters of 12 and 24 pixels are with respect to the scattering plane. (h–h0) is the position angle measured for each polarized components, and the stability of of the polarization plane in the equatorial reference system. The intensity values is controlled for the whole series. If a difference position angle F of the scattering plane (given by the solar direction greater than 1% is detected, the image is rejected. in the equatorial reference system) is known for each date (Ta- A center of gravity algorithm is used to find the position of the ble 3). For symmetry reasons, UF is defined between 0° and 180°, optocenter of the comet on each polarized image. To avoid artifacts and the sign inside the parentheses of (7) is chosen to ensure the in the polarization maps, the polarized components are centered condition 0° <(U ±90°) < 180° so that the plane perpendicular to with a precision of 0.1 pixel before any addition or calculation. the scattering plane is correctly oriented with respect to the defini- The signal-to-noise ratio is increased through addition of the tion of Pr (or equivalently to give the correct orientation of h in Eq. individual images for each orientation of the polarized filters. In (6)). As the linear polarization plane is either close to the scattering some series star trails are quite visible in the inner coma region plane or perpendicular to it, hr is about 0° or 90° and Pr is equal to after the images have been centered and added. In that case, when about P or P respectively. it is possible, the images with stars in the inner coma are not used The polarization values obtained for standard stars with the two in the sum. When the number of images is sufficiently large (5–10 instruments are comparable to values obtained in previous publi- images in a series), a median sum is applied. It allows suppressing E. Hadamcik et al. / Icarus 222 (2013) 774–785 777

Table 3 Log of the observations. D, distance to Earth; R, solar distance; a, phase angle; U, scattering plane position angle.

Date 2010 Days to perihelion D (AU) R (AU) a (°) U (°) Filters Exposure timesa (s) OHP 5–6 October 22 0.16 1.1 46.4 18 CR; R; CB 60 20; 60 18; 240 11 6–7 October 21 0.16 1.1 46.8 21 R; CR; I; 60 11; 60 20; 30 10; CB 180 12 IUCAA 2–3 November 5 0.15 1.06 58.6 102 Rb; CR; CB 300 2; 900 2; 1200 2 OHP 3–4 November 7 0.15 1.06 58.8 104 R; CR 30 10; 30 20 4–5 November 8 0.16 1.06 58.8 104 I; CR; CB 30 20; 30 20; 60 14 5–6 November 9 0.16 1.06 58.8 106 R; CR 30 15; 30 25

a For each series used to build a polarization map (time for one exposure x number of exposures). At IUCAA with two series, four polarization values can be calculated.

Table 4 Polarization values for different apertures (diameters) and wavelengths.

