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A&A
Volume 668, December 2022
Article Number A97
Number of page(s) 10
Section Planets and planetary systems
DOI https://doi.org/10.1051/0004-6361/202244853
Published online 09 December 2022

© Y. G. Kwon et al. 2022

Licence Creative CommonsOpen Access article, published by EDP Sciences, under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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1 Introduction

Comets preserve the least-altered planetesimals from the nascent solar system. In particular, comets from the Oort cloud (Oortcloud comets, OCCs), namely, a reservoir of objects at the outskirts of the solar system (~103–5 au from the Sun; Dones et al. 2015), have spent most of their lives in locations where sunlight can hardly reach them. This sets these objects among the most important classes of primitive objects to relate their properties back to the early solar system environment. This motivates the upcoming “Comet Interceptor” mission, nominally scheduled for launch in 2029, to explore the early solar system environment via OCCs (Snodgrass & Jones 2019).

Observations over recent decades have revealed a heterogeneous nature in OCCs. Departing from the traditional view of comets extending a conspicuous dust tail thousands of kilometers from the nuclei (Krishna Swamy 2010), OCCs in a low-to-moderate activity level (e.g., Licandro et al. 2019; Garcia et al. 2020), or even those without dust tails (so-called Manx comets; Meech et al. 2016) have been found, implying a variety of origins for comets found in the present-day Oort cloud. Given the lack of in situ data on OCCs, characterization of their dust constituents and context are of particular importance for a better understanding of their formation and subsequent evolutionary history and support of future space missions.

Here, we report a new near-infrared spectropolarimetric observation of Oort-cloud comet C/2020 T2 (hereafter T2) obtained about two weeks before perihelion (q = 2.05 au). Spectropolarimetry provides a degree of linear polarization as a function of wavelength, which is independent of the number density of dust particles but sensitively depends on their microphysical properties (size, structure, and composition; Kiselev et al. 2015; Kwon et al. 2022). Together with archival data and modeling of both the light scattered by dust and the dust motion in the coma, we aim to constrain the dust environment of T2 and compare its properties with other OCC observations.

2 Observations and data analysis

A one-epoch low-resolution (λ/Δλ ~ 100) near-infrared 1.9 μm) spectropolarimetric observation of T2 was conducted on UT 2021 June 26.2 using a spectropolarimeter attached to the 5.1-m diameter Hale Telescope at Palomar Observatory 116°51′54″W, 33°21′23″N 1712m). Specifically, WIRC+Pol is a newly commissioned spectropolarimetry mode of the Wide-field InfraRed Camera (WIRC; Wilson et al. 2003), with a field of view of 4′.3 × 4′.3 and seeing-limited angular resolution of ≃1″.2, located at the prime focus of the telescope (Tinyanont et al. 2019). It measures full linear Stokes parameters (I, Q, and U) as a function of wavelength with one exposure, making resulting datasets free from sky rotation during a sequential exposure of images. A half-wave plate (HWP) rotates the incoming light by four different angles (2θHWP, where θHWP = 0°, 22°.5, 45°, 67°.5) that allows for beam-swapping and improved calibration. Each of the four beams then passes a quarter-wave plate, a so-called PG (acting as a beam-splitting polarizer and a grating simultaneously) and an opaque mask in a row, thereby leaving their trace in four quadrants on the CCD, 3′ away from one another (Fig. 1 in Tinyanont et al. 2019). We obtained 32 dithered images (16 each in slit A and B positions) in the J and H bands to subtract background signals. More details of the observing strategy can be found in Masiero et al. (2022). The observation journal is summarized in Table 1.

Basic calibration, spectral extraction, and polarimetric calculation were all performed in the WIRC+Pol Data Reduction Pipeline1, whose details are described in Tinyanont et al. (2019). An additional correction was made to residual offsets in polarization angle by adding 6°.101 101 and 3°. 416 to the J- and H-band values, respectively, based on Fig. 3 from Masiero et al. (2022). In the pipeline, Stokes parameters were corrected from the instrumental polarization and polarization efficiency. We extracted the spectra of each resolution element in three aperture sizes, corresponding to 850 km, 1140 km, and 1420 km in cometocentric distances at the time of our observation, yet the latter of which was discarded due to the significant background emission at ≳ 1.7 μm. To increase the signal-to-noise ratio (S/N), we smoothed the extracted spectra over five spectral bins and eliminated the measurements deviating by more than 5σ. The resulting polarimetric parameters (the degree of linear polarization and its position angle) were transformed into the scattering plane (a plane containing the Sun-comet-Earth) in the same manner as Chernova et al. (1993).

Table 1

Geometry and instrument settings of the observations of C/2020 T2 (Palomar).

