A Search for Water Masers in the Saturnian System

We searched for H2O 6(1,6)-5(2,3) maser emission at 22.235 GHz from several Saturnian satellites with the Nobeyama 45m radio telescope in May 2009. Observations were made for Titan, Hyperion, Enceladus and Atlas, for which Pogrebenko et al. (2009) had reported detections of water masers at 22.235 GHz, and in addition for Iapetus and other inner satellites. We detected no emission of the water maser line for all the satellites observed, although sensitivities of our observations were comparable or even better than those of Pogrebenko et al.. We infer that the water maser emission from the Saturnian system is extremely weak, or sporadic in nature. Monitoring over a long period and obtaining statistical results must be made for the further understanding of the water maser emission in the Saturnian system.Comment: 8 pages, 2 figures, accepted for publication in PASJ (Letter

The origin of planetary impactors in the inner solar system

New insights into the history of the inner solar system are derived from the impact cratering record of the Moon, Mars, Venus and Mercury, and from the size distributions of asteroid populations. Old craters from a unique period of heavy bombardment that ended $\sim$3.8 billion years ago were made by asteroids that were dynamically ejected from the main asteroid belt, possibly due to the orbital migration of the giant planets. The impactors of the past $\sim$3.8 billion years have a size distribution quite different from the main belt asteroids, but very similar to the population of near-Earth asteroids.Comment: 12 pages (including 4 figures

The invasion of a free floating planet and the number asymmetry of Jupiter Trojans

This paper extends our previous study (Li et al. 2023) of the early evolution of Jupiter and its two Trojan swarms by introducing the possible perturbations of a free floating planet (FFP) invading the Solar System. In the framework of the invasion of a FFP, we aim to provide some new scenarios to explain the number asymmetry of the L4 and L5 Jupiter Trojans, and some other observed features. We investigate two different cases: (i) The indirect case, where Jupiter experiences a scattering encounter with the FFP and jumps outwards at a speed that is much higher than that considered in(Li et al. 2023), resulting in a change in the numbers of the L4 (N4) and L5 (N5) Trojans swarms. (ii) The direct case, in which the FFP traverses the L5 region and affects the stability of the local Trojans. In the indirect case, the outward migration of Jupiter can be fast enough to make the L4 islands disappear temporarily, inducing a resonant amplitude increase of the local Trojans. After the migration is over, the L4 Trojans come back to the re-appeared and enlarged islands. As for the L5 islands, they always exist but expand even more considerably. Since the L4 swarm suffers less excitation in the resonant amplitude than the L5 swarm, more L4 Trojans are stable and could survive to the end. In the direct case, the FFP could deplete a considerable fraction of the L5 Trojans, while the L4 Trojans at large distances are not affected and all of them could survive. Both the indirect and direct cases could result in a number ratio of R45=N4/N5~1.6 that can potentially explain the current observations. The latter has the advantage of producing the observed resonant amplitude distribution. For achieving these results, we propose that the FFP should have a mass of at least of a few tens of Earth masses and its orbital inclination is allowed to be as high as 40 degrees.Comment: Accepted for publication in A&A, 13 pages, 7 figures, 5 table

Time-series photometry of Earth flyby asteroid 2012 DA14

Context. The object 2012 DA14 is a near-Earth asteroid with a size of several tens of meters. It had approached closely the Earth on 15 February, 2013 UT, providing an opportunity for precise measurements of this tiny asteroid. Aims. The solar phase angle of 2012 DA14 had varied widely around its closest approach but was almost constant during the following night. We performed time-series photometric observations on those two nights to determine the rotational properties and phase effect. Methods. The observations were carried out using the 0.55-m telescope at Saitama University, Japan. The R-band images were obtained continuously over a 2 hr period at the closest approach and for about 5 hr on the next night. Results. The lightcurve data from the second night indicates a rotational period of 11.0 +1.8/-0.6 hr and a peak-to-peak amplitude of 1.59 +/- 0.02 mag. The brightness variation before and after the closest approach was separated into two components that are derived from the rotation and phase effect. We found that the phase curve slope of this asteroid is significantly shallower than those of other L-type asteroids. Conclusions. We suggest that 2012 DA14 is coated with a coarse surface that lacks fine regolith particles and/or a high albedo surface.Comment: 4 pages, 4 figures, accepted for publication in Astronomy and Astrophysic