Date Day (UT) Aperture (km) 200 400 600 1200 2400 4800 7200 9600 Filter OHP 6.05 CR 9.6 ± 0.4 9.1 ± 0.3 8.8 ± 0.3 8.3 ± 0.3 8.1 ± 0.4 7.9 ± 0.5 October 6.16 R 9 ± 0.5 8.9 ± 0.3 8.9 ± 0.4 8.5 ± 0.4 8.15 ± 0.4 7.9 ± 0.4 2010 6.19 CB 8.2 ± 0.6 7.9 ± 0.6 7.5 ± 0.5 6.8 ± 0.5 6.5 ± 0.6 6.3 ± 0.7 (a = 46.8°) 6.93 R 10.9 ± 0.4 9.9 ± 0.4 9 ± 0.4 8.6 ± 0.5 8.1 ± 0.5 7.9 ± 0.6 6.99 CR 12.4 ± 0.4 10.6 ± 0.4 9.6 ± 0.6 9.4 ± 0.5 8.7 ± 0.4 8.6 ± 0.4 7.05 I 7.5 ± 0.2 7.6 ± 0.2 7.5 ± 0.3 7 ± 0.4 6.9 ± 0.5 6.7 ± 0.5 7.08 CB 8.9 ± 0.5 8.5 ± 0.5 8.3 ± 0.5 6.8 ± 0.5 6.4 ± 0.6 6.2 ± 0.8 IUCAA 2.9 Rb 15.2 ± 0.5 14.9 ± 0.5 13.3 ± 0.5 12.6 ± 0.6 11.6 ± 0.8 November 2.96 CR 15.9 ± 0.5 15.6 ± 0.4 15.1 ± 0.5 14.2 ± 0.6 12.9 ± 0.8 2010 3.03 CB – 11.6 ± 0.6 10.1 ± 0.5 10.5 ± 0.5 9 ± 0.8 (a = 58.6°) OHP 4.16 R 16.3 ± 0.4 15 ± 0.4 13.7 ± 0.5 11.9 ± 0.6 10.9 ± 0.7 10.1 ± 0.7 November 4.24 CR 17.1 ± 0.3 15.8 ± 0.3 15.3 ± 0.4 12.8 ± 0.4 10.9 ± 0.5 10.4 ± 1.0 2010 5.15 I 11.7 ± 0.3 11.5 ± 0.3 11.4 ± 0.4 11.6 ± 0.5 11.4 ± 0.4 11 ± 0.6 (a = 58.8°) 5.19 CR 16.7 ± 0.4 15.9 ± 0.4 14.5 ± 0.5 12.3 ± 0.4 11.2 ± 0.4 10.6 ± 0.5 5.24 CB 12.3 ± 0.8 13.2 ± 0.8 12.3 ± 1.0 9.1 ± 1.0 8.7 ± 1.2 8.2 ± 1.3 6.14 R 15.4 ± 0.3 15 ± 0.3 14.2 ± 0.4 12.8 ± 0.4 12.5 ± 0.4 11.4 ± 0.4 6.2 CR 15.2 ± 0.5 14.8 ± 0.3 13.5 ± 0.4 13.3 ± 0.4 12.7 ± 0.5 11.5 ± 0.6

any object, which has a different position in the different images 3. Results (e.g. background stars). The signal-to-noise ratio remains the same than for the individual images and it is necessary to make an First, the intensity variations and coma morphologies from addition of second and third series... The problem of faint star October to November are presented in this section. Short jets are trails crossing the inner cometary region is also observed at IUCAA detected in the sunward direction. Than, variations in the polariza- with the cometary tracking-mode and cannot always be avoided. tion are pointed out. The comparison between the intensities measured at the four posi- tions of the HWP allows to control the eventual effect. With the 3.1. Intensity images two telescopes, the equality of the two final intensities (Eq. (3)) is verified. 3.1.1. Coma morphology The correct subtraction of the sky background level is controlled The coma is elongated tailward. Typical examples are given in by a nominal 1 azimuthal integrated decrease of intensity at rel- Fig. 1 (first row, with isophotes superimposed on the images). atively large distances from the optocenter (see Section 3.1.2) be- The resolution is better for IUCAA than for OHP, while the field fore the polarization maps are built. of view is smaller (images are scaled to the same units for The uncertainties have different origins: (1) In the central part, comparison). An important tailward feature and some short fea- the centering can become imprecise due to the presence of a faint tures (smaller than 1000 km) are detected on the intensity images star in some of the images (only detectable when building the treated by a rotational gradient method (this emphasizes the polarization maps), (2) incorrect determination of the sky back- intensity gradients but can introduce some distortion of the ground e.g. by the presence of remaining stars in the cometary features, Samarasinha and Larson, 2011); the observed features coma (a medium sum is not always sufficient in the inner region). are confirmed on the isophotes. Examples can be found in Fig. 1 The S/N ratio (SNR) at the images side is about 10 for each individ- (bottom row). ual image and 16 in average for the polarized components in the The directions of the features are studied for each wavelength same region after addition of the individual images (the SNR is domain and each date of observation. All position angles, PA, are measured on the image itself); in the inner part of the coma it is given North to East. The main feature is antisunward, at a about 200. The error bars are obtained by considering all the inten- PA = (202 ± 3)° in October and increases from (282 ± 1)° to sity images and polarization maps for a given date; they corre- (286 ± 2)° in November as the solar direction changes with the date spond to a confidence level of about 95% (2r). (Table 3). Close to the optocenter, this feature presents a width of 778 E. Hadamcik et al. / Icarus 222 (2013) 774–785

Fig. 1. Intensity images with superimposed isophotes in log scale (684 nm, Dk 9 nm). (a) October 6.05, (b) November 2.94, (c) November 4.24. Same images treated by the rotational gradient method emphasizing the structures (d–f). Field of view 9000 km (a, c, d, and f) and 3000 km (b and e). Remaining star-trails marked on the image by :(a and d) horizontal, (e and f) oblique. Optocenter: +. Sun direction: solid line. Jets: dashed lines. Tailward structure extension limits: dashed lines.

