Oral presentations and abstracts

Long-periodic comet C/2011 KP36 (Spacewatch), defined by Bauer et al. 2013 (Aph. J. 773:22) as a Scattered Disc Object, has a period of around 238 years and perihelion distance q=4.88 au. Its semi-major axis (a=38.41 au) is larger than aphelion distance of Neptune (a_N=30.07 au). Comprehensive observations of the comet were carried out at the 6-m BTA telescope of the Special Astrophysical Observatory (Russia) with the multimode focal reducer SCORPIO-2.

Long-slit spectra in the visible and photometric and linear polarimetric images with the g-sdss and r-sdss filters were obtained on November 25, 2016, after perihelion passage, when heliocentric and geocentric distances of the comet were 5.06 au and 4.47 au, respectively. The observations were done at the phase angle 9.57 deg. Two strong jet-like structures in solar and antisolar directions and two short and narrow jet features in a direction almost perpendicular to the Sun–comet direction were revealed in the coma. The cometary activity was characterized by Afρ values of 1065±11 cm in the g-sdss filter and 1264±17 in the r-sdss filter.

The spectrum of comet C/2011 KP36(Spacewatch) we observed shows emissions from the ion CO⁺. It is second comet, for which we detected emissions at large heliocentric distances more 4 au.

In the near-nucleus coma, the polarization is about 5% (in absolute value), then it decreases, reaching the minimal values about 2% in the region of 10000–20000 km. The polarization increases up to 3.7–5% at the distance of 25000–45000 km, and then the degree of polarization changes with wave-like variations up to 1–4%, depending on the cut direction, with increasing distance up to ~10⁵ km. See Figure 1 for details.

On average, the (g–r) color index of the dust varies with increasing distance from the nucleus. In the near-nucleus region, the color index is about 0.7^m. At distances from about 10000 to 20000 km from the photocenter, dust color is bluer (~0.33^m) in comparison with the Sun color (color indices (g–r) for the Sun is 0.44^m

To characterize the dust in this comet, we modeled both polarization and color in the directions of all jets as well as for the regular coma. We considered a variety of materials typical for cometary dust and Centaurs (including porous particles) and their mixtures and modeled the dust as ensembles of rough spheroids of a variety of obliquity and size. Our modeling allowed us to reproduce not only specific values of color and polarization but also their change with the distance from the nucleus using realistic ideas about possible changes in particle size and composition as they move through the coma and also to compare the dust in different cometary features.

Figure 1 A spatial distribution of polarization degree in comet C/2011KP36 (Spacewatch) in the r-sdss filter obtained at the phase angle 9.6 deg. The left image shows the polarization map for the whole coma with its associated scale bar in percentage on the top of the image, while the image on the right displays the coma area around the nucleus in an enlarged scale. The location of the optocenter is marked with a black cross, and jets with black lines. The arrows point in the direction to the Sun, North (N), East (E), and negative velocity vector of the comet as seen in the observer’s plane of sky (V). Negative distance is in the solar direction.

How to cite: Ivanova, O., Kolokolova, L., Luk’yanyk, I., Kreshenok, V., Rosenbush, V., Kiselev, N., Afanasiev, V., and Kirk, Z.: Scattering properties of dust in C/2011 KP36 (Spacewatch), Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-346, https://doi.org/10.5194/epsc2020-346, 2020.

Abstract

We present detailed polarimetric analysis of pre-perihelion STEREO observations of Kreutz group comet C/2010 E6 (STEREO), focusing on the changes in polarisation with decreasing heliocentric distance in the pre-perihelion STEREO spacecraft hourly observations of the comet in March 2010. We utilise a bespoke image analysis method for the coronagraph data, finding the Stokes parameters for comet and its tail. The results show a clear variability in polarisation, which can be attributed to sublimation of refractory material.

1. Introduction

Polarimetry is a useful tool for remote observations of comet refractory material, since it can provide constraints on its structure and composition. Near-Sun comets like C/2010 E6 (STEREO) are interesting partly for their frequency – Kreutz group comets represent the single largest comet family [1] – but mostly for the intense near-Sun environment they experience. The comet’s approach to perihelion is likely accompanied by sublimation of refractory material, therefore we can, by studying comet's changing polarimetric signature during its approach to the Sun, extract information about the composition of refractory material in cometary dust. The full analysis is presented in [2].

2. Observations

Comet C/2010 E6 (STEREO) belongs to the Kreutz family of sungrazing comets, which share very similar orbital parameters and are thought to be the remnants of a larger parent comet [1,3,4].