3 Results and discussion

Figure 1 shows a J-band composite image of T2, notably appearing as a featureless, spherical coma that is distinct from the morphology of typical OCCs (e.g., Bauer et al. 2015). Most of the coma signal comes from the central part within ~5″ from the photocenter. Similar trends throughout the apparition of the comet are confirmed from r-band archival data (Appendix A).

3.1 Polarimetrie properties of the coma dust

Figure 2 shows the degree of linear polarization, Pr, and its position angle with respect to the normal direction of the scattering plane, θr, extracted from two different aperture sizes, ρ, as a function of wavelength, λ. The 1σ intervals of θr are distributed close to zero, which precludes significant dust alignment over the region analyzed and supports the reliability of our data reduction. The two spatial resolutions have almost the same Pr values at a given wavelength and similar spectral dependence: a slight increase in the J-band (red polarimetric color; 4.13±2.11% μm−1 for ρ = 850 km and 8.68+3.01% μm−1 for ρ = 1 140 km) and inflection point around 1.4 μm, followed by a decrease in the H-band (blue polarimetric color; −1.59±1.99% μm−1 for ρ = 850 km and −1.58±1.72% μm−1 for ρ = 1140 km). There is a local peak of Pr (≈ 1.5σ significance) at ~ 1.65 μm for the inner coma (ρ = 850 km) data. This region corresponds to an absorption peak of crystalline water-ice (Grundy & Schmitt 1998) hosted by a broader 1.5-μm band that has been observed for several active comets (e.g., Kawakita et al. 2004). We tested different smoothing parameters, but the feature is always present and becomes statistically indistinguishable in the larger aperture size. Although polarimetry is sensitive to the rapid change of the refractive index of scattering materials, in the absence of data covering the region where deeper bands of water ice are expected to be (1.5 and 2.0 μm; Mastrapa et al. 2008), we will not consider the weak 1.65 μm peak in the following analysis.

The Polarimetrie color Pr(λ) of T2 is then compared with those of other OCCs observed at similar geometries (Fig. 3). We selected archival data for which observations at multiple wavelengths were conducted simultaneously (or at least on the same night). For comparison, representative Pr values of T2 were used by averaging data points in each band: 2.91±0.15% for the J and 2.84±0.14% for the H bands over ρ = 850 km; and 2.80±0.19% for the J and 2.62±0.18% for the H bands over ρ = 1140 km. Since the two apertures offer consistent values within the errors, we averaged the Pr at the same band and derived the overall Polarimetrie color as −0.31±0.14% μm−1. T2’s average Pr values themselves are comparable to those of 1P/Halley, which seems natural for cometary dust in this small α region, as Pr begins to show discernible deviations between comets at larger α (≳40°; Kiselev et al. 2015), except for the exceptionally high Pr of Hale-Bopp (Hadamcik et al. 1997). However, the blue Pr(λ) of T2 is in contrast to the dust of OCCs that generally exhibit red Polarimetrie color over the J and H bands.

T2’s lower H-band Pr as a result of the blue Pr(λ) indicates that its dust has a heterogeneity larger than other cometary dust at the given wavelength (Bohren & Huffman 1983). This can be achieved by its compositional (lower absorptivity, for instance, lower fractions of amorphous carbon or abundant silicates; Rouleau & Martin 1991; Greenberg & Li 1996) and/or mechanical aspects (e.g., higher packing density; Kolokolova & Kimura 2010) to consequently enhance electromagnetic interactions among constituting grains. Compact water ice larger than a few tens of micrometers yields blue Pr(λ) but also increases overall Pr values (Warren 1984, 2019) and so is not consistent with T2. If the dust is small (of order ~0.1–1 μm) or fluffy (porosity as low as 2%) it will behave similarly to individual constituting grains in dynamics (Mukai et al. 1992; Skorov et al. 2016) and in light scattering (Kolokolova 2011). It will then provide a red Pr(λ) similar to that of lP/Halley and Hale-Bopp (Fig. 3). In this case, its enhanced thermal emission close to the Sun (rH ≲ 1 au) can depolarize signals longward of the H-band (Oishi et al. 1978), but this would not be significant for our T2 observations at 2.06 au from the Sun. Together with the coma morphology (Fig. 1), it is evident for T2 that the observed Polarimetrie behaviors of its coma cannot be explained by the typical dust properties used to characterize observations of active OCCs.

thumbnail Fig. 1

J-band composite image of T2. The image was boxcar smoothed in the 3-pixel width and overlapped with contours at 90, 50, 25, and 5% of T2’s peak brightness. The “S” marks a background star. A radial profile is given over the inner coma region (enclosed by a dashed-line square of 20″ × 20″) in the lower right corner. The negative velocity (−v) and solar radius (r⊙) vectors are given.
thumbnail Fig. 2