Fig. 2. Isophotes in log scale (1) and in negative, emphasized intensity features (2). + optocenter. Field of view 3000 km. Sun direction indicated by a solid line, jets directions indicated by dashed lines. North is up and East is left. (a) October 6.16. (b) October 6.99. (c) November 2.96. (d) November 4.24. (e) November 5.19. (f) November 6.20.

about ± 10°; further away (above 3000 km), it seems to be wider at about a constant PA of (85 ± 10)° and (135 ± 5)° are present; and more diffuse. their extension varies with the date. The latter jet is slightly curved Jets with different orientations and extensions are detected on backward and extends to more than 1000 km from the optocenter. the images (Figs. 1 and 2). Their PA are measured for all the treated The PA and extension of the jets may be related to the rotation of intensity images obtained on the same night. On October 6.05 and the nucleus. On November 5.19 the extension of the jets is reduced 6.16 images, their PA are at (5° ±10°) and (50° ±10°) and PA as compared to the other dates with just a faint jet at about PA (50° ±5°) on October 6.93 and 6.99. On November images, two jets 135° (Fig. 2e). The jets are observable on October and November E. Hadamcik et al. / Icarus 222 (2013) 774–785 779

Fig. 4. Integrated polarization obtained through the cometary red CR and the Fig. 3. Intensity profiles through the coma (normalized at optocenter, curves broad-band filters R and Rb. For the November 2.9 and 2.96 images, the field of view vertically shifted for clarity). The intensity is not shown in the blurred inner region. is limited to an aperture (diameter) of 3000 km. Curves with a lower polarization CR and I filters. On 4.24 November, the undulations at about d = 3200 km through apertures smaller than 3000 km diameter are in red; curves with a higher (logd = 3.5) may be attributed to faint stars. polarization in the same region are in black. Numbers in green color indicate the fraction of nucleus rotation for each date if the origin is taken on November 2.96. (For interpretation of the references to colours in this figure legend, the reader is as shown on Fig. 2 (Fig. 2a and c with a preferred direction at about referred to the web version of this paper.) 20° clockwise from the solar direction and Fig. 2b, d, and f at about 30° anticlockwise). polarization values through small apertures (diameters smaller 3.1.2. Radial decrease in intensity than about 3000 km) are lower for the first group than for the sec- In Fig. 3, the azimuthal integrated profiles are compared to pro- ond group; opposite behavior is found for large apertures (Fig. 4). files obtained sunward and tailward for three different dates (2.96, Although these differences are small, typically less than 1%, they 4.24, 5.15 November). The very good resolution and seeing at may be a clue to the propagation of dust with different physical IUCAA provides results at less than 100 km from the optocenter. properties. The emitted dust is less polarized on November 2.96, The intensity for an isotropic coma would decrease with a slope 6.14, 6.20 than on November 4.16, 4.24 and 5.19. This result seems to be in good agreement with the orientation of the jets when the of 1 in log–log scale; this slope is also generally found for the integrated azimuthal intensity at relatively large optocentric distances. For Hartley 2, between 200 km and 400 km (out of the region blurred by seeing) a slope of 0.60 ± 0.05 is found; further away the average slope is 1 ± 0.05 for all the dates. The slope of the sunward profile is 1.2 ± 0.1 and the slope of the tailward profile is 0.85 ± 0.05. As compared to the slope for the azimuthally integrated profile, the change in slope is, as expected for an asymmetric coma, steeper in the sunward direction due to radiation pressure and gentler in the tailward profile.

