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eScholarship
Open Access Publications from the University of California

UCSC is one of the world's leading centers for both observational and theoretical research in astronomy and astrophysics. The department was recently ranked first in the country in research impact, based on citation studies. Faculty and students in the department and our affiliated research centers are building and using first-rank telescopes and instrumentation—on Earth and in space—extending humanity’s vision to planets orbiting nearby stars and the first stirrings of the Universe.

The department includes 24 faculty members, whose research interests range from our solar system and the Milky Way to the most distant galaxies in the Universe and the most fundamental questions of cosmology.

UCSC is a leader in astrophysics education, and we attract some the best graduate students in the country, enrolling approximately 40 students working towards the Ph.D. degree.

Currently this page is for hosting only ISIMA (International Summer Institute for Modeling in Astrophysics) conference proceedings.

Cover page of Multi-phase turbulent ISM: Theory Confronting Observations

Multi-phase turbulent ISM: Theory Confronting Observations

(2011)

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.

Cover page of Abundance and evolution of gas around Beta Pictoris

Abundance and evolution of gas around Beta Pictoris

(2011)

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.

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

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

(2011)

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.

Cover page of Analytical studies of fragmentation during gravitational collapse

Analytical studies of fragmentation during gravitational collapse

(2011)

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.

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

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

(2011)

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.

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

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

(2011)

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.

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

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

(2011)

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.

Cover page of Convectively generated zonal jets by thunderstorms on Jupiter

Convectively generated zonal jets by thunderstorms on Jupiter

(2011)

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.

Cover page of Disk Dynamos in Simulations of Collapsing Cores

Disk Dynamos in Simulations of Collapsing Cores

(2011)

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.

Cover page of Detecting the earliest stages of giant planet formation in scattered light

Detecting the earliest stages of giant planet formation in scattered light

(2011)

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 factor. Qualitatively, the more massive and more flared the disk is, the brighter the disk is. The gap is only visible when the disk is visible, and the deeper and wider the gap is, the larger the contrast level of the gap is.