The comet was imaged by the twin STEREO (Solar Terrestrial Relations Observatory) spacecraft in March 2010; A (Ahead) and B (Behind). In this work only COR2 visible light (bandpass 650-750 nm) coronagraph imagery is used [5].

Permanently in the light path of COR2 is a linear polariser which can be set at three angles: 0º, 120º, and 240º relative to a reference position. A sequence (triplet) of images using the three different polariser angles in quick succession is taken once per hour.

3. Methodology

The image analysis was conducted using a bespoke semi-automated routine fine-tuned for the use with STEREO COR2 data.

Firstly, the image triplets undergo standard pre-processing using the SECCHI_PREP routine from the SolarSoft library for IDL [6]. Then the comet head is found interactively in the image triplets.

Orbital parameters are used to find the plane of the comet's orbit, in which the dust tail is assumed to lie; all data points in the images are then mapped onto this plane. From this, the phase angle - between the Sun, the comet, and the spacecraft – can be determined.

Comet tail is then traced in each separate image. Its transverse cross-sections first have their background removed using a cubic fit and are then truncated (integrated) to a single point for each longitudinal step.

The three resulting polarised intensity vectors from the image triplet are then aligned, and the polarimetric properties (Stokes parameters Q, U, and I) calculated. The method is similar to, but distinct from, the one used by [7,8].

4. Results

We present two sets of plots of STEREO/SECCHI/COR2 observations of comet C/2010 E6 (STEREO): polarimetric (Figure 1) and photometric (Figure 2). All data is plotted against heliocentric distance. Since the data would contain considerable overlap otherwise, three different offsets are used to stagger the data.

The phase angle information is lost in these plots. STEREO-A phase angle observations range between ~35º and ~25º (right to left). The phase angle range in STEREO-B observations is ~135º to ~105º (right to left). The phase angle variation along the tail is small for each observation.

5. Discussion & Conclusions

Prior to perihelion, at which it likely disintegrated, comet C/2010 E6 (STEREO) exhibited a variety of peculiar behaviours - both photometric and polarimetric - which distinguish it from most other comet observations [9].

Most notable is the sudden drop in polarisation of the comet nucleus at ~20 R_⨀, which then spread slowly along the comet tail. This is accompanied later (~15 R_⨀) by a broadening of the intensity peak from the nucleus further down the tail. The broadening and brightening are consistent with those observed by e.g. [4].

The polarimetric signature of near-zero (STEREO-B, high phase angles) or negative (STEREO-A, low phase angles) polarisation spreading from the near-nucleus region along the tail is consistent with sublimation of silicate particles at local temperatures exceeding their sublimation point. This effect propagates slowly both through the ablating surface and through the dust tail, as sublimation acts as a source of cooling. Thus the observed negative polarisation at low phase angle may explained as the response of the as of yet unsublimated fluffy dust particles of primarily silicate composition.

Acknowledgements

We would like to thank M. Knight, K. Battams, and V. Andretta for their help and collaboration. This research is supported through the UK Science and Technology Facilities Council and has made use of data and/or services provided by the IAU’s Minor Planet Center.

References

^{[1] Battams, K. and Knight, M. M.: SOHO comets: 20 years and 3000 objects later, Phil. Trans. R. Soc. A, Vol. 375: 20160257, 2017}

^{[2] Nežič, R. et al.: Polarimetric analysis of pre-perihelion STEREO observations of Kreutz group comet C/2010 E6 (STEREO), 2020, in prep.}

^{[3] Jones, G. H. et al.: The Science of Sungrazers, Sunskirters, and Other Near-Sun Comets, Space Sci Rev, Vol. 214: 20, 2018}

^{[4] Knight, M. M. et al.: Photometric Study of the Kreutz Comets Observed by SOHO from 1996 to 2005, AJ, Vol. 139: 3, pp. 926-949, 2010}

^{[5] Howard, R. A., Moses J. D., and Socker D. G.: Sun Earth Connection Coronal and Heliospheric Investigation (SECCHI), Proceedings of SPIE, Vol. 4139-26, 2000}

^{[6] Freeland, S. L. and Handy, B. N.: Data Analysis with the SolarSoft System, Solar Physics, Vol. 182, pp. 497-500, 1998}

^{[7] Thompson, W. T.: Linear polarization measurements of Comet C/2011 W3 (Lovejoy) from STEREO, Icarus, Vol. 261, pp. 122-132, 2015}

^{[8] Thompson, W. T.: Changing linear polarization properties in the dust tail of comet C/2012 S1 (ISON), Icarus, Vol. 338, pp. 113533, 2020}

^{[9] Kiselev, N. et al.: Comets, Chapter 22 in Polarimetry of Stars and Planetary Systems (ed.: Kolokolova, L. et al.), CUP, pp. 379-404, 2015}

How to cite: Nezic, R., Bagnulo, S., and Jones, G.: Effects of heliocentric distance on polarisation of comet C/2010 E6 (STEREO), Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-817, https://doi.org/10.5194/epsc2020-817, 2020.