3.2. Linear polarization

Aperture polarization is obtained from the integrated ﬂux through apertures measured in the polarized intensity images. The polarization is computed using Eq. (4) and varies at a given phase angle as a function of aperture and wavelength. The values are then compared to other comets. In the polarization maps, regions are tentatively detected where dust properties are different from those in the surrounding regions.

3.2.1. Aperture polarization Values of the integrated polarization, P, through the cometary and broadband filters are computed and compared for different dates (Table 4 and Fig. 4). The polarization decreases with increasing aperture and remains stable for apertures larger than about 5000 km. On a given date, the polarization values through the R (Gunn) filter are similar to the values through the cometary red filter (CR). For the Rb (Bessel) filter, the polarization is slightly smaller possibly due to some gaseous species contamination. The observations may be classified in two groups according to their rotation phase (in fraction of the rotation period of the nucleus, with an origin taken on 2.96 November): group 1 (Novem- ber 2.96, 6.14, 6.20), phase smaller than 0.2 and group 2 (Novem- Fig. 5. Integrated polarization vs aperture (diameter) at different wavelengths with ber 4.16, 4.25, 5.19) with phase between 0.6 and 0.9. The fits (dotted lines) representing the trends. (a) a = 46.8°. (b) a = 58.8°. 780 E. Hadamcik et al. / Icarus 222 (2013) 774–785

phase angle of 46.8° and Fig. 5b at 58.8° (the variation in phase within each observing epoch is smaller than 1°). The decrease in polarization with increasing aperture is observed through the red and blue ﬁlters. Unexpectedly the polarization is about constant through the near infrared ﬁlter for both phase angles. In this wavelength domain (810 nm, Dk 150 nm), the gaseous contaminations are expected to be negligible and the comparison to other comets values may help to draw conclusions.

3.2.2. Polarimetric phase curves In Fig. 6, the polarization values for Hartley 2 are compared to the polarimetric phase curves obtained for other comets, so-called ‘high-polarization comets’ and ‘low-polarization comets’ for clarity of the figures; Hale–Bopp, the comet with the highest polarization ever measured is not shown (Levasseur-Regourd, 1999; Hadamcik and Levasseur-Regourd, 2003b). The data are taken for sufficiently large apertures to allow the polarization values to be about constant with the aperture. Hartley 2 values for a = 46.8° and 58.8° are plotted on the graphs (aperture 4800 km diameter). In the blue–green domain (Fig. 6a) and in the red wavelength domain (Fig. 6b) the data points for Hartley 2 are between the fits for high-

and low-Pmax comets but significantly closer to the low-Pmax comets fit (as defined in Levasseur-Regourd et al. (1996), classification). For the near–infrared phase curve (Fig. 6c), the average wavelength

for low-Pmax comets is (900 ± 100) nm. The Hartley 2 data are just below the ﬁt. One can notice the small number of data for this wavelength region and in consequence the uncertainty on the ﬁt-

ted curve in the intermediate region (the low-Pmax comets fit is obtained through the data shown on the graph plus a number of 5 ones in the 85–95° range). To better characterize the wavelength dependence of the polarization values. Fig. 7a and b present P as a function of wavelength for two apertures. The polarization values are compared to Comet Halley polarization at a =50° (Fig. 7b) and to other comets (a = 46.8° for Hartley 2) in Fig. 8. The values for Halley are above the polarization values for Hartley 2 at a phase angle of about 50°. These comparisons confirm the classification of Hartley 2

among the low-Pmax comets.

3.2.3. Polarization angle

As expected at such phase angles, the polarization angle hr is about equal to 90° (91° ±7°) in October, (90° ±3°) at IUCAA on November 2.96 and (89° ±5)° at OHP in November.

