The interplay between radiation pressure and the photoelectric instability in optically thin disks of gas and dust

Previous theoretical works have shown that in optically thin disks, dust grains are photoelectrically stripped of electrons by starlight, heating nearby gas and possibly creating a dust clumping instability, the photoelectric instability (PeI), that significantly alters global disk structure. In the current work, we use the Pencil Code to perform the first numerical models of the PeI that include stellar radiation pressure on dust grains in order to explore the parameter regime in which the instability operates. In models with gas surface densities greater than $\sim$$10^{-4}~\mathrm{g}~\mathrm{cm}^{-2}$, we see a variety of dust structures, including sharp concentric rings and non-axisymmetric arcs and clumps that represent dust surface density enhancements of factors of $\sim$$5-20$ depending on the run parameters. The gas distributions show various structures as well, including clumps and arcs formed from spiral arms. In models with lower gas surface densities, vortices and smooth spiral arms form in the gas distribution, but the dust is too weakly coupled to the gas to be significantly perturbed. In one high gas surface density model, we include a large, low-order gas viscosity, and, in agreement with previous radiation pressure-free models, find that it observably smooths the structures that form in the gas and dust, suggesting that resolved images of a given disk may be useful for deriving constraints on the effective viscosity of its gas. Broadly, our models show that radiation pressure does not preclude the formation of complex structure from the PeI, but the qualitative manifestation of the PeI depends strongly on the parameters of the system. The PeI may provide an explanation for unusual disk morphologies such as the moving blobs of the AU Mic disk, the asymmetric dust distribution of the 49 Ceti disk, and the rings and arcs found in the disk around HD 141569A.Comment: 13 pages, 13 figures; submitted to Ap

Luminosity, Energy and Polarization Studies for the Linear Collider: Comparing e+e- and e-e- for NLC and TESLA

We present results from luminosity, energy and polarization studies at a future Linear Collider. We compare e+e- and e-e- modes of operation and consider both NLC and TESLA beam parameter specifications at a center-of-mass energy of 500 GeV. Realistic colliding beam distributions are used, which include dynamic effects of the beam transport from the Damping Rings to the Interaction Point. Beam-beam deflections scans and their impact for beam-based feedbacks are considered. A transverse kink instability is studied, including its impact on determining the luminosity-weighted center-of-mass energy. Polarimetry in the extraction line from the IP is presented, including results on beam distributions at the Compton IP and at the Compton detector.Comment: 17 pages, 12 figures. Presented at 5th International Workshop on Electron-Electron Interactions at TeV Energies, December 12-14, 2003, Santa Cruz, C

Eccentricity Growth Rates of Tidally Distorted Discs

We consider discs that orbit a central object and are tidally perturbed by a circular orbit companion. Such discs are sometimes subject to an eccentric instability due to the effects of certain resonances. Eccentric instabilities may be present in planetary rings perturbed by satellites, protostellar discs perturbed by planets, and discs in binary star systems. Although the basic mechanism for eccentric instability is well understood, the detailed response of a gaseous disc to such an instability is not understood. We apply a linear eccentricity evolution equation developed by Goodchild and Ogilvie. We explore how the eccentricity is distributed in such a disc and how the distribution in turn affects the instability growth rate for a range of disc properties. We identify a disc mode, termed the superhump mode, that is likely at work in the superhump binary star case. The mode results from the excitation of the fundamental free precession mode. We determine an analytic expression for the fundamental free mode precession rate that is applicable to a sufficiently cool disc. Depending on the disc sound speed and disc edge location, other eccentric modes can grow faster than the superhump mode and dominate.Comment: 11 pages, 15 figures to be published on MNRA