Summary

Linear polarization observations have suggested the presence of dust particles that scatter solar light within cometary comae and the interplanetary dust cloud. Recent progresses result from in-situ observations or remote observations from large telescopes, while avoiding contamination from atmospheric airglow. The interpretation of polarimetric observations, at given phase angles and wavelengths, is a powerful tool to better understand the composition and physical properties of these irregular dust particles and their formation processes. It now provides clues to the origin of the Solar System and to some characteristics of dust in stellar systems, as discussed in [1].

Along a line-of-sight within the interplanetary dust cloud, both solar distance (R) and phase angle (a) vary, meaning that the intensity and linear polarization (P) of solar light scattered by interplanetary dust (i.e. zodiacal light) need to be inverted to retrieve some local properties. In the near-ecliptic symmetry plane of the cloud, P(a) phase curves are smooth, with a slightly negative branch below 20° and a maximum by 90°. For a constant a=90°, P increases with R, at least between 0.5 au and 1.5 au. Numerical and experimental simulations (in the laboratory and under microgravity conditions) of such results suggest i) the presence in dust of low-absorbing minerals and absorbing complex organics, with a significant amount of fluffy aggregates, ii) some sublimation of semi-volatile organics with decreasing R and increasing temperatures [2; also 3-5 for reviews].

In situ observations have, up to now, only been done on board Giotto during its flybys, allowing us to suggest the presence of a fragment in the coma of 26P/Grigg-Skjellerup and to derive very low values for geometric albedos (about 3%) and densities (50 to 500 kg m⁻³) of 1P/Halley dust particles [6-7].

Dust whole-coma remote observations, which avoid contamination from gaseous emissions with narrow interference filters, have been used by various teams to obtain P(a) phase curves. As for the zodiacal light, they are smooth; also P (whenever positive) increases with wavelenght for a given a. Numerical and experimental simulations are consistent with a dust population of irregular and possibly fractal structure, with complex optical indices suggesting mixtures of minerals and more absorbing organics [8-10]. Polarimetric in situ observations and remote imaging of comae (at given phase and wavelenght) suggest that the properties of dust are quite different in dust jets than elsewhere in a dust coma, and in Oort-cloud comets than in periodic comets [9, 11-12]. Such trends fairly agree with the ground-truth provided by the Rosetta rendezvous with 67P/Churyumov-Gerasimenko [13 for a review]. Finally, experimental simulations of 67P pre-perihelion brightness phase curves provide clues to an important amount of organic compounds and to fluffy aggregates with sizes above 10 micrometres [14].

Back in time, polarimetric results on cometary dust contribute to a better understanding of the formation of dust particles, in external regions of the protosolar disk. They likely resulted from submicron-sized grains accreted at low collision velocities, with further addition of minerals processed near the proto-Sun [e.g. 15-16]. At the Late Heavy Bombardment epoch, enormous influxes of interplanetary dust might then have provided a massive delivery of cometary carbonaceous compounds on telluric planets [1; 17].

Beyond in space, properties of interplanetary dust may suggest improvements in the modelling of observations of protoplanetary and debris disks. Updating results on the modelling of the dust continuum emission of the protoplanetary disk around MWC 758 [18], with moderately porous spherical particles and the above-mentioned phase functions retrieved by Rosetta, provides a better agreement between synthetic and observed maps of emission at sub-millimetre wavelengths. Besides, observations of debris disks have now shown that light scattering properties of dust in debris disks are far from those predicted by Mie theory and in favour of irregular dust particles, as pointed out essentially by phase functions analysis [19-20]. These results can greatly benefit from in situ and remote cometary observations and from advanced modelling of cometary dust, and at the same time provide examples of dust properties in other stellar systems to put our own Solar System into context [1].

Polarimetric observations of dust clouds in the Solar System provide clues to the properties of dust particles within cometary comae and the zodiacal cloud. Such results are already noticed to improve the modelling of dust in protoplanetary and debris disks, through the use of irregular porous particles properties instead of more conventional compact spherical ones.

Comet Interceptor, ESA F-Class mission to an Oort-cloud comet, to be launched in 2028, should provide a better understanding of its local polarimetric properties with EnVisS experiment. Meanwhile, remote polarimetric observations of comets might contribute to the target selection.

References

[1] Levasseur-Regourd, A.C. et al., PSS, 186, 104896, 2020.