3.2.4. Polarization maps Fig. 6. Polarization values (aperture diameter 4800 km) for Hartley 2 compared to Polarization maps are retrieved for both epochs from OHP and previously observed comets. Data in infrared at an average wavelength of (900 ± 100) nm, from Nogushi et al. (1974), Oishi et al. (1978), Michalsky (1981), IUCAA observations. Fig. 9a presents polarization maps through Kikuchi et al. (1987), Kikuchi (2006), Eaton et al. (1988, 1992), and Chernova et al. the CR filter for 3 dates (October 6.05, November 2.96, November (1993). Dashed lines for each wavelength domain represent the trends for 4.24). Features close to the optocenter with a higher polarization previously observed comets. are observed on all the maps for the red filters (see Table 4 and Fig. 4). In the red wavelength domain, the central polarization is between 12% and 13% for October observations (a = 46.8°) and between 16% and 17% for November observations (a = 58.8°); it de- nucleus rotation phase is considered. The two jets seem to eject creases further away to respectively 6% in October and 11% in different materials with a different composition and/or materials November. The solar–antisolar asymmetry observed tailward in with a different size distribution. A higher polarization seems to intensity is also observed in the polarization maps; however, the be produced from the materials ejected at about 30° anticlockwise highest polarization region corresponds to sunward jets. Fig. 9b from the projected solar direction and lower polarization from the shows the inner part of the polarization maps for the same six materials ejected at about 20° clockwise. dates than in Fig. 2 for comparison. The jets locations are similar The decrease in polarization with increasing aperture was fre- to the ones in the intensity images (treated by the rotational gradi- quently noticed for numerous comets, just as the variability in ent method). Their extension changes with time and the dust polarization in the inner part of the coma (Renard et al., 1996; Had- seems to be sensitive to radiation pressure, and pushed tailward, amcik and Levasseur-Regourd, 2003b). indicating dust with higher b ratio (ratio between solar radiation Fig. 5 compares the polarization as a function of aperture at dif- pressure force and solar gravity), i.e. rather micron-sized and/or ferent wavelengths (OHP observations). Fig. 5a corresponds to a low density (typically fluffy) dust. E. Hadamcik et al. / Icarus 222 (2013) 774–785 781

