Recent ucsc_astro_isima items

Analytical studies of fragmentation during gravitational collapse

Mon, 11 Jun 2012 00:00:00 +0000

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.

Improving the grain growth model in the outer part of circumstellar disks

Mon, 11 Jun 2012 00:00:00 +0000

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.

Charge exchange in the colliding winds of Hot Jupiters and their host stars

Mon, 11 Jun 2012 00:00:00 +0000

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.

Day-night cold traps for TiO in hot Jupiter atmospheres

Mon, 11 Jun 2012 00:00:00 +0000

Temperature inversion leading to a hot stratosphere have been observed in some hot-Jupiter. Theoretical models predict that such a temperature inversion can be caused by the presence of a strong absorber in the visible in the high atmosphere. Titanium oxide have been proposed to be a good candidate for being this extra-absorber. Although the temperature in the day side of these planets can be high enough to maintain titanium oxide in a gaseous phase, it is not the case in the night side. In this work we discuss how the day/night temperature contrast can lead to the depletion of titanium oxide in the high atmosphere of hot-Jupiter. Using 1D and 3D models we found some constraints on the vertical diffusion coefficient needed to maintain enough titanium oxide in the upper atmosphere to create a temperature inversion. These constraints are similar to the ones given by Spiegel et al. (2009) for the vertical cold trap but hold for all the planets, even the ones that are too hot to be...

Planetesimal formation: shear instabilities at the dust-rich mid-plane

Mon, 11 Jun 2012 00:00:00 +0000

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.

Ohmic dissipation in hot Jupiters

Mon, 11 Jun 2012 00:00:00 +0000

We present an isolated analytical model for the ohmic heating in the interior of hot jupiters, treating the wind zone as a parameterized boundary condition. Under a conserved estimation of the strength of induced field and the assumption of an isothermal-convective planet model, we conclude that the mechanism of ohmic heating may not explain the over-inflated radius of hot jupiters along. We also develop a new time dependent evolution model for hot jupiters with ohmic heating, further show that ohmic heating is important only when the planet mass is small or the planet is at late stage of evolution.

Gap formation in transitional and pre-transitional disks: dust filtration in the presence of coagulation and fragmentation.

Mon, 11 Jun 2012 00:00:00 +0000

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...

A simple model for understanding the day-night temperature constrast on Hot Jupiters

Mon, 11 Jun 2012 00:00:00 +0000

We examine the dynamical mechanisms that control heat transport in atmospheres of tidally locked planets. Current estimates of heat redistribution in atmospheres of hot Jupiters generally only consider the equilibrium of stellar irradiation and advection. In this work, we show that gravity waves can effectively transport heat on global scales in atmospheres of tidally locked planets. A simple 1D atmospheric shallow-water model is used to study the day/night temperature contrast as a function of the radiative, advective, frictional, and wave-travel timescales of the atmosphere. We qualitatively compare our results with those made by 2D non-linear shallow-water simulations.

Abundance and evolution of gas around Beta Pictoris

Mon, 11 Jun 2012 00:00:00 +0000

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.

Disk Dynamos in Simulations of Collapsing Cores

Mon, 11 Jun 2012 00:00:00 +0000

We present simulations of the collapse of a massive rotating protostellar core assuming conditions characteristic of Population III star formation. Starting with an initially weak magnetic field, we find that the combined action of compression and dynamo processes amplify the field to nearly equipartition levels. At late times, we find the magnetic field is able to buoyantly rise above and below the protostellar disk, producing a large-scale magnetic field.

Convectively generated zonal jets by thunderstorms on Jupiter

Mon, 11 Jun 2012 00:00:00 +0000

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.

Multi-phase turbulent ISM: Theory Confronting Observations

Mon, 11 Jun 2012 00:00:00 +0000

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...

Detecting the earliest stages of giant planet formation in scattered light

Mon, 11 Jun 2012 00:00:00 +0000

Using Whitney’s Monte Carlo radiative transfer code, we simulate the near IR scattered light images in both intensity and polarized intensity for a series of axisymmetric protoplanetary disk models. By measuring the properties of the images, we study the detectability of both the disks and the features of giant planet formation at early stage (i.e. gaps opened by the planets) in real observations, and the connection between the detected disk structure and the intrinsic properties of the system. We use real point spread functions of the Subaru telescope to convolve the images, in order to synthesize realistic images with the smallest spatial resolution and inner working angle which ground based instruments can provide at present. In the models without gaps, the effects of the disk depletion factor, mass, and flareness on the images are investigated, while for the models with a gap, we focus on the dependence of the detectability of the gap on the gap position, width, and depletion...

Planetary dynamics in collisional particle disks

Mon, 11 Jun 2012 00:00:00 +0000

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.

Destroying resonance between Neptune and its resonant Kuiper Belt Objects by stochastic planetesimal scatterings

Mon, 11 Jun 2012 00:00:00 +0000

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.

Sweet-Parker Reconnection with Anomalous Resistivity — A Toy Model