Degree of linear polarization of the dust of T2 Pr and its position angle θr at a phase angle of 28°.5 are given as a function of wavelength λ in panels (a) and (b), respectively. Pr(λ) extracted from different aperture sizes (ρ) are offset for clarity. In panel a, datapoints in each J- and H-band region were fitted by a linear least-square function (given as thick solid and dashed lines), where upper and lower dotted lines indicate 1σ intervals. In panel (b), the 1σ regions of θr and their central values are given.
thumbnail Fig. 3

Spectral dependence of the Pr of T2 and other OCCs: C/1995 Ol (Hale–Bopp) and 1P/Halley. Measurements for other OCCs were obtained from the NASA/PDS comet Polarimetrie archive (Kiselev et al. 2017): Hale-Bopp from Hasegawa et al. (1997) and 1P/Halley from Brooke et al. (1987) with squares and Kikuchi et al. (1987) with diamonds, whose phase angles are given in parentheses. The horizontal bars cover the J and H bands, while the vertical bars indicate errors in the average value of all data points in each band.

3.2 Quantitative estimations of the optical and dynamical properties of the 12 dust

To reproduce the blue Polarimetrie color Pr(λ) of T2, we conducted light-scattering modeling of dust aggregates. Presuming that cometary dust is hierarchical (Guttler et al. 2019), a >100-μm dust agglomerate consists of ~10–100-μm aggregates, each of which is in turn composed of ~0.1 μm grains (monomers). In this regard, near-infrared wavelengths are most sensitive to the intermediate-scale structure (i.e., dust aggregate). We thus considered two types of ballistic aggregates in the same fractal dimension (≈3)2 and mass but in different porosities (Shen et al. 2008): ballistic aggregate (BA, porosity P≈87% and characteristic cluster radius R ~ 15.8 μm) and BA with two migrations (BAM2, P≈66% and R~11.5 μm). The Rosetta mission to comet 67P/Churyumov-Gerasimenko showed that monomer size is distributed over ~0.05–0.6 μm but weighted toward smaller size with a mean of ~0.1 μm (Mannel et al. 2019). Due to computational limitations, however, we cannot probe a large aggregate of ≲0.1-μm monomers. Shen et al. (2008, 2009) verified that the size of monomers would be secondary as long as they are smaller than the wavelength considered; thus, we considered a simple case of dust clusters consisting of 4096 spherical monomers of 0.5-μm in radius3. Their geometry (Shen et al. 2008)4 was randomly generated and averaged over four realizations. For each dust realization, we defined 64 scattering planes (each plane has a constant azimuthal angle ϕ) and then averaged the outputs. We repeated these processes at 128 random orientations of each dust cluster and averaged the results. We also compared two composition cases: an average complex refractive index of mr = 1.6 and mi = 0.1, which is in the range of the typical cometary dust (e.g., Moreno et al. 2018), and slightly more transparent one (mi = 0.01). The 4 × 4 Mueller matrix was calculated using the fast superposition T-matrix method (FaSTMM; Markkanen & Yuffa 2017).

Figure 4 shows orientation- and realization-averaged Pr of the modeled dust as a function of wavelength. In the case of the average composition (mi = 0.1), BA has red Pr(λ), whereas less porous BAM2 steadily yields blue Pr(λ). This P–Pr(λ) relationship is in line with previous studies of dust-rich comets whose high Pr and red Pr(λ) are associated with the presence of highly porous particles in the coma and for Hale-Bopp with extremely fluffy aggregates (Kiselev et al. 2015 and references therein). In the case of less absorbing dust (mi = 0.01), BA shows slight red Pr(λ) despite large fluctuations due to enhanced interparticle scattering which contributes to an unstable trend5, while BAM2 gives either inconsistent or a comparably shallow slope of Pr(λ) to our results. In the absence of other information on T2, this leads us to prefer the standard composition. Slightly higher Pr values of BAM2 than the observation might not be critical because in reality scattering dust should be ensembles of aggregates distributed in size, structure, and composition which contain larger heterogeneities than our models and could decrease resulting Pr (Bohren & Huffman 1983). We do not claim that our BAM2 dust is unique, but it can indeed reproduce the characteristic blue Pr(λ) of T2. Our results thus suggest that the optical properties of the coma dust of T2 are compatible with BAM2-like lower-P (~66%) dust aggregates that are deficient in higher-P (≳87%) ones used to describe active comets.