Detection of the evolutionary stages of variables in M3

The large number of variables in M3 provides a unique opportunity to study an extensive sample of variables with the same apparent distance modulus. Recent, high accuracy CCD time series of the variables show that according to their mean magnitudes and light curve shapes, the variables belong to four separate groups. Comparing the properties of these groups (magnitudes and periods) with horizontal branch evolutionary models, we conclude that these samples can be unambiguously identified with different stages of the horizontal branch stellar evolution. Stars close to the zero age horizontal branch (ZAHB) show Oosterhoff I type properties, while the brightest stars have Oosterhoff II type statistics regarding their mean periods and RRab/RRc number ratios. This finding strengthens the earlier suggestion of Lee et al. (1990) connecting the Oosterhoff dichotomy to evolutionary effects, however, it is unexpected to find large samples of both of the Oosterhoff type within a single cluster, which is, moreover, the prototype of the Oosterhoff I class globular clusters. The very slight difference between the Fourier parameters of the stars (at a given period) in the three fainter samples spanning over about 0.15 mag range in M_V points to the limitations of any empirical methods which aim to determine accurate absolute magnitudes of RR Lyrae stars solely from the Fourier parameters of the light curves.Comment: 4 pages, 4 figures. Submitted to Astrophys. J. Letter

Kepler-16b: safe in a resonance cell

The planet Kepler-16b is known to follow a circumbinary orbit around a system of two main-sequence stars. We construct stability diagrams in the "pericentric distance - eccentricity" plane, which show that Kepler-16b is in a hazardous vicinity to the chaos domain - just between the instability "teeth" in the space of orbital parameters. Kepler-16b survives, because it is close to the stable half-integer 11/2 orbital resonance with the central binary, safe inside a resonance cell bounded by the unstable 5/1 and 6/1 resonances. The neighboring resonance cells are vacant, because they are "purged" by Kepler-16b, due to overlap of first-order resonances with the planet. The newly discovered planets Kepler-34b and Kepler-35b are also safe inside resonance cells at the chaos border.Comment: 17 pages, including 5 figure

A Second Order Godunov Method for Multidimensional Relativistic Magnetohydrodynamics

We describe a new Godunov algorithm for relativistic magnetohydrodynamics (RMHD) that combines a simple, unsplit second order accurate integrator with the constrained transport (CT) method for enforcing the solenoidal constraint on the magnetic field. A variety of approximate Riemann solvers are implemented to compute the fluxes of the conserved variables. The methods are tested with a comprehensive suite of multidimensional problems. These tests have helped us develop a hierarchy of correction steps that are applied when the integration algorithm predicts unphysical states due to errors in the fluxes, or errors in the inversion between conserved and primitive variables. Although used exceedingly rarely, these corrections dramatically improve the stability of the algorithm. We present preliminary results from the application of these algorithms to two problems in RMHD: the propagation of supersonic magnetized jets, and the amplification of magnetic field by turbulence driven by the relativistic Kelvin-Helmholtz instability (KHI). Both of these applications reveal important differences between the results computed with Riemann solvers that adopt different approximations for the fluxes. For example, we show that use of Riemann solvers which include both contact and rotational discontinuities can increase the strength of the magnetic field within the cocoon by a factor of ten in simulations of RMHD jets, and can increase the spectral resolution of three-dimensional RMHD turbulence driven by the KHI by a factor of 2. This increase in accuracy far outweighs the associated increase in computational cost. Our RMHD scheme is publicly available as part of the Athena code.Comment: 75 pages, 28 figures, accepted for publication in ApJS. Version with high resolution figures available from http://jila.colorado.edu/~krb3u/Athena_SR/rmhd_method_paper.pd

Nonlinear Development of Streaming Instabilities In Strongly Magnetized Plasmas

The nonlinear development of streaming instabilities in the current layers formed during magnetic reconnection with a guide field is explored. Theory and 3-D particle-in-cell simulations reveal two distinct phases. First, the parallel Buneman instability grows and traps low velocity electrons. The remaining electrons then drive two forms of turbulence: the parallel electron-electron two-stream instability and the nearly-perpendicular lower hybrid instability. The high velocity electrons resonate with the turbulence and transfer momentum to the ions and low velocity electrons.Comment: Accepted by PR