in color may be the result of sublimation of ice and/or fragmentation of the particles. It is consistent with the presence of large particles observed by radar (Harmon et al., 2011) or with Deep Impact (A’Hearn et al., 2011). It is also consistent with the decrease in polarization with increasing apertures. Change in the production of dust and gas in relation with the rotation of the nucleus is also noticed by Tozzi et al. (2011b) and the position angle of the jets are similar in the two series of November observations. Mainly in the inner coma region the polarization may change from one night to the next one, even if the phase angle does not change. The values of integrated polarization given in Meech et al., 2011 are similar to our values; taking into account the error bars and the variation in polarization probably related to the nucleus rotation. In Meech et al., 2011, the polarization value is about 13% for a Fig. 7. Polarization vs wavelength, through two apertures (diameters). Comparison with trend (dotted line) derived for 1P/Halley observations at 50°. 9600 km diameter aperture at a =59°, in the red wavelength domain. In the present paper, the equivalent average value is (10.8 ± 1.5)%. For an aperture of 3000 km, the polarization at 59° is in average 14% (this paper). If a constant ratio is assumed between the values found for the two apertures and on the polarization synthetic phase curve between 59° and 30°, the polarization should be about 6%, i.e. equal to the value given in Meech et al., 2011. The results in the blue domain are also similar. The near- infrared polarization map seems to be flat with perhaps a small region with increased polarization in the solar direction. The low polarization in the (810 ± 150) nm filter could suggest a contamination mainly by some faint CN bands (Feldmann et al., 2004). The flat polarization map and the constancy of the values with increasing aperture seems to indicate that the contamination is higher close to the nucleus than far from it when the dust-to-gas ratio usually decreases with a decrease of the polarization value. As a matter of fact, HCN (a parent of CN) presents a spatial distribution similar to that of the dust (Dello Russo et al., 2011; Meech Fig. 8. Comparison between near-infrared polarization for Hartley 2 (a = 46.8° at et al., 2011). Another interpretation is thus to be found for the fea- k = 810 nm, Dk = 150 nm) and other comets (different a) k = (900 ± 100) nm. Error tureless polarization map. Kelley et al. (2004) observed different bars for Hartley 2 values smaller than markers. comets at 2200 nm. They measured a polarization for Hartley 2 Through the narrow band cometary blue filter (CB), the polariza- of (12.5 ± 0.7)% at a = 48.9° with an aperture of about 7200 km tion maps are noisy and artifacts can hardly be avoided; however, diameter very close to the high-Pmax cometary synthetic phase the polarization values as a function of aperture indicate a behavior curve in red wavelength (see Fig. 6b for the polarimetric phase similar to the one noticed in the red spectral domain (Section 3.2.1, curve in red). A neutral polarimetric color was also found using Fig. 5). The maps in the near-infrared domain do not present any the NASA Infrared Telescope Facility (Meech et al., 2011). Kelley major feature around the optocenter confirming the fact that the et al. (2004) also compared the different comets they observed polarization does not vary with increasing aperture (Fig. 4). and found two low-Pmax comets, while Hartley 2 seems to be a high-Pmax in that comparison. They also integrated the polarization as a function of the nucleus distance and did not find any variation 4. Discussion as in our near IR results (Fig. 5). Similarly, they did not find any difference between the polarization in the jets and in the background 4.1. Comparison to other observations of Hartley 2 coma. They interpret their observations by light scattered by ‘grain aggregates’ larger than 2 lm, which do not break up within several Numerous large icy and dusty particles are present in the inner days after leaving the nucleus. coma, they have very slow motions and weak interaction with the The polarimetric spectral gradient is positive in the visible do- solar radiation field (A’Hearn et al., 2011). The slope smaller than main (increasing polarization values with increasing wavelength) the nominal value of 1, found in the azimuthally integrated and neutral in the near infrared domain as usually observed for intensity profiles close to the optocenter, may be due to large slow comets. The behavior of the spectral gradient in the near infrared particles detected in situ. A similar behavior is suggested for Comet domain seems to depend on the size of the grains in the aggregates 67P/Churyumov–Gerasimenko by remote observations (Hadamcik and probably on the average composition of the dust, e.g., for Co- et al., 2010; Tozzi et al., 2011a). Another interpretation is an in- met Hale–Bopp it increases up to a maximum reached for a wave- crease in brightness due to dust particles fragmentation. The high length of about 1200 nm; for comet Halley the positive gradient is activity of the comet and the important presence of ice chunks in smaller and stabilizes at about 1000 nm (Hadamcik and Levasseur- its inner coma suggest dust fragmentation from ice sublimation Regourd, 2003a). as the driving process for the radial intensity behavior (Lisse et al., 2011). 4.2. Comparison with other JFCs A red color around the optocenter was deduced from the photometric observations of Lara et al. (2011) in the visible domain, on Hartley 2 presents some characteristics such as low dust-to-gas October 27–28 (between our two observational rounds); the red ratio and strength of the mid-infrared silicate feature similar to color decreased further away. The authors suggest that the change other JFC (Kelley and Wooden, 2009; Meech et al., 2011). The high- 782 E. Hadamcik et al. / Icarus 222 (2013) 774–785

Fig. 9. Polarization maps. North is up and East is left. + optocenter. (a) Field of view 9000 km (a and c) and 3000 km (b). (a) October 6.05 (horizontal star-trails), (b) November 2.96, (c) November 4.24. (b) Field of view 3000 km. Sun direction indicated by a solid black line, jets directions (from Fig. 2) indicated by white lines. (a) October 6.16. (b) October 6.99. (c) November 2.96. (d) November 4.24. (e) November 5.19. (f) November 6.20. (For interpretation of the references to colours in this figure legend, the reader is referred to the web version of this paper.) er value of P in the inner coma may be attributed to large dark par- perihelion had a limited activity but features were detected in ticles as in the coma of 9P/Tempel 1 or to very fluffy particles made intensity in the coma without any correlated difference in polariza- of micron- to submicron-sized grains as in the ‘plume’ of Comet 9P/ tion between the different parts of the coma. The presence of large Tempel 1 or in the dust of Comet 67P/Churyumov–Gerasimenko particles was suggested (Hadamcik et al., 2010; Tozzi et al., 2011a). after perihelion (Hadamcik et al., 2007a, 2010). In these cases the motion of the fluffy particles in the coma was fast and the high 4.3. Tentative interpretation of light scattering results by simulations polarization was also detected in features at large distances from the optocenter. This is not the case for Comet Hartley 2 (except Actually, this is the first time that solid particles in the inner the tailward feature). Comet Churyumov–Gerasimenko before coma are observed as mixtures of clean ice particles and dark dust E. Hadamcik et al. / Icarus 222 (2013) 774–785 783