Next, we focus on the coma morphology of T2. At the region of ≳1 000 km from the nucleus, dust trajectories in the coma result from a tug-of-war between outward solar radiation pressure and inward solar gravity forces (Finson & Probstein 1968a; Burns et al. 1979). Their mutual effect is parameterized by β, which is the ratio of the former to the latter:

(1)

where L⊙ is the solar luminosity, Qpr is the dimensionless coefficient of radiation pressure (≈1), G is the gravitational constant, M⊙ is the solar mass, c is the speed of light, and a and ρd are the radius and density of the dust. Assuming spherical dust with uniform ρd, K is ~5.71 × 10−4 kg m−2 and β becomes a direct function of a and ρd (which is pertinent to P). With Eq. (1), we can specify in each coma region the β of dust having different physical characteristics (syndyne) and ejection times (synchrone). From r-band archival images showing the onset of discernible activity for T2 began around 3.1 au (Appendix A), we set a simple model with zero ejection velocity where the dust ejection started ~180 days prior to our observation. Additional forces that can alter dust motions in the coma (e.g., sublimation and fragmentation) are not considered here.

Figure 5 compares the modeled synchrones and syndynes with the T2 image that is the same as Fig. 1 but with a twice larger field of view. Only the β = 10−4 syndyne reproduces the compact central part of the coma. Its featureless morphology suggests that dust particles in the coma were ejected over a wide range of times. The small working β of ~10−4 is in accordance with dust that makes up dust trails (~mm-sized dust with β <10−4; Ishiguro et al. 2007) and thus implies that T2 accommodates dust that is less sensitive to solar radiation pressure in its near-nucleus region. This is certainly not typical for the majority of OCCs whose significant dust tails tend to have larger β (Moreno 2022).

In tandem with our light-scattering modeling showing that viable dust in the T2 coma has P of ~66% on the scale of a dust aggregate (order of ~ 10–100 μm; Güttler et al. 2019) in a typical range of the dust composition of comets (Levasseur-Regourd et al. 2018), if we assume that the ice-free dust consists of amorphous carbon (75 vol.%) and Mg-rich silicates (25 vol.%), its approximate density (ρd = fCPC + fSiρSi, where fC and fSi are the volume fractions of amorphous carbon and Mg-rich silicates, respectively, normalized as fC + fSi + P = 1) is ~652 kg m−3. Here, we use densities of ρC = 1435 kg m−3 (Jäger et al. 1998) and ρSi = 3360 kg m−3 (Dorschner et al. 1995), and P of 66%. Substituting the retrieved density in to ρd in Eq. (1), β ~ 10-4 corresponds to the dust size of a ~ 270 μm. However, we should be cautious in extrapolating our P of dust aggregates to larger dimensions since cometary dust is likely hierarchical structures of heterogeneous dust aggregates rather than a homogeneous conglomerate of sub-micrometer grains (Skorov et al. 2018; Blum et al. 2022).

thumbnail Fig. 4

Orientation- and realization-averaged Pr as a function of wavelength. The upper and lower rows show results for BA and BAM2, respectively, whose geometry is visualized on the rightmost side of the micrometer scale. The left columns display outputs in the standard composition (mr = 1.6 and mi = 0.1), while the right columns show those with a lower absorptivity (mi = 0.01). Errors indicate 1σ for the four realizations of the clusters.
thumbnail Fig. 5

(a) Synchrones and syndynes of the T2 coma on UT 2021 June 26.2. The dashed lines are synchrones, indicating the locations of dust ejected at different times prior to the observation: 180, 120, 90, 60, 45, 30, 15, and 5 days from right to left. The solid curves are syndynes, where each has a constant β varying from 10−4 (low-mobility dust) to 1 (high-mobility dust) anticlockwise from the rightmost. A close-up image is given on the bottom right. (b) Same as T2 in panel a but with an image magnified five times.

3.3 Possible relationships between the dynamical & dust properties of Oort-cloud comets

Finally, to search for a relationship between the dust properties and orbital distribution of OCCs, information diagnosing their dust characteristics was gleaned from previous observations made in polarimetry and/or mid-infrared spectroscopy. Following the criteria suggested by Kolokolova et al. (2007) and Kwon et al. (2021), we classify comets as “Type II” when they have (1) higher Pr than the average trend of OCCs at given α, (2) red Pr(λ) and/or (3) strong intensity of a 10-μm silicate emission feature (≳1.5; Hanner & Bradley 2004), indicative of high-porosity coma dust; otherwise, they are classified as “Type I” that predominantly eject T2-like lower-porosity dust. Comets with a low-intensity ratio of the C2 emission feature to the dust continuum (<500; Krishna Swamy 2010) and a sharp anti-solar dust tail are also grouped as Type II. Six Manx-comet candidates are classified in Type I as their (nearly) tailless morphology (akin to T2) indicates that they presumably lack high-porosity dust in the coma (Meech et al. 2016). We only considered comets whose perihelion distance q is less than 3.1 au for this classification6. As a result, a total of 43 OCCs (= 20 Type I + 23 Type II) are selected. Their orbital elements, types, and references are tabulated in Table B.1.

