Extremely Low Linear Polarization of Comet C/2018 V1 (Machholz–Fujikawa–Iwamoto)

Extremely low linear polarization of comet C/2018 V1 (Machholz–Fujikawa–Iwamoto) Evgenij Zubko a,*, Ekaterina Chornaya b,c, Maxim Zheltobryukhov c, Alexey Matkin c, d,e,f g Oleksandra V. Ivanova , Dennis Bodewits , Anton Kochergin b,c, Gennady Kornienko c, Igor Luk’yanyk d, Dean C. Hines h, Gorden Videen i,j a Humanitas College, Kyung Hee University, 1732, Deogyeong-daero, Giheung-gu, Yongin-si, Gyeonggi-do 17104, South Korea b Far Eastern Federal University, 8 Sukhanova St., Vladivostok 690950, Russia c Institute of Applied Astronomy of RAS, 10 Kutuzova Emb., Saint-Petersburg 191187, Russia d Astronomical Observatory, Taras Shevchenko National University of Kyiv, 3 Observatorna St., Kyiv, 04053, Ukraine e Astronomical Institute of the Slovak Academy of Sciences, SK-05960 Tatranska� Lomnica, Slovak Republic f Main Astronomical Observatory of National Academy of Sciences, 27 Akademika Zabolotnoho St., Kyiv, 03143, Ukraine g Auburn University, Physics Department, Auburn, AL 36849-5319, USA h Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA i Space Science Institute, 4750 Walnut Street, Boulder Suite 205, CO 80301, USA j Department of Astronomy and Space Science, Kyung Hee University, 1732, Deogyeong-daero, Giheung-gu, Yongin-si, Gyeonggi-do 17104, South Korea We measured the degree of linear polarization P of comet C/2018 V1 (Machholz-Fujikawa-Iwamoto) with the broadband Johnson V filter in mid-November of 2018. � � Within a radius of ρ � 17,000 km of the inner coma, we detected an extremely low linear polarization at phase angles α � 83 –91.2 and constrained the polarization maximum to Pmax � (6.8 � 1.8)%. This is the lowest Pmax ever measured in a comet. Using model agglomerated debris particles, we reproduced the polarimetric response of comet C/2018 V1. Four retrieved refractive indices closely match what was experimentally found in Mg-rich silicates with little or no iron content. Moreover, the size distribution of the agglomerated debris particles appears in good quantitative agreement with the in situ findings of comet 1P/Halley. The dust model of polarization of comet C/2018 V1 suggests a strongly negative polarization with amplitude |Pmin| � 5%–7%; whereas, an interpretation based on gaseous emission requires no negative polarization at small phase angles. This dramatic difference could be used to discriminate gaseous- emission and dust explanations in low-Pmax comets in future. Introduction percent. Clearly, P in Eq. (1) is a sign-dependent characteristic whose negative value corresponds to the case of I? < I|| and positive value to I? When initially unpolarized solar radiation is scattered from the gas > I||. and dust surrounding a comet, it acquires a partial linear polarization. The degree of linear polarization P varies significantly with the Its quantity is described in terms of the degree of linear polarization P: observation/illumination geometry of the comet. For instance, at small � phase angles (α < 25 ), the polarization is predominantly negative. At Q I? À Ik � P ¼ À ¼ (1) larger phase angles (α > 25 ), P takes on positive values (e.g., Chernova I I? þ Ik et al., 1993; Levasseur-Regourd et al., 1996). In both domains of α, the Here, Q and I denote the Stokes parameters (Bohren and Huffman, angular profile of polarization tends to appear in a form of a simple, 1983); I? and I|| are the intensity of components of the scattered light bell-shape branch. While the branch of negative polarization is charac- that are polarized perpendicularly to the scattering plane and within the terized by a minimum polarization Pmin at phase angle αmin, the positive scattering plane, respectively. The polarization P is often expressed in polarization branch reaches its maximum Pmax at a phase angle αmax. It is interesting to note that laboratory measurements of light scattering by particles and highly absorbing particles. Within this two-component single cometary-analog particles and meteorite particles demonstrate framework, the coma in comets with low Pmax predominantly consists ~ qualitatively similar behavior (e.g., Munoz et al., 2000; Frattin et al., of Mg-rich silicates; whereas, in the high-Pmax comets, it is abundant 2019), although specific values of Pmin, αmin, Pmax, and αmax may with carbonaceous materials. significantly differ from one target to another. What emerges from Fig. 1 is a disparity between the number of high- Comets demonstrate a significant dispersion of theirP max (Chernova Pmax comets and low-Pmax comets. There are well over a dozen repre- et al., 1993) and were initially classified as high-Pmax or low-Pmax sentatives of Pmax > 20%, and only a few comets with Pmax � 10%. There (Levasseur-Regourd et al., 1996). A third class was established based on are three comets, 23P/Brorsen–Metcalf, 27P/Crommelin, and C/1975 the very high Pmax, observed in comet C/1995 O1 (Hale-Bopp) N1 (Kobayashi–Berger–Milon) whose polarimetric response appears (Hadamcik and Levasseur-Regourd, 2003a). However, these segregated with confidence at the bottom limit inFig. 