Chemistry in a gravitationally unstable protoplanetary disc

Until now, axisymmetric, alpha-disc models have been adopted for calculations of the chemical composition of protoplanetary discs. While this approach is reasonable for many discs, it is not appropriate when self-gravity is important. In this case, spiral waves and shocks cause temperature and density variations that affect the chemistry. We have adopted a dynamical model of a solar-mass star surrounded by a massive (0.39 Msun), self-gravitating disc, similar to those that may be found around Class 0 and early Class I protostars, in a study of disc chemistry. We find that for each of a number of species, e.g. H2O, adsorption and desorption dominate the changes in the gas-phase fractional abundance; because the desorption rates are very sensitive to temperature, maps of the emissions from such species should reveal the locations of shocks of varying strengths. The gas-phase fractional abundances of some other species, e.g. CS, are also affected by gas-phase reactions, particularly in warm shocked regions. We conclude that the dynamics of massive discs have a strong impact on how they appear when imaged in the emission lines of various molecular species.Comment: 10 figures and 3 tables, accepted for publication in MNRA

Simulated Observations of Young Gravitationally Unstable Protoplanetary Discs

The formation and earliest stages of protoplanetary discs remain poorly constrained by observations. ALMA will soon revolutionise this field. Therefore, it is important to provide predictions which will be valuable for the interpretation of future high sensitivity and high angular resolution observations. Here we present simulated ALMA observations based on radiative transfer modelling of a relatively massive (0.39 M_solar) self-gravitating disc embedded in a 10 M_solar dense core, with structure similar to the pre-stellar core L1544. We focus on simple species and conclude that C17O 3-2, HCO+ 3-2, OCS 26-25 and H2CO 404-303 lines can be used to probe the disc structure and kinematics at all scales.Comment: 12 pages, 15 figures, Accepted by MNRA

3D Radiative Hydrodynamics for Disk Stability Simulations: A Proposed Testing Standard and New Results

Recent three-dimensional radiative hydrodynamics simulations of protoplanetary disks report disparate disk behaviors, and these differences involve the importance of convection to disk cooling, the dependence of disk cooling on metallicity, and the stability of disks against fragmentation and clump formation. To guarantee trustworthy results, a radiative physics algorithm must demonstrate the capability to handle both the high and low optical depth regimes. We develop a test suite that can be used to demonstrate an algorithm's ability to relax to known analytic flux and temperature distributions, to follow a contracting slab, and to inhibit or permit convection appropriately. We then show that the radiative algorithm employed by Meji\'a (2004) and Boley et al. (2006) and the algorithm employed by Cai et al. (2006) and Cai et al. (2007, in prep.) pass these tests with reasonable accuracy. In addition, we discuss a new algorithm that couples flux-limited diffusion with vertical rays, we apply the test suite, and we discuss the results of evolving the Boley et al. (2006) disk with this new routine. Although the outcome is significantly different in detail with the new algorithm, we obtain the same qualitative answers. Our disk does not cool fast due to convection, and it is stable to fragmentation. We find an effective $\alpha\approx 10^{-2}$. In addition, transport is dominated by low-order modes.Comment: Submitted to Ap

Modeling of Laser-Induced Metal Combustion

Experiments involving the interaction of a high-power laser beam with metal targets demonstrate that combustion plays an important role. This process depends on reactions within an oxide layer, together with oxygenation and removal of this layer by the wind. We present an analytical model of laser-induced combustion. The model predicts the threshold for initiation of combustion, the growth of the combustion layer with time, and the threshold for self-supported combustion. Solutions are compared with detailed numerical modeling as benchmarked by laboratory experiments

