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 multiband polarimetry. A close approach of the comet to the Earth in March 2016 (~0.036 au) pro
vided a rare opportunity for the sampling of the comet with a high spatial resolution. Methods. We made NIR JHKS bands polarimetric observations of the comet for 12 days near perihelion, interspersed between broadband optical 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. Before the first enhancement, the NIR polarization degrees 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 % / um on average) and bluing of both J-H and H-Ks dust color. Throughout the event, the polarization vector was marginally aligned perpendicular to the scattering plane. 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 Rc 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, 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.
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. We attempt
to derive the continuum 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. 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 75.7 deg. We find that the continuum polarization degree with respect to the scattering plane is 33.8+/-2.7 % at the effective wavelength of 0.815 um, which is significantly higher than those of cometary dust in a high-Pmax group at similar phase angles. Assuming that an ensemble polarimetric response of 2P/Enckes dust as a function of phase angle is morphologically similar with those of other comets, its maximum polarization degree is estimated to > 40 % at the phase angle of ~100 deg. In addition, we obtain the polarization degrees of the C2 swan bands (0.51-0.56 um), the NH2 alpha bands (0.62-0.69 um) and the CN-red system (0.78-0.94 um) 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 deg over a wavelength range of 0.50-0.97 um. 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.
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 $alpha$=52.1 deg - 53.1 deg. Additionally, we obtained an optical spectrum and multi-ba
nd 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 $R_{rm C}$ and 3 to 18 % in the $I_{rm C}$ bands, depending on the position in the coma. We derive the `gas-free dust polarization degrees of 13.8$pm$1.0 % in the $R_{rm C}$ and 12.5$pm$1.1 % in the $I_{rm C}$ bands and a gray polarimetric color, i.e., -8.7$pm$9.9 % $mu mathrm{m}$$^{-1}$ in optical and 1.6$pm$0.9 % $mu mathrm{m}$$^{-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 $R_{rm C}$-band polarization degree of the southeast dust tail, which consists of large dust particles (100 $mu mathrm{m}$ - 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.
The JAXA Hayabusa-2 mission was approved in 2010 and launched on December 3, 2014. The spacecraft will arrive at the near-Earth asteroid 162173 Ryugu in 2018 where it will perform a survey, land and obtain surface material, then depart in Dec 2019 an
d return to Earth in Dec 2020. We observed Ryugu with the Herschel Space Observatory in Apr 2012 at far-IR thermal wavelengths, supported by several ground-based observations to obtain optical lightcurves. We reanalysed previously published Subaru-COMICS and AKARI-IRC observations and merged them with a Spitzer-IRS data set. In addition, we used a large set of Spitzer-IRAC observations obtained in the period Jan to May, 2013. The data set includes two complete rotational lightcurves and a series of ten point-and-shoot observations. The almost spherical shape of the target together with the insufficient lightcurve quality forced us to combine radiometric and lightcurve inversion techniques in different ways to find the objects key physical and thermal parameters. We find that the solution which best matches our data sets leads to this C class asteroid having a retrograde rotation with a spin-axis orientation of (lambda = 310-340 deg; beta = -40+/-15 deg) in ecliptic coordinates, an effective diameter (of an equal-volume sphere) of 850 to 880 m, a geometric albedo of 0.044 to 0.050 and a thermal inertia in the range 150 to 300 Jm-2s-0.5K-1. Based on estimated thermal conductivities of the top-layer surface in the range 0.1 to 0.6 WK-1m-1, we calculated that the grain sizes are approximately equal to between 1 and 10 mm. The finely constrained values for this asteroid serve as a `design reference model, which is currently used for various planning, operational and modelling purposes by the Hayabusa2 team.