Fig. 10. (Left) Pmax vs decreasing size parameter of grains in aggregates with an average 50 lm size. Transparent material: silica and absorbing material carbon. Right. Pmax vs particles size: 1. Compact quartz particles. 2. Mg–silicate aggregates with lm-sized and larger grains. 3. Fluffy SiO2 aggregates with grains smaller than 0.2 lm. 4. Dark poly- HCN aggregates with of 20 lm average grains size. 5. Carbon aggregates with 20 lm average grains. particles (A’Hearn et al., 2011). The latter might be mixtures of and ice materials have not exactly the same refractive indices dark organics and transparent materials such as Mg–silicates. From but they are transparent materials and the general trend for the the EPOXI observations results, the icy grains size seems to be lar- polarization is probably similar. ger than 1 lm and may form relatively compact aggregates; a sub- As a general rule, the polarization values decrease with the de- stantial porosity is suggested for rock and ice mixtures. The actual crease of the size of the particles (aggregates or compact particles spatial distribution of the clean ice around the dark dust particles is larger than the wavelength). However, polarization behaves the unknown. However, hints about ice distribution can be found from opposite way for transparent materials in fluffy aggregates (poros- the following observations. The clean ice particles progressively ity >95%) with grain size smaller than 0.2 lm (size parameter disappear after ejection from the nucleus, with a sublimation time- smaller than 1; Fig. 10b) or for submicron-sized compact particles. scale within 0.67 and 67 h, depending on size and composition of This is not the case for the ice particles in the coma. When the ice the initial particles (Lisse et al., 2011). Then, ice particles should grains sublimate, P would increase (Fig. 10a), but their spatial not be found farther than 200 km from the nucleus. If ice material number density decreases and the light they scatter decreases. coats the dust particles, ice sublimation can cause the particles to The size of the pure ice aggregates decreases by fragmentation or fragment in pieces of porous particles (Aumatell and Wurm, collapse and P decreases overall (as on curve 2 or 1 in Fig. 10b). 2011), which eventually move along the tail direction. This process If the albedo is considered, the majority of the particles are dark. results in a decrease of the average particle size, and an increase of The size of the rocky and refractory organics grains do not change the average absorption coefficient. The evolution of the particle a lot for the dark aggregates. They fragment with the sublimation porosity is not clear: in some cases, one may expect particles to of the ice inclusions and P decreases. collapse because of the stresses due to sublimation, while in other Even if the complex refractive index is about constant as a func- cases, one may argue that the big voids of the initial fluffy structure tion of the wavelength, it is important for the polarization behavior could be recovered through gentle ice evaporation. (Kimura et al., 2003; Petrova et al., 2004) and its spectral gradient The light scattering properties of an ensemble of particles, such (Kimura et al., 2006). Hadamcik et al. (2007b) built a cometary- as the linear polarization, depend on the grain-size distribution, particle analog with mixtures of transparent vapor-condensed aggregates-size distribution, refractive indices of the constituents magnesio-silicates, grey iron–silicates and dark and fluffy carbon- and the aggregates structure. This is established through the vari- black (with submicronic grains inside the aggregates). They ous numerical models, as well as through laboratory experiments. showed that carbon-black particles are necessary to reproduce From dust aggregation processes, grains can make fluffy porous the polarimetric phase curve shape and the positive spectral gradi- aggregated particles – e.g. ballistic random cluster–cluster aggre- ent. Kolokolova and Kimura (2010) attribute the positive spectral gation (BCCA)–, or compact particles – e.g. aggregation with two gradient in the visible range to the presence of fluffy aggregates migrations (BAM2)–, or particles of intermediate porosity – e.g. with submicron-sized grains and the neutral polarization in the