Figure 6a shows a distribution of the reciprocal original semimajor axes 1/aori that has a definite correlation with the number of approaches to the Sun (Everhart 1972) and the reciprocal osculating semimajor axes 1/aosc of the 43 OCCs. The 1/aori values are quoted from the Warsaw Catalogue of near-parabolic comets (Królikowska 2014), Minor Planet Center database search engine7 or Nakano note8. The comets are divided into two dynamical groups: Oort-spike comets (OSCs, black circles) inside the classical Oort peak at aori ~ 104 au (Oort 1950) and long-period comets (LPCs, brown circles) outside the peak. The former is often assumed to be dynamically new, though recent studies suggest that the fiducial line of classification should be located farther than the classical one (Dybczyński & Królikowska 2015). Another boundary is marked at aori ~ 250 au, inside of which the effect of resonances from Neptune outweigh the Oort-cloud processes (e.g., perturbations of passing stars and the Galactic tide; Brasser et al. 2012; Fouchard et al. 2017). The aori of the Manxes and T2 are inside the inner edge of the Oort-cloud at ~ 1500 au (Vokrouhlický et al. 2019).

The OSCs and LPCs are distributed rather homogeneously in the plane of the current perihelion, qosc, and inclination, cos(iosc) (Fig. 6b). For Type I (filled black) and Type II (hatched black) comets, there are about 1.5 times as many Type I comets in prograde orbits (Fig. 6c), though the number of comets per bin here would hardly be related to the real architecture of the Oort cloud since we selected comets only for which decent dust analysis has been made. Nonetheless, the relatively high fraction of Type I comets among the considered ones within ±30° from the ecliptic plane, particularly the clustering of all Manx comets and T2 toward the plane, is noteworthy. This apparent clustering in space with their similar 1/aori points might reflect their unique origins (Meech et al. 2016) or evolutionary pathways, different from the majority of OCCs ejected from the region of ice-giant planets (Vokrouhlický et al. 2019). More observations are required to exclude any possible observational biases (e.g., less active comets might be easier to be detected in low relative velocity space to the Earth). This may be an interesting topic for future studies, with the aim of examining whether this trend can be retained in larger datasets and, thus, whether it is more probable to observe less-evolved OCCs near perpendicular to the ecliptic plane.

thumbnail Fig. 6

Correlations between the dust and dynamical properties of the selected 43 Oort-cloud comets. (a) Distribution of the reciprocal original semimajor axis 1/aori and reciprocal osculating semimajor axis 1/aori of 43 selected OCCs. The Oort Spike (~104 au) and the boundary (~250 au) of planetary regions are marked as the left and right vertical lines, respectively. (b) Distribution of the comets in the plane of the osculating perihelion distance qosc and inclination cos(iosc). (c) Distribution of the comets grouped in 30° inclination bins. OSCs and LPCs are shown in the left and right panels, respectively, therein the hatched and filled blocks denote Type I and Type II comets. Detailed classification criteria are explained in the text.

4 Conclusions

This paper reports our tailless Oort-cloud comet C/2020 T2 (Palomar) analysis. The main results are as follows.

  1. The J-band (1.25 μm) image of T2 on UT 2021 June 26.2 exhibits tailless morphology, where more than 95% of light is concentrated in ≲ 104 km from the nucleus center, reminiscent of the coma morphology of Manx comets (Meech et al. 2016). The secular evolution of its r-band activity (Appendix A) supports the overall low activity of the comet.

  2. Average Pr of T2 at α = 28°.5 is 2.91±0.15% for the J and 2.84±0.14% for the H bands over ρ = 850 km; and 2.80±0.19% for the J and 2.62±0.18% for the H bands over ρ=1140km. The aperture-averaged polarimetric color over the J–H bands is blue (−0.31±0.14% μm−1), opposite to the red polarimetric color of active comets observed at similar α.

  3. From our light-scattering modeling of ballistic aggregates distributed in compositions and porosities and dust dynamic modeling, we suggest that the coma dust of T2 in the near-infrared is compatible with low-porosity (~66%) dust with typical dust composition (ρd ~ 625 kg m−3). If we assume the uniform distribution in density over the 10–100 μm aggregate scale, the constrained β ~ 10−4 corresponds to the viable dust aggregate size of ~270 μm.

  4. We found that Manx comets and T2 share dynamical properties to some extent, showing clustering near the ecliptic plane. It appears that a higher percentage of Type I comets occurs within 30° of the plane, which needs to be confirmed with more observations.

This paper suggests the heterogeneous nature of the Oort cloud through distinct dust properties of comet C/2020 T2 from most active OCCs. The apparent clustering of the comet and Manx comets, whose scattered light is likely dominated by T2-like low-porosity dust particles, toward the ecliptic plane suggests the intriguing potential of cometary dust as a probe to trace cometary dynamics.

