Determination of resonance locations in NGC 613 from morphological arguments

In this paper, we present BVRI imaging data of NGC 613. We use these data to determine the corotation radius of the bar, using the photometric phase crossing method. This method uses the phase angle of the spiral structure in several wavebands, and looks for a crossing between the blue (B) light and the redder wavebands (e.g., R or I). For NGC 613, we find two phase crossings, an outer phase crossing at 136 +/- 8 arcsec and an inner phase crossing at 16 +/- 8 arcsec. We argue that the outer phase crossing is due to the bar corotation radius, and from the bar length of $R_{\rm bar}=90.0\pm4.0$ arcsec we go on to calculate a relative bar pattern speed of R = 1.5 +/- 0.1, which is consistent with the results of previous methods described in the literature. For a better understanding of the inner phase crossing, we have created structure maps in all four wavebands and a B-R color map. All of our structure maps and our color map highlight a nuclear ring of star formation at a radius of ~4 arcsec, which had also been observed recently using ALMA. Furthermore, the radius of our inner phase crossing appears to be consistent with the size of a nuclear disk of star formation that has been recently detected and described in the literature. We therefore suggest that the phase crossing method can be used to detect the size of nuclear star formation regions as well as the location of corotation resonances in spiral galaxies.Comment: 8 pages accepted for publication in MNRA

The Formation and Role of Vortices in Protoplanetary Disks

We carry out a two-dimensional, compressible, simulation of a disk, including dust particles, to study the formation and role of vortices in protoplanetary disks. We find that anticyclonic vortices can form out of an initial random perturbation of the vorticity field. Vortices have a typical decay time of the order of 50 orbital periods (for a viscosity parameter alpha=0.0001 and a disk aspect ratio of H/r = 0.15). If vorticity is continuously generated at a constant rate in the flow (e.g. by convection), then a large vortex can form and be sustained (due to the merger of vortices). We find that dust concentrates in the cores of vortices within a few orbital periods, when the drag parameter is of the order of the orbital frequency. Also, the radial drift of the dust induces a significant increase in the surface density of dust particles in the inner region of the disk. Thus, vortices may represent the preferred location for planetesimal formation in protoplanetary disks. We show that it is very difficult for vortex mergers to sustain a relatively coherent outward flux of angular momentum.Comment: Sumitted to the Astrophysical Journal, October 20, 199

Effects of Dust Scattering in Expanding Spherical Nebulae

The mean intensity of planetary nebulae with an expanding atmosphere is modeled by considering dusty and dust-free atmospheres. The bulk matter density is determined from the adopted velocity field through the equation of continuity. The gas is assumed to consist of hydrogen and helium and the gas-to-dust mass ratio is taken to be $3\times10^{-4}$. The Rayleigh phase function is employed for atomic scattering while the full Mie theory of scattering is incorporated for determining the dust scattering and absorption cross-section as well as the phase function for the angular distribution of photons after scattering. It is shown that in a dust free atmosphere, the mean intensity increases with the increase in the expansion velocity that makes the medium diluted. The mean intensity profile changes significantly when dust scattering is incorporated. The increase in forward scattering of photons by the dust particles yields into an increase in the mean intensity as compared to that without dust. The mean intensity increases as the particle size is increased. Thus it is shown that both the expansion of the medium and the presence of dust play important role in determining the mean intensity of a planetary nebulae.Comment: 18 pages, Elseveir style (cls file included), 5 postscript figures, Accepted for publication in New Astronom

Measurement, Dissipation, and Quantum Control with Superconducting Circuits

The interaction between a superconducting circuit and its environment can cause decoherence. However, interactions with an environment are necessary for quantum state preparation and measurement. Through the dynamics of open quantum systems, the environment is a resource to control and readout superconducting circuit states. I present an experimental result demonstrating qubit state stabilization from engineered dissipation with a microwave photonic crystal. In addition, I discuss the statistical arrow of time in the dynamics of continuous quantum measurement. These results demonstrate an interplay between open quantum system dynamics and statistics, which highlights the role of both dissipation and measurement for quantum control

Using Noncooperative Potential Games to Improve Network Security

Our work puts forth a game theoretic global security mechanism to optimize security in a large heterogeneous network consisting of autonomous devices. Our work is applicable to a network that includes various computing devices such as PCs, cell phones, sensors, and control systems. Constraint satisfaction is used to fulfill the requirements of the differing computers in the network. Security metrics are used to quantify network security in a meaningful way. Attack tree analysis of the quantified security measurements is performed for decision-making to maximize security by altering links that form the network. Coalitions of the computers forming the network are used to improve efficiency, as well as give a broader and greater overall security than would be possible in their absence. Side payments are used to induce a computer to move beyond its selfish motivations to benefit another computer. In keeping with noncooperative game rules, costs to form links are imposed only on the initiator of the link.Computer Science Departmen