ballistic particle-cluster aggregation (BPCA) – (Kimura, 2001; Lasue near infrared range to more compact particles. and Levasseur-Regourd, 2006; Das and Sen, 2011; Das et al., 2011). For Hartley 2, the decrease in polarization with larger nucleus In Hadamcik et al. (2006, 2009), the authors discuss the case of par- distances is consistent with progressive ice sublimation and se- ticles made of mixtures of transparent silica and absorbing organic quences of particle fragmentations. The positive spectral gradient compound, using laboratory simulations. They show how the dif- in the visible domain is due to the presence of fluffy dark aggregates ferent physical parameters determine light polarization properties. with submicron- to micron-sized grains. The neutral polarization in The positive polarization phase curve is indicative of the maximum the infrared and the absence of structure on the polarization maps polarization value. Fig. 10a shows Pmax as a function of the grain indicate that the aggregates are larger than 2 lm in the whole field size for sizes ranging from nanometers to thousands of microme- of view and behave as large compact particles in the infrared. ters. The average size of the aggregates is in the 40–60 lm range. Using the size parameter (X = pd/k) where d is the typical diameter of the grains in Fig. 10a, or of the aggregates in Fig. 10b, and k the 5. Conclusion wavelength, allows exploring a larger range of sizes and wavelengths. For grey materials as silica or carbon-black (about con- Light-scattering observations of Comet 103P/Hartley 2 were stant refractive indices in the visible wavelength range), the data performed in France and India during two periods: on October points for each material follow the same trends (Fig. 10a). Silica 2010 about 20 days before perihelion and on November 2010 784 E. Hadamcik et al. / Icarus 222 (2013) 774–785 about 8 days after perihelion at the time of the EPOXI mission. A Hadamcik, E., Levasseur-Regourd, A.C., 2003b. Imaging polarimetry of cometary solar-antisolar asymmetry is observed on both the intensity and dust: Different comets and phase angles. J. Quant. Spec. Radiat. Trans. 79–80, 661–679. polarization images. The azimuthal integrated intensity decreases Hadamcik, E., Levasseur-Regourd, A.C., 2009. Optical properties of dust from Jupiter in a nominal way (slope 1) for the two periods at optocentric dis- family comets. Planet. Space Sci. 57, 1118–1132. tances larger than few hundred kilometers. The linear polarization Hadamcik, E., Renard, J.-B., Levasseur-Regourd, A.C., Lasue, J., 2006. Light scattering by fluffy particles with the PROGRA2 experiment: Mixtures of materials. J. measured through narrow-band cometary blue and red filters cor- Quant. Spec. Radiat. Trans. 100, 143–156. respond to low-Pmax comets and decreases with increasing aper- Hadamcik, E., Levasseur-Regourd, A.C., Leroi, V., Bardin, D., 2007a. Imaging tures. The spectral gradient in polarization is positive in the polarimetry of the dust coma of Comet Tempel 1 before and after Deep Impact at Haute-Provence Observatory. Icarus 190, 459–468. visible domain (as generally observed for comets) and neutral in Hadamcik, E., Renard, J.-B., Rietmeijer, F.M., Levasseur-Regourd, A.C., Hill, H.G.M., the near-infrared. The polarization near the optocenter changes et al., 2007b. Light scattering by fluffy Mg–Fe–SiO and C mixtures as cometary from day to day, this variation being possibly related to variations analogs (PROGRA2 experiment). Icarus 190, 660–671. Hadamcik, E., Renard, J.-B., Levasseur-Regourd, A.C., Lasue, J., Alcouffe, G., Francis, in activity due to the nucleus rotation. Short jets ejected in the so- M., 2009. Light scattering by agglomerates: Interconnecting size and absorption lar direction are more polarized than the tailward feature, which is effects (PROGRA2 experiment). J. Quant. Spec. Radiat. Trans. 110, 1755–1770. itself more polarized than the surrounding coma. Two jets are seen Hadamcik, E., Sen, A.K., Levasseur-Regourd, A.C., Gupta, R., Lasue, J., 2010. to be ejected in two preferred projected direction at about 20° Polarimetric observations of Comet 67P/Churyumov–Gerasimenko during its 2008–2009 apparition. Astronom. Astrophys. 517, A86. clockwise and 30° anticlockwise from the solar direction. 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