Acknowledgements

Y.G.K. gratefully acknowledges the support of the Alexander von Humboldt Foundation. J.M. acknowledges funding from the European Union’s Horizon 2020 research and innovation program under grant agreement no. 75390 CAstRA. This paper is based in part on observations obtained at the Hale Telescope, Palomar Observatory as part of a continuing collaboration between the California Institute of Technology, NASA/JPL, Yale University, and the National Astronomical Observatories of China.

Appendix A r-band photometry of C/2020 T2 (Palomar)

Secular evolution of cometary activity as a comet approaches the Sun provides insight into how much volatile ices surface retains and thus the degree of processing of the surface layer (Gundlach et al. 2015, 2020). To put our near-infrared spectropolarimetric observation in a broader context, we utilized the r-band broadband imaging data of T2 from the Zwicky Transient Facility (ZTF) archive (Masci et al. 2019)9. The data cover heliocentric distances rH ranging from 3.575 au to 2.075 au between UT 2020

October 23 and UT 2021 August 04, with the comet passing perihelion at q = 2.055 au on UT 2021 July 11.110. ZTF is a 1.2-m diameter time-domain survey telescope, scanning the entire northern visible sky with a pixel scale of 1″.0 pixel−1 and a field of view (FoV) of 47 deg2 to a 5-σ median magnitude limit of r ~ 21.012 mags11. Since ZTF applies a unified integration time of 30 seconds (Masci et al. 2019), we made use of r-band data that generally displays the best signal-to-noise ratio for cometary dust. The archive provides images that are already pre-processed with bias removal, dark subtractions, and flattening. Figure A.1 summarizes the change in the coma morphology of T2 during this period.

thumbnail Fig. A.1

Coma morphology of T2 in its inbound (Panels a to h) and outbound (Panel i) orbit taken from the ZTF archive. All panels cover a FoV of 4′ × 4′ and provide a close-up image on the bottom right with FoV of 1′. The negative velocity vector (−v), solar radius vector (r⊙), and 1′ scale bar are given in each panel. A zoom-in image was processed by the boxcar smoothing with a width of 3 pixels to better visualize coma features. Five contour levels stratify the brightness within 2-σ of the peak brightness on a logarithmic scale, except for the image in panel a, where we could not make aperture photometry due to its weak contrast to the background signal. The heliocentric distance for the comet in each panel is (a) 3.004 au (inbound), (b) 2.624 au (inbound), (c) 2.422 au (inbound), (d) 2.329 au (inbound), (e) 2.267 au (inbound), (f) 2.129 au (inbound), (g) 2.068 au (inbound), (h) 2.055 au (inbound, two days before perihelion), and (i) 2.075 au (outbound). North is up and east is to the left.
thumbnail Fig. A.2

Secular evolution of the reduced r-band magnitude (Panel a) and Afρ parameter (Panel b) as a function of heliocentric distance rH. All points correspond to the photometric parameters measured from the aperture size of 10 000 km from the comet nucleus. The star symbol in Panel b denotes the time of our spectropolarimetric observation (UT 2021 June 26.2). The vertical dashed line indicates the perihelion distance at rH = 2.055 au. An increase in the brightness of T2 accelerates around 2.4 au from the Sun. A linear least-square fit to the heliocentric evolution of the reduced r magnitudes gives different slopes (‘sl’ in panel a) before and after the acceleration.

T2 became bright enough in the ZTF images to conduct aperture photometry on UT 2020 October 23 at rH ~ 3.6 au and began to show an extended coma signal around 3.1 au from the Sun. Throughout the apparition, T2 lacked a significant dust tail but showed a modest elongation of the dust coma (Fig. A.1). The oval-shaped coma containing most of the light shows no preference in the elongation direction. In the absence of significant coma features whose signal exceeds the background uncertainty, the coma morphology in the visible light supports the nearly tailless feature of T2 in the near-infrared (Fig. 1).

The evolution of the photometric parameters of T2 is shown in Figure A.2. r-band magnitudes of T2 were measured with a fixed aperture size of 10 000 km from the comet center and then corrected by differential magnitudes of background stars by comparing the instrumental magnitude of the comet with the star magnitude provided by the SDSS-12 catalog (Alam et al. 2015). The conversion between magnitudes in different catalogs was made based on the transformation parameters provided by Tonry et al. (2012) and Medford et al. (2020). Reduced r-band magnitude as a function of heliocentric distance rH, mr(rH), was derived by correcting the effect of varying geocentric distance and phase angle over the period as

(A.1)

where mr(rH, ∆, α) is the apparent magnitude, which is corrected from the differential photometry using the background star catalog and Φ(α) is the phase function of the coma dust. We adopted a commonly used empirical scattering phase function (2.5 log10(Φ(α)) = bα), where the phase coefficient of b = 0.035 mag deg−1 was assumed (e.g., Lamy et al. 2004). Given the apparent magnitude, we further derived the so-called Afρ parameter (A is the albedo of dust particles and f is their packing density within the aperture radius of ρ; A’Hearn et al. 1984), a proxy of the dust production rate (Fulle et al. 2022), using Eq. 8 in Kwon et al. (2019), where ρ = 10 000 km throughout the analysis.

