-3 -Multiple outbursts of a Jupiter-family comet, 15P/Finlay, occurred from late 2014 to early 2015. We conducted an observation of the comet after the first outburst and subsequently witnessed another outburst on 2015 January 15.6-15.7. The gas, consisting mostly of C 2 and CN, and dust particles expanded at speeds of 1,110 ± 180 m s −1 and 570 ± 40 m s −1 at a heliocentric distance of 1.0 AU. We estimated the maximum ratio of solar radiation pressure with respect to the solar gravity β max = 1.6 ± 0.2, which is consistent with porous dust particles composed of silicates and organics. We found that 10 8 -10 9 kg of dust particles (assumed to be 0.3 µm-1 mm) were ejected through each outburst. Although the total mass is three orders of magnitude smaller than that of the 17P/Holmes event observed in 2007, the kinetic energy per unit mass (10 4 J kg −1 ) is equivalent to the estimated values of 17P/Holmes and 332P/2010 V1 (Ikeya-Murakami), suggesting that the outbursts were caused by a similar physical mechanism. From a survey of cometary outbursts on the basis of voluntary reports, we conjecture that 15P/Finlay-class outbursts occur >1.5 times annually and inject dust particles from Jupiter-family comets and Encke-type comets into interplanetary space at a rate of ∼10 kg s −1 or more.

The near-Earth asteroid (3200) Phaethon is the parent body of the Geminid meteor stream. Phaethon is also an active asteroid with a very blue spectrum. We conducted polarimetric observations of this asteroid over a wide range of solar phase angles α during its close approach to the Earth in autumn 2016. Our observation revealed that Phaethon exhibits extremely large linear polarization: P = 50.0 ± 1.1% at α = 106.5°, and its maximum is even larger. The strong polarization implies that Phaethon’s geometric albedo is lower than the current estimate obtained through radiometric observation. This possibility stems from the potential uncertainty in Phaethon’s absolute magnitude. An alternative possibility is that relatively large grains (~300 μm in diameter, presumably due to extensive heating near its perihelion) dominate this asteroid’s surface. In addition, the asteroid’s surface porosity, if it is substantially large, can also be an effective cause of this polarization.

We present an optical and near-infrared (hereafter NIR) polarimetric study of a comet C/2013 US10 (Catalina) observed on UT 2015 December 17-18 at phase angles of α=52.1 • -53.1 • . Additionally, we obtained an optical spectrum and multi-band images to examine the influence of gas emission. We find that the observed optical signals are significantly influenced by gas emission, that is, the gas-to-total intensity ratio varies from 5 to 30 % in the R C and 3 to 18 % in the I C bands, depending on the position in the coma. We derive the 'gas-free dust polarization degrees' of 13.8±1.0 % in the R C and 12.5±1.1 % in the I C bands and a gray polarimetric color, i.e., -8.7±9.9 % µm −1 in optical and 1.6±0.9 % µm −1 in NIR. The increments of polarization obtained from the gas correction show that the polarimetric properties of the dust in this low-polarization comet are not different from those in high-polarization comets. In this process, the cometocentric distance dependence of polarization has disappeared. We also find that the R C -band polarization degree of the southeast dust tail, which consists of large dust particles (100 µm-1 mm), is similar to that in the outer coma where small and large ones are mixed. Our study confirms that the dichotomy of cometary polarization does not result from the difference of dust properties, but from depolarizing gas contamination. This conclusion can provide a strong support for similarity in origin of comets.

