No abstract

Strange-looking dust cloud around asteroid (596) Scheila was discovered on 2010 December 11.44-11.47. Unlike normal cometary tails, it consisted of three tails and faded within two months. We constructed a model to reproduce the morphology of the dust cloud based on the laboratory measurement of high velocity impacts and the dust dynamics. As the result, we succeeded in the reproduction of peculiar dust cloud by an impact-driven ejecta plume consisting of an impact cone and downrange plume. Assuming an impact angle of 45 • , our model suggests that a decameter-sized asteroid collided with (596) Scheila from the direction of (α im , δ im ) = (60 • , -40 • ) in J2000 coordinates on 2010 December 3. The maximum ejection velocity of the dust particles exceeded 100 m/s. Our results suggest that the surface of (596) Scheila consists of materials with low tensile strength.

Based on optical and infrared observations, we study the albedo and the temperature of the dust grains associated with the spectacular 2007 outburst of Jupiter-family comet 17P/Holmes. We found that the albedo at the solar phase angle ∼16 • was 0.03-0.12. While the color temperature around 3-4 μm was 360 ± 40 K, the color temperature at 12.4 μm and 24.5 μm was ∼200 K, which is consistent with that of a blackbody. We studied the equilibrium temperature of the dust grains at 2.44 AU and found that the big discrepancy in the temperature was caused by the heterogeneity in particle size, that is, hotter components consist of submicron absorbing grains whereas colder components consist of large ( 1 μm) grains. The contemporaneous optical and mid-infrared observations suggest that the albedo and the temperature could decrease within ∼ 3 days after the outburst and stabilized at typical values of the other comets. We estimated the total mass injected into the coma by the outburst on the basis of the derived albedo and the optical magnitude for the entire dust cloud, and found that at least 4 × 10 10 kg (equivalent to a few meter surface layer) was removed by the initial outburst event. The derived mass suggests that the outburst is explainable by neither the exogenetic asteroidal impact nor water ice sublimation driven by solar irradiation, but by an endogenic energy source. We conclude that the outburst was triggered by the energy sources several meters or more below the nuclear surface.

The short-period comet P/2010 V1 (Ikeya-Murakami, hereafter "V1") was discovered visually by two amateur astronomers. The appearance of the comet was peculiar, consisting of an envelope, a spherical coma near the nucleus and a tail extending in the anti-solar direction. We investigated the brightness and the morphological development of the comet by taking optical images with ground-based telescopes. Our observations show that V1 experienced a large-scale explosion between UT 2010 October 31 and November 3. The color of the comet was consistent with the Sun (g − R C = 0.61 ± 0.20, R C − I C = 0.20 ± 0.20, and B − R C = 0.93 ± 0.25), suggesting that dust particles were responsible for the brightening. We used a dynamical model to understand the peculiar morphology, and found that the envelope consisted of small grains (0.3-1 μm) expanding at a maximum speed of 500 ± 40 m s −1 , while the tail and coma were composed of a wider range of dust particle sizes (0.4-570 μm) and expansion speeds 7-390 m s −1 . The total mass of ejecta is ∼5 × 10 8 kg and kinetic energy ∼5 × 10 12 J. These values are much smaller than in the historic outburst of 17P/Holmes in 2007, but the energy per unit mass (1 × 10 4 J kg −1 ) is comparable. The energy per unit mass is about 10% of the energy released during the crystallization of amorphous water ice suggesting that crystallization of buried amorphous ice can supply the mass and energy of the outburst ejecta.

-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.