At around 2.4 au, the slope of brightening in the reduced r-band magnitude begins to steepen at a rate that is more than two times higher than at further distances (Fig. A.2). The Afρ parameter accordingly exhibits an acceleration around that point, indicative of the discontinuous dust ejection into the coma. The dust production rate, approximated by the Afρ parameter, of T2 is ≳ 100 times weaker than other OCCs at similar rH (Mazzotta Epifani et al. 2016; Garcia et al. 2020; Fulle et al. 2022).

Different types of ice particles sublimate at different temperature environments and thus dominate cometary activity at distinctive distance regimes from the Sun. Supervolatile ice (particularly CO2 ice) sublimation is significant until ~4 au, followed by the regime where water ice starts to dominate the dust ejection at ~2.5–2.7 au (Blum et al. 2014; Gundlach et al. 2015; Bauer et al. 2015; Gundlach et al. 2020). The observed transitional point of T2 at ~2.4 au would signify the onset point of water ice sublimation-driven dust activity of the comet. The very low-activity level outside the transition point implies a dearth of supervolatile ices near the surface dust layer and/or that the upper dust layer through which gas molecules diffuse outward is more consolidated compared to those of other active comets. Both cases indicate that there is a processed surface environment on T2’s nucleus. These surface conditions seem to be far different from those of comets that have just completed their gravitational aggregation in the protoplanetary system (Blum et al. 2022 and references therein) or those of comets in early evolutionary phases where their surfaces deplete inherent ice by sublimation, but have not yet undergone significant compression that may preserve highly fluffy structures (Skorov & Blum 2012; Poch et al. 2016a,b).

Putting all the results together, T2 appears to share several aspects with so-called Manx comets: they are not only similar locations in orbital space (Fig. 6), but they also exhibit relatively low activity in scattered light than active dust-rich OCCs, along with a (nearly) tailless coma morphology (Meech et al. 2016; Piro et al. 2021) and the presence of a discontinuous, transitional point of the dust ejection in their inbound orbits (e.g., Molnar-Bufanda et al. 2019). All the observations suggest the paucity of highly porous (or ensuing ~0.1–1 μm-sized small) dust particles in their pre-perihelion comae. Observations of low-activity OCCs in thermal emission (particularly in the mid-infrared of 5–20 μm) will provide independent information on how the internal structure and composition of their dust constituents (Gehrz & Ney 1992) differentiate from active OCCs. This will help reveal diversities embedded in the formation and subsequent evolutionary history of comets consisting of the present-day Oort cloud.

Appendix B Ancillary information for the comets used in Figure 6

Table B.1

Properties of the comets used in Figure 6 and their references

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2

Rosetta observations of coma dust showed particle strength can vary across a large agglomerate, where smaller dust constituents (~15–40 μm) have higher strength (Hornung et al. 2016). This makes us exclude fluffy dust (fractal dimension of 1.5–2.5; Güttler et al. 2019), which is relevant to a ~millimeter-sized parent dust (Fulle et al. 2015; Mannel et al. 2016), from the study at hand.

3

We expect Pr to be independent of the size for larger aggregate scales of interest in typical Halley-dust composition (Mackowski & Kolokolova 2022). This makes us consider a simple dust cluster of monodisperse monomers.

5

In part, resonance effects inside monomers due to the monomer size considered here could contribute to the enhancement of fluctuation.

6

As OCCs with q > 3.1 au are located outside the water ice line (Blum et al. 2014; Gundlach et al. 2020), their relatively low activity and lack of dust features shaped by sufficient solar radiation pressure make the clas- sification criteria adopted here hardly applicable. In addition, given that these distant comets are likely to undergo a rapid change of their q by planet perturbations (Fouchard et al. 2017; Vokrouhlický et al. 2019), we anticipated that their current orbital elements would be less informative.

All Tables

Table 1

Geometry and instrument settings of the observations of C/2020 T2 (Palomar).