Context. Spectropolarimetry is a powerful technique for investigating the physical properties of gas and solid materials in cometary comae without mutual contamination, but there have been few spectropolarimetric studies to extract each component. Aims. We attempt to derive the continuum (i.e., scattered light from dust coma) polarization degree of comet 2P/Encke, free from influence of molecular emissions. The target is unique in that it has an orbit dynamically decoupled from Jupiter like main-belt asteroids, while ejecting gas and dust like ordinary comets. Methods. We observed the comet using the Higashi-Hiroshima Optical and Near-Infrared Camera attached to the Cassegrain focus of the 150-cm Kanata telescope on UT 2017 February 21 when the comet was at the solar phase angle of α=75 • .7. Results. We find that the continuum polarization degree with respect to the scattering plane is P cont,r =33.8±2.7 % at the effective wavelength of 0.815 µm, which is significantly higher than those of cometary dust in a high-P max group at similar phase angles. Assuming that an ensemble polarimetric response of 2P/Encke's dust as a function of phase angle is morphologically similar with those of other comets, its maximum polarization degree is estimated to P max 40 % at α max ≈100 • . In addition, we obtain the polarization degrees of the C 2 swan bands (0.51-0.56 µm), the NH 2 α bands (0.62-0.69 µm) and the CN-red system (0.78-0.94 µm) in a range of 3-19 %, which depend on the molecular species and rotational quantum numbers of each branch. The polarization vector aligns nearly perpendicularly to the scattering plane with the average of 0 • .4 over a wavelength range of 0.50-0.97 µm. Conclusions. From the observational evidence, we conjecture that the large polarization degree of 2P/Encke would be attributable to a dominance of large dust particles around the nucleus, which have remained after frequent perihelion passages near the Sun.

Context. Comets have been resurfacing under solar radiation while preserving their primordial interiors. Multi-epoch observations of comets enable us to characterize a change in sublimation pattern as a function of heliocentric distance, which in turn provides information on the dust environment of comets. Aims. We aim to constrain the size and porosity of ejected dust particles from comet 252P/LINEAR and their evolution near the perihelion via near-infrared (NIR) multiband polarimetry. A close approach of the comet to the Earth in March 2016 (∼0.036 au) provided a rare opportunity for the sampling of the comet with a high spatial resolution. Methods. We made NIR JHK S bands (1.25-2.25 µm) polarimetric observations of the comet for 12 days near perihelion, interspersed between broadband optical (0.48-0.80 µm) imaging observations over four months. In addition, dynamical simulation of the comet was performed 1000 yr backward in time.Results. We detected two discontinuous brightness enhancements of 252P/LINEAR. Before the first enhancement, the NIR polarization degrees of the comet were far lower than those of ordinary comets at a given phase angle. Soon after the activation, however, they increased by ∼13 % at most, showing unusual blue polarimetric color over the J and H bands (−2.55 % µm −1 on average) and bluing of both J − H and H − K S dust color. Throughout the event, the polarization vector was marginally aligned perpendicular to the scattering plane (i.e., θ r = 4.6 • -10.9 • ). The subsequent postperihelion reactivation of the comet lasted for approximately 1.5 months, with a factor of ∼30 times pre-activation dust mass-loss rates in the R C band. Conclusions. The marked increase in the polarization degree with blue NIR polarimetric color is reminiscent of the behaviors of a fragmenting comet D/1999 S4 (LINEAR). The most plausible scenario for the observed polarimetric properties of 252P/LINEAR would be an ejection of predominantly large (well in geometrical optics regime), compact dust particles from the desiccated surface layer. We conjecture that the more intense solar heating that the comet has received in the near-Earth orbit would cause the paucity of small, fluffy dust particles around the nucleus of the comet.

The investigation of asteroids near the Sun is important for understanding the final evolutionary stage of primitive Solar system objects. A near-Sun asteroid, (155140) 2005 UD, has orbital elements similar to those of (3200) Phaethon (the target asteroid for the JAXA’s DESTINY+ mission). We conducted photometric and polarimetric observations of 2005 UD and found that this asteroid exhibits a polarization phase curve similar to that of Phaethon over a wide range of observed solar phase angles (α = 20-105○) but different from those of (101955) Bennu and (162173) Ryugu (asteroids composed of hydrated carbonaceous materials). At a low phase angle (α ≲ 30○), the polarimetric properties of these near-Sun asteroids (2005 UD and Phaethon) are consistent with anhydrous carbonaceous chondrites, while the properties of Bennu are consistent with hydrous carbonaceous chondrites. We derived the geometric albedo, pV ∼ 0.1 (in the range of 0.088-0.109); mean V-band absolute magnitude, HV = 17.54 ± 0.02; synodic rotational period, Trot = 5.2388 ± 0.0022 hours (the two-peaked solution is assumed); and effective mean diameter, Deff = 1.32 ± 0.06 km. At large phase angles (α ≳ 80○), the polarization phase curve are likely explained by the dominance of large grains and the paucity of small micron-sized grains. We conclude that the polarimetric similarity of these near-Sun asteroids can be attributed to the intense solar heating of carbonaceous materials around their perihelia, where large anhydrous particles with small porosity could be produced by sintering.