Model of the influence of magnetic fields on a plasma electrode Pockels cell

We describe a model which gives the effects of magnetic fields on a plasma electrode Pockels cell. The fields arise from the return currents to the cathode as well as from neighboring devices such as amplifier flashlamps. In effect, electrons are treated as a static, planar fluid moving under the influence of magnetic fields, the electric field of the discharge, electron pressure gradients, and electron-atom elastic collisions. This leads to a closed two- dimensional equation for the electron density, which is solved subject to appropriate boundary collisions. The model is applied to four cases-. the baseline NIF configuration with magnetic fields due to balanced return currents; a case with unbalanced return currents; the reverser configuration containing an external field parallel to the main plasma current; and a configuration with a field perpendicular to both the current and the optical direction

Modeling Antimortar Lethality by a Solid-State Heat-Capacity Laser

We have studied the use of a solid-state heat-capacity laser (SSHCL) in mortar defense. This type of laser, as built at LLNL, produces high-energy pulses with a wavelength of about 1 {micro}m and a pulse repetition rate of 200 Hz. Currently, the average power is about 26 kW. Our model of target interactions includes optical absorption, two-dimensional heat transport in the metal casing and explosive, melting, wind effects (cooling and melt removal), high-explosive reactions, and mortar rotation. The simulations continue until HE initiation is reached. We first calculate the initiation time for a range of powers on target and spot sizes. Then we consider an engagement geometry in which a mortar is fired at an asset defended by a 100-kW SSHCL. Propagation effects such as diffraction, turbulent broadening, scattering, and absorption are calculated for points on the trajectory, by means of a validated model. We obtain kill times and fluences, as functions of the rotation rate. These appear quite feasible

Hydraulic/Shock-Jumps in Protoplanetary Disks

In this paper, we describe the nonlinear outcome of spiral shocks in protoplanetary disks. Spiral shocks, for most protoplanetary disk conditions, create a loss of vertical force balance in the post-shock region and result in rapid expansion of the gas perpendicular to the disk midplane. This expansion has characteristics similar to hydraulic jumps, which occur in incompressible fluids. We present a theory to describe the behavior of these hybrids between shocks and hydraulic jumps (shock bores) and then compare the theory to three-dimensional hydrodynamics simulations. We discuss the fully three-dimensional shock structures that shock bores produce and discuss possible consequences for disk mixing, turbulence, and evolution of solids.Comment: 39 pages, 18 figures, 1 table. Edited to match as closely as possible the ApJ proofs, which resulted in the correction of several typos. In addition, section 5.3 was slightly altered because an error in an analysis tool was discovered; the differences between the entropy gradient method and the Schwarzschild criterion method are minor. Figure 18 now only includes what was Figure18

Numerical determination of the material properties of porous dust cakes

The formation of planetesimals requires the growth of dust particles through collisions. Micron-sized particles must grow by many orders of magnitude in mass. In order to understand and model the processes during this growth, the mechanical properties, and the interaction cross sections of aggregates with surrounding gas must be well understood. Recent advances in experimental (laboratory) studies now provide the background for pushing numerical aggregate models onto a new level. We present the calibration of a previously tested model of aggregate dynamics. We use plastic deformation of surface asperities as the physical model to bring critical velocities for sticking into accordance with experimental results. The modified code is then used to compute compression strength and the velocity of sound in the aggregate at different densities. We compare these predictions with experimental results and conclude that the new code is capable of studying the properties of small aggregates.Comment: Accepted for publication in A&

The Thermal Regulation of Gravitational Instabilities in Protoplanetary Disks III. Simulations with Radiative Cooling and Realistic Opacities

This paper presents a fully three-dimensional radiative hydrodymanics simulation with realistic opacities for a gravitationally unstable 0.07 Msun disk around a 0.5 Msun star. We address the following aspects of disk evolution: the strength of gravitational instabilities under realistic cooling, mass transport in the disk that arises from GIs, comparisons between the gravitational and Reynolds stresses measured in the disk and those expected in an alpha-disk, and comparisons between the SED derived for the disk and SEDs derived from observationally determined parameters. The mass transport in this disk is dominated by global modes, and the cooling times are too long to permit fragmentation for all radii. Moreover, our results suggest a plausible explanation for the FU Ori outburst phenomenon.Comment: 45 pages, 17 figures; submitted to Ap