午餐會報 (2021)

I will present a new method to measure the color-dependent source size in lensed quasars using the microlensing time delays inferred from multi-band light curves. In practice, the relevant observable is the differential microlensing time delays between different bands. I will show from simulation using the facility as Vera C. Rubin Observatory that if this differential time delay between bands can be measured with a precision of 0.1 days in any given lensed image, the disk size can be recovered to within a factor of 2. If four lensed images are used, our method is able to achieve an unbiased source measurement within the error of the order of 20%, which is comparable with other techniques.

Recent observations have revealed the existence of multiple-planet systems composed of the Earth-mass planets around late M dwarfs. Planets around late M dwarfs tend to be in or near resonant chains, while those around FGK dwarfs are not. We investigate the formation of multiple-planet systems in resonant chains around low-mass stars. A time-evolution model of the multiple-planet formation via pebble accretion in the early phase of the disk evolution is constructed based on the formation model for the TRAPPIST-1 system. We find that there is a range of the protoplanet formation timescale for forming the stable multiple-planet systems in resonant chains. This range depends on the stellar mass. We suggest that the protoplanet formation timescale is a key parameter for the formation of multiple-planet systems with planets in resonant chains around low-mass stars.

The arrival of ISOs -- and specifically the appearance of 1I/'Oumuamua -- points to a significant number density of free-floating bodies in the Solar neighborhood. We study the details of 'Oumuamua's pre-encounter galactic orbit and find that it encountered our Sun at very nearly its maximum vertical and radial excursion relative to the galactic plane. 'Oumuamua's kinematics, moreover, strongly resemble those of nearby Young Associations. To obtain an order-of-magnitude a-priori age estimate, we compared 'Oumuamua's orbit to the orbits of 13,066 F-type stars drawn from the Gaia DR2, and with a simple diffusion model, we find tau_`O ~35 Myr. We then compare 'Oumuamua's orbit with the trajectories of nearby moving groups, confirming that its motion is consistent with its membership in the Carina and Columba associations, whose ages are 45 Myr. We conducted Monte Carlo simulations that trace the orbits of test particles ejected from the stars in the Carina and Columba associations. We find that to achieve the inferred Pan-STARRS number density of 0.2 ISOs per au^3, the required ejection mass is ~ 100 M_{Jup} per known star within the associations. The Pan-STARRS observation is thus in significant tension with scenarios that posit 'Oumuamua's formation and ejection from a protostellar disk.

I will briefly discuss the occurrence and detection of spiral density waves and the viscous overstability in Saturn's rings. The former typically being excited by moons orbiting outside the rings, with typical wavelengths of some tens of kilometers, the latter being a spontaneous instability of the ring flow, instigating axisymmetric density wave trains on a 100m scale. The study of these wave structures can provide insights in principles of disk-satellite interaction and disk dynamics, and is therefore of fundamental interest for astrophysical disks. For instance, the actions of moons that orbit interior and exterior to Saturn’s rings bear many similar- ities to those of planets forming in proto-planetary disks. Furthermore, by utilizing a fluid description of the rings, the analysis of these wave structures in Saturn’s rings can provide important physical pa- rameters which are otherwise hard to constrain, such as the local mass surface density and the viscosity of the rings.