1; all these observations were classes seemingly result from limited data, as a recent analysis of the reported in Chernova et al. (1993). One more comet, C/1996 Q1 � extended dataset in Zubko et al. (2016) suggests a continuous transition (Tabur), had P � 13–15% at α � 85.5 during a period of normal activity. from the low-Pmax comets to the high-Pmax comets, and further to However, for no obvious reason, this comet suddenly lost its activity and C/1995 O1 (Hale-Bopp). This can be seen in Fig. 1 where we reproduce a coma. This process was accompanied with a decrease of polarization to � compilation of polarimetric measurements of 23 different comets P � 10% at α � 85 (Kikuchi, 2006). Thus, strictly speaking, only three considered by Zubko et al. (2016). comets revealed a persistent maximum of polarization Pmax � 10%. It is The dispersion was initially interpreted in terms of the different worth noting that the last of them was observed about thirty years ago, relative contributions of the gaseous emission into light scattering from in 1989. In this manuscript we report our polarimetric observations of a cometary coma (Chernova et al., 1993) because such emission is long newly discovered comet C/2018 V1 (Machholz–Fujikawa–Iwamoto) at � � known to be highly depolarized. For instance, complex gaseous mole- large phase angles, α � 83 –91.2 This is only the fourth example of a cules have Pmax ¼ 7.7% (e.g., Le Borgne et al., 1987 and therein). A few comet with such a low maximum of positive polarization (Fig. 1). years later, Levasseur-Regourd et al. (1996) attributed the dispersion of Pmax in comets to different microphysical properties of their dust. Until Observations and data processing recently, both these interpretations were considered to be possible (e.g., Kiselev et al., 2015). Zubko et al. (2016) demonstrated that the gaseous We conducted polarimetric observations of comet C/2018 V1 shortly emission model is inconsistent with several existing observations of after its discovery on November 7, 2018. The comet passed perihelion at comets, and they developed a simple model that can satisfactorily 0.387 au on December 3, 2018. Weather conditions were favorable on reproduce the dispersion of Pmax in comets using rigorous simulations of three dates, November 16, 17, and 18. However, on the first date, the light scattering by particles with highly irregular and fluffy morphology, observations were interrupted by clouds when only two series of images and that the different polarizations are a result of having different ma- were taken. Nevertheless, they appear in very good accordance with terial compositions. The modeling was based on an assumption that the results obtained on the other two nights and, therefore, we include them light-scattering signal was dominated by two different types of particles, into the analysis. Observations were made with the 22-cm telescope one with low (Im(m) � 0.01) and another with high (Im(m) > 0.4) located at the Ussuriysk Astrophysical Observatory (code C15), which imaginary part of complex refractive index. While the former compo- operates within the International Scientific Optical Network (ISON). We nent is consistent with Mg-rich silicates (Dorschner et al., 1995), the used CCD detector FLI ProLine PL4301E that has a resolution of latter one appears consistent with some organics (Jenniskens, 1993) and 2084 � 2084 pixels and pixel size of 24 μm. The field of view of the CCD amorphous carbon (Duley, 1984). The presence of these species in detector is 326 � 326 arcmin with angular resolution of 9.39 � 9.39 comets is well established (e.g., Fomenkova et al., 1992; Cochran et al., arcsec per pixel. The Johnson V filter was used for polarimetric obser- 2015). The dispersion of polarization in comets can be explained then by vation of the comet. We use a dichroic polarization filter (analyzer), � � variation of a sole parameter, the volume ratio of the weakly absorbing which rotated sequentially through three fixed position angles 0 , þ60 , Fig. 1. Degree of linear polarization P measured with continuum filters in the blue-green part of the spectrum in 23 different comets as a function of phase angleα . Compilation is adapted from Zubko et al. (2016), where references and some details of the observation of each specific comet are given. � and þ120 The details of observations are summarized in Table 1. the polarimetric response in C/2018 V1 appears accidently consistent Morphology of the C/2018 V1 coma can be seen in Fig. 2. with the angular profile of polarization of fluorescence from complex The obtained data have been processed using the Image Reduction molecules and ions (shown with the dashed line), like C2 molecules (Le and Analysis Facility (IRAF) software system for reduction and analysis Borgne et al., 1987).

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