International Summer Institute for Modeling in Astrophysics

We have studied the stability of the thin dust-rich midplane in a protostellar disk. The layer tends to be broken apart by the Kelvin-Helmholtz instability due to differential velocities in the vertical direction, but is stabilized by the stretching of this instability thanks to the Coriolis force and radial shear. We found two trends depending on the metallicity at the midplane, which support previous studies and go further on which criteria is relevant to study these layers numerically.

In this document, we report our recent study on the turbulence inside the multi-phase ISM. First, we quantify the turbulence inside the molecular phase ISM by adopting a pixel-by-pixel line fitting strategy and studying the statistics of the fittings results of the ¹³CO lines. The histogram of the ¹³CO line amplitude and ¹³CO line FWHM show power-law behavior, indicative of turbulence. Especially, the histogram of the FWHM of the ¹³CO line show a dN/dv proportional to v^-2.45 at high velocity end, which seems to be universal. By plotting the 2D histogram of the ¹³CO line strength versus the ¹³CO line width, a lower limit of the ¹³CO line width for a given ¹³CO line strength can be identified. We argue that such a lower limit is due to the self-gravity of the molecular cloud. Second, with the combination of ¹³CO data from the GRS survey and the Hi 21cm data from the VGPS survey, we study the connection between the molecular gas probed with the ¹³CO line emission and the cold Hi gas probed by the Hi 21cm self-absorption feature. We found that the Hi gas that is associated with the molecular clouds is also turbulent. The molecular clouds that have detectable Hi envelope are more turbulent than clouds that do not show such envelope. Our results support the idea the molecular cloud turbulence is driven from outside, e.g. by the large-scale converging flow.

A simple, fast algorithm which simulates collisions between inelastic particles in an optically thin disk orbiting a central mass is implemented to the N-body simulation code MERCURY. The hybrid symplectic integrator is used to simulate a moonlet in the Saturn ring scenario, and produced a propeller structure around the moonlet which opens a partial gap in the ring.

A forced-dissipative shallow water model is adopted to simulate the jet streams, especially the equatorial ow, on Jupiter. Two types of forcing, the local mass pulse and vorticity pulse, are used to parameterize the small scale moist convection such as thunderstorms, respectively. In the mass-forced dissipative model without the frictional drag, it is unable to produce a prograde ow at equator. The reason could be that the anticyclonic features are favored by the off-equator positive mass forcing. In the simulations with the vorticity-type forcing, equatorial superrotation could be produced under some condition, although the physical mechanism is not fully understood.

We investigate the growth of linear perturbations on self-similar gravitational collapse solutions of an isothermal sphere. The perturbation equations are derived analytically, for exponentially growing modes, as well as oscillatory modes and the resulting system of differential equations is solved numerically, using different algorithms.

The transitional disks around young stars are protoplanetary disks with inner holes that are relatively empty of small dust grains, as inferred from the excess of far-infrared emission in their spectral energy distribution (SED) (Espaillat et al. 2007,2010). Recently, a new class of 'pre-transitional disks' are identified as exhibiting substantial emission from an optically thick inner disk separated from an optically thick outer disk by an optically thin gap (Espaillat et al. 2010). One plausible model for gap opening in these disks is by multiple giant planets (Zhu et al. 2011). However, two major problems remain to be solved. Firstly, micron-sized dust grains are not removed efficiently enough from the giant planet's gap to explain the observed low disk emission at near/mid-infrared wavelengths. Secondly, the presence of multiple Jupiter mass planets in resonance is not likely in standard disk models. We have developed a simple but robust coagulation-fragmentation model showing that piled-up material at the outer gap edge acts as a very efficient filter for micron-sized grains. Its reduction of the particle flow by two orders of magnitude provides excellent agreement with observational data. We can also produce high local surface density of particles at the outer edge of the gap, which may trigger planet formation in the outer disk.

Observations of T-Tauri circumstellar discs show the presence of mm or cm size dust grains at large distances from the central star (r > 10s AU). There empirical data challenge the currently mainstream grain growth theory, that disfavours the formation of such large grains in the outer disc and, despite formation, predicts their rapid inward migration due to coupling with the gas on short timescales. In this work, we develop some improvements in the grain growth theory and implement them in GrOG (Growth Of Grains), a new numerical solver for the coagulation and fragmentation of grains inside a circumstellar disc. Our results revise conclusions from previous theoretical models, as we are able to growth particles of significantly larger size.

Recent observations have shown that carbon in the gas around beta Pictoris is more than 100 times over- abundant with respect to the solar abundance. Although it is thought that such an overabundance in carbon is crucial to retain the metal elements in the disk, its origin is however unclear. In this paper, we establish a simple analytical model to study gas the removal process and thus calculate the abundance of various elements in the gas disk around beta Pictoris. The gas removal rate is controlled by the inward flow from viscous accretion and the outward radiation-induced drift. If the disk viscosity (using classical alpha-disk model) is low, radiation drift dominates the gas loss, and carbon can become highly overabundant. In order to produce the observed overabundance of carbon, a low viscosity of alpha < 10^-3 and a gas production with solar abundance are preferred.

This project investigates the charge exchange process between the atmospheric escape of a hot jupiter and the wind of its host star as a possible explanation for the Lyman alpha high-velocity absorption of the stellar spectrum observed during transits. We use 2D hydrodynamical simulation to follow the dynamic of the two fluids (shocks, compression layers...) and we implement a chemistry module to compute the quantity of neutral hydrogen produced by charge exchange. The simulations show that Kelvin Helmholtz instabilities develop at the interface between the two flows leading to a mixing that gives enough energetic neutral hydrogen to explain the observed absorption. However works still need to be done, especially the implementation of coriolis force due to the orbital motion of the planet, to achieve a steady state and get the correct geometry for the interface.

We revisit the destruction of resonance between Neptune and Kuiper Belt Objects (KBOs) by random planetesimal scatterings, which has been studied by Murray-Clay and Chiang (2006) previously. In this work, we consider the encounters between Neptune's resonant KBOs and planetesimals and the Levy flight behavior of resonant KBOs corresponding to a single big kick. The analysis in this work is based on order-of-magnitude estimation.