[2] Hadamcik, E. et al., PSS, 183, 104527, 2020.

[3] Levasseur-Regourd, A.C. et al.: In Interplanetary dust, E. Grün et al. eds, Springer, 57-94, 2001.

[4] Lasue, J. et al., In Polarimetry of stars and planetary systems, L. Kolokolova et al. eds, CUP, 419-436, 2015.

[5] Lasue et al., PSS, 104973, 2020 (in press).

[6] Levasseur-Regourd et al., PSS, 41, 167-169, 1993.

[7] Fulle, M. et al., AJ, 119, 1968-1977, 2000.

[8] Lasue, J. et al., Icarus, 199, 129-144, 2009.

[9] Kiselev, N. et al., In Polarimetry of stars and planetary systems, L. Kolokolova et al. eds, CUP, 379-404, 2015.

[10] Muñoz, O. et al., ApJ, 247:19, 2020.

[11] Hadamcik, E. et al., MNRAS, 462, S507-S515, 2017.

[12] Rosenbush, V.K., et al., MNRAS, 469, S475-S491, 2017.

[13] Levasseur-Regourd, A.C. et al., Space Sci. Rev., 214:64, 2018.

[14] Levasseur-Regourd, A.C. et al., A&A, 630, A20, 2019.

[15] Fulle, M. & Blum, J., MNRAS. 469, S39-S44, 2017.

[16] Lasue, J. et al., A&A, 630, A28, 2019.

[17] Nesvorný, D., et al., ApJ, 713, 816-836, 2010.

[18] Baruteau, C., et al., MNRAS, 486, 304-319, 2019.

[19] Milli, J., et al., A&A, 599, A108, 2017.

[20] Milli, J., et al., A&A, 626, A54, 2019.

How to cite: Levasseur-Regourd, A.-C., Baruteau, C., Lasue, J., Milli, J., and Renard, J.-B.: Linear polarization as a tool to characterize interplanetary, cometary, and extrasolar dust particles , Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-391, https://doi.org/10.5194/epsc2020-391, 2020.

Context

Debris discs represent the ultimate stage of planetary formation, after the initial gas-rich protoplanetary disc dissipated and potential giant planets have already formed in the system. Infrared observations have shown that at least 20% of main-sequence stars host a dusty debris disc [1], i.e. a massive analog to the Edgeworth-Kuiper belt in the solar system, typically detected at tens to hundreds of astronomical units from their parents stars. Considering the short lifetime of the dust particles compared to the system age, this dust population is believed to originate from collisions of a population of kilometre-sized parent bodies, called planetesimals, reminiscent of Kuiper belt objects or comets in our solar system. Collisions constantly replenish the disc and create smaller and smaller dust particles, until the smallest micron-sized particles leave the system expelled by the radiation pressure of the central star. 

Polarimetry as a tool to study debris discs in scattered light

Extreme adaptive optics instruments working at optical / near-infrared wavelengths have revealed exquisite details on debris discs [2,3], allowing to extract the optical properties of the dust particles such as the phase function, the degree of polarisation and the spectral reflectance. These are three powerful diagnostic tools to understand the physical properties of the dust population : the size, shape and composition of the micron-sized dust particles. 

An ideal laboratory: HR4796

It is however very rare to be able to combine all those three observables for the same system, because this requires different high-contrast imaging techniques to suppress the starlight and reveal the faint scattered light emission from the dust. Due to its properties, the narrow ring detected at 77 astronomical units around the A-type star HR4796 is a notable exception, with both unpolarised and polarised images covering the optical and near-infrared wavelengths [3]. Two new studies have revealed the intriguing properties on this system. The first one [4] revisited optical polarised light images obtained earlier with SPHERE/Zimpol [5] (left figure) and proposed an original technique to retrieve with enhanced fidelity the scattering phase function of the dust in polarised light (inset of the right figure). This method more accurately accounts for projection effects that can enhance the surface brightness of a disc along the semi-major axis if the scale height of the ring is sufficiently large.  The second study [5] extracted the degree of polarisation of the disc in the near-infrared from 20 to 120 degrees. Neither of those two studies can fully reconcile the degree of polarisation of the dust population, its scattering phase function and the particles blow-out size with a single scattering theory (right figure). 
In this contribution, we propose to discuss the dust properties compatible with current and new observations of this disc. We will base the discussion on laboratory experiments and analogies with cometary dust particles, rather than relying on scattering theories assuming spherical dust particles. We show that this interdisciplinary approach moving away from the Mie theory enables to find interesting candidates to reconcile the optical properties and thermal behaviour of the dust particles in this system.