Radiation Pressure Supported Starburst Disks and AGN Fueling

We consider the structure of marginally Toomre-stable starburst disks under the assumption that radiation pressure on dust grains provides the dominant vertical support against gravity. This is particularly appropriate when the disk is optically thick to its own IR radiation, as in the central regions of ULIRGs. Because the disk radiates at its Eddington limit, the Schmidt-law for star formation changes in the optically-thick limit, with the star formation rate per unit area scaling as Sigma_g/kappa, where Sigma_g is the gas surface density and kappa is the mean opacity. We show that optically thick starburst disks have a characteristic flux and dust effective temperature of F ~ 10^{13} L_sun/kpc^2 and T_eff ~ 90K, respectively. We compare our predictions with observations and find good agreement. We extend our model from many-hundred parsec scales to sub-parsec scales and address the problem of fueling AGN. We assume that angular momentum transport proceeds via global torques rather than a local viscosity. We account for the radial depletion of gas due to star formation and find a strong bifurcation between two classes of disk models: (1) solutions with a starburst on large scales that consumes all of the gas with little fueling of a central AGN and (2) models with an outer large-scale starburst accompanied by a more compact starburst on 1-10 pc scales and a bright central AGN. The luminosity of the latter models is in many cases dominated by the AGN. We show that the vertical thickness of the starburst disk on pc scales can approach h ~ r, perhaps accounting for the nuclear obscuration in some Type 2 AGN. We also argue that the disk of young stars in the Galactic Center may be the remnant of such a compact nuclear starburst.Comment: 26 pages, 9 figures, emulateapj, accepted to ApJ, minor changes, discussion tightened, references adde

The effect of small elongations on the electronic and optical signatures in InAs nanocrystal quantum dots

We present a detailed theoretical investigation of the electronic structure and optical properties of InAs nanocrystals at the transition from spheres to rods. Using a semiempirical pseudopotential approach, we predict that, despite the qualitative similarity of both intra- and inter-band optical spectra, for NCs with R > 15 °A even slight elongations should result in shifts of the order of hundreds of meV in the spacings between STM peaks measured in the positive bias regime, in the position of the intra- band absorption peaks associated with transitions in the conduction band and in the separation between the first and the fifth peak in PLE experiments. Our results show that, based on the spectroscopic data, it should be possible to discriminate between spherical and elongated NCs with aspect ratios of length over diameter as small as 1.2. Indeed our results suggest that many nominally spherical experimental samples contained a large fraction of slightly elongated structures

Estimation over Communication Networks: Performance Bounds and Achievability Results

This paper considers the problem of estimation over communication networks. Suppose a sensor is taking measurements of a dynamic process. However the process needs to be estimated at a remote location connected to the sensor through a network of communication links that drop packets stochastically. We provide a framework for computing the optimal performance in the sense of expected error covariance. Using this framework we characterize the dependency of the performance on the topology of the network and the packet dropping process. For independent and memoryless packet dropping processes we find the steady-state error for some classes of networks and obtain lower and upper bounds for the performance of a general network. Finally we find a necessary and sufficient condition for the stability of the estimate error covariance for general networks with spatially correlated and Markov type dropping process. This interesting condition has a max-cut interpretation

Stratigraphical evidence of Elysium sea ice from HiRise images

Abstract not available

The Migration and Growth of Protoplanets in Protostellar Discs

We investigate the gravitational interaction of a Jovian mass protoplanet with a gaseous disc with aspect ratio and kinematic viscosity expected for the protoplanetary disc from which it formed. Different disc surface density distributions have been investigated. We focus on the tidal interaction with the disc with the consequent gap formation and orbital migration of the protoplanet. Nonlinear hydrodynamic simulations are employed using three independent numerical codes. A principal result is that the direction of the orbital migration is always inwards and such that the protoplanet reaches the central star in a near circular orbit after a characteristic viscous time scale of approximately 10,000 initial orbital periods. This was found to be independent of whether the protoplanet was allowed to accrete mass or not. Inward migration is helped through the disappearance of the inner disc, and therefore the positive torque it would exert, because of accretion onto the central star.Our results indicate that a realistic upper limit for the masses of closely orbiting giant planets is approximately 5 Jupiter masses, because of the reduced accretion rates obtained for planets of increasing mass. Assuming some process such as termination of the inner disc through a magnetospheric cavity stops the migration, the range of masses estimated for a number of close orbiting giant planets (Marcy, Cochran, & Mayor 1999; Marcy & Butler 1998) as well as their inward orbital migration can be accounted for by consideration of disc--protoplanet interactions during the late stages of giant planet formation. Maximally accreting protoplanets reached about four Jovian masses on reaching the neighbourhood of the central star.Comment: 19 pages, 16 figures, submitted to MNRAS. A version of this paper that includes high resolution figures may be obtained from http://www.maths.qmw.ac.uk/~rpn/preprint.htm

Pure multiplicative stochastic resonance of anti-tumor model with seasonal modulability

The effects of pure multiplicative noise on stochastic resonance in an anti-tumor system modulated by a seasonal external field are investigated by using theoretical analyses of the generalized potential and numerical simulations. For optimally selected values of the multiplicative noise intensity quasi-symmetry of two potential minima and stochastic resonance are observed. Theoretical results and numerical simulations are in good quantitative agreement.Comment: 5 pages, 5 figure

Morphological evidence for a sea-ice origin for Elysium Planitia platy terrain

Abstract not available

Discovery of disc precession in the M31 dipping X-ray binary Bo 158

We present results