This paper reports a new optical observation of 17P/Holmes one orbital period after the historical outburst event in 2007. We detected not only a common dust tail near the nucleus, but also a long narrow structure that extended along the position angle 274.6 • ± 0.1 • beyond the field of view of the Kiso Wide Field Camera, i.e., >0.2 • eastward and >2.0 • westward from the nuclear position. The width of the structure decreased westward with increasing distance from the nucleus. We obtained the total cross section of the long extended structure in the field of view, C FOV = (2.3 ± 0.5) × 10 10 m 2 . From the position angle, morphology and the mass, we concluded that the long narrow structure consists of materials ejected during the 2007 outburst. On the basis of the dynamical behavior of dust grains in the solar radiation field, we estimated that the long narrow structure would be composed of 1 mm-1 cm grains having an ejection velocity of >50 m s −1 . The velocity was more than one order of magnitude faster than that of millimeter -centimeter grains from typical comets around a heliocentric distance r h of 2.5 AU. We considered that sudden sublimation of a large amount of water ice (≈10 30 mol s −1 ) would be responsible for the high ejection velocity. We finally estimated a total mass of M TOT =(4-8) × 10 11 kg and a total kinetic energy of E TOT =(1-6)×10 15 J for the 2007 outburst ejecta, which are consistent with those of previous studies that conducted soon after the outburst.

Context. Comets are conglomerates of ice and dust particles, the latter of which encode information on changes in the radiative and thermal environments. Dust displays distinctive scattered and thermal radiation in the visible and mid-infrared (MIR) wavelengths, respectively, based on its inherent characteristics. Aims. We aim to identify a possible correlation between the properties of scattered and thermal radiation from dust and the principal dust characteristics responsible for this relationship, and therefrom gain insights into comet evolution. Methods. We use the NASA/PDS archival polarimetric data on cometary dust in the red (0.62−0.73 μm) and K (2.00−2.39 μm) domains to leverage the relative excess of the polarisation degree of a comet to the average trend at the given phase angle (Pexcess) as a metric of the dust’s scattered light characteristics. The flux excess of silicate emissions to the continuum around 10 μm (FSi/Fcont) is adopted from previous studies as a metric of the dust’s MIR feature. Results. The two observables – Pexcess and FSi/Fcont – show a positive correlation when Pexcess is measured in the K domain (Spearman’s rank correlation coefficient ρ = 0.71−0.19+0.10). No significant correlation was identified in the red domain (ρ = 0.13−0.15+0.16). The gas-rich comets have systematically weaker FSi/Fcont than the dust-rich ones, and yet both groups retain the same overall tendency with different slope values. Conclusions. The observed positive correlation between the two metrics indicates that composition is a peripheral factor in characterising the dust’s polarimetric and silicate emission properties. The systematic difference in FSi/Fcont for gas-rich versus dust-rich comets would instead correspond to the difference in their dust size distribution. Hence, our results suggest that the current MIR spectral models of cometary dust, which search for a minimum χ2 fit by considering various dust properties simultaneously, should prioritise the dust size and porosity over the composition. With light scattering being sensitive to different size scales in two wavebands, we expect the K-domain polarimetry to be sensitive to the properties of dust aggregates, such as size and porosity, which might have been influenced by evolutionary processes. On the other hand, the red-domain polarimetry reflects the characteristics of sub-micrometre constituents in the aggregate.