In the text
Table B.1

Properties of the comets used in Figure 6 and their references

In the text

All Figures

thumbnail Fig. 1

J-band composite image of T2. The image was boxcar smoothed in the 3-pixel width and overlapped with contours at 90, 50, 25, and 5% of T2’s peak brightness. The “S” marks a background star. A radial profile is given over the inner coma region (enclosed by a dashed-line square of 20″ × 20″) in the lower right corner. The negative velocity (−v) and solar radius (r⊙) vectors are given.
In the text
thumbnail Fig. 2

Degree of linear polarization of the dust of T2 Pr and its position angle θr at a phase angle of 28°.5 are given as a function of wavelength λ in panels (a) and (b), respectively. Pr(λ) extracted from different aperture sizes (ρ) are offset for clarity. In panel a, datapoints in each J- and H-band region were fitted by a linear least-square function (given as thick solid and dashed lines), where upper and lower dotted lines indicate 1σ intervals. In panel (b), the 1σ regions of θr and their central values are given.
In the text
thumbnail Fig. 3

Spectral dependence of the Pr of T2 and other OCCs: C/1995 Ol (Hale–Bopp) and 1P/Halley. Measurements for other OCCs were obtained from the NASA/PDS comet Polarimetrie archive (Kiselev et al. 2017): Hale-Bopp from Hasegawa et al. (1997) and 1P/Halley from Brooke et al. (1987) with squares and Kikuchi et al. (1987) with diamonds, whose phase angles are given in parentheses. The horizontal bars cover the J and H bands, while the vertical bars indicate errors in the average value of all data points in each band.
In the text
thumbnail Fig. 4

Orientation- and realization-averaged Pr as a function of wavelength. The upper and lower rows show results for BA and BAM2, respectively, whose geometry is visualized on the rightmost side of the micrometer scale. The left columns display outputs in the standard composition (mr = 1.6 and mi = 0.1), while the right columns show those with a lower absorptivity (mi = 0.01). Errors indicate 1σ for the four realizations of the clusters.
In the text
thumbnail Fig. 5

(a) Synchrones and syndynes of the T2 coma on UT 2021 June 26.2. The dashed lines are synchrones, indicating the locations of dust ejected at different times prior to the observation: 180, 120, 90, 60, 45, 30, 15, and 5 days from right to left. The solid curves are syndynes, where each has a constant β varying from 10−4 (low-mobility dust) to 1 (high-mobility dust) anticlockwise from the rightmost. A close-up image is given on the bottom right. (b) Same as T2 in panel a but with an image magnified five times.
In the text
thumbnail Fig. 6

Correlations between the dust and dynamical properties of the selected 43 Oort-cloud comets. (a) Distribution of the reciprocal original semimajor axis 1/aori and reciprocal osculating semimajor axis 1/aori of 43 selected OCCs. The Oort Spike (~104 au) and the boundary (~250 au) of planetary regions are marked as the left and right vertical lines, respectively. (b) Distribution of the comets in the plane of the osculating perihelion distance qosc and inclination cos(iosc). (c) Distribution of the comets grouped in 30° inclination bins. OSCs and LPCs are shown in the left and right panels, respectively, therein the hatched and filled blocks denote Type I and Type II comets. Detailed classification criteria are explained in the text.
In the text
thumbnail Fig. A.1

Coma morphology of T2 in its inbound (Panels a to h) and outbound (Panel i) orbit taken from the ZTF archive. All panels cover a FoV of 4′ × 4′ and provide a close-up image on the bottom right with FoV of 1′. The negative velocity vector (−v), solar radius vector (r⊙), and 1′ scale bar are given in each panel. A zoom-in image was processed by the boxcar smoothing with a width of 3 pixels to better visualize coma features. Five contour levels stratify the brightness within 2-σ of the peak brightness on a logarithmic scale, except for the image in panel a, where we could not make aperture photometry due to its weak contrast to the background signal. The heliocentric distance for the comet in each panel is (a) 3.004 au (inbound), (b) 2.624 au (inbound), (c) 2.422 au (inbound), (d) 2.329 au (inbound), (e) 2.267 au (inbound), (f) 2.129 au (inbound), (g) 2.068 au (inbound), (h) 2.055 au (inbound, two days before perihelion), and (i) 2.075 au (outbound). North is up and east is to the left.
In the text
thumbnail Fig. A.2

Secular evolution of the reduced r-band magnitude (Panel a) and Afρ parameter (Panel b) as a function of heliocentric distance rH. All points correspond to the photometric parameters measured from the aperture size of 10 000 km from the comet nucleus. The star symbol in Panel b denotes the time of our spectropolarimetric observation (UT 2021 June 26.2). The vertical dashed line indicates the perihelion distance at rH = 2.055 au. An increase in the brightness of T2 accelerates around 2.4 au from the Sun. A linear least-square fit to the heliocentric evolution of the reduced r magnitudes gives different slopes (‘sl’ in panel a) before and after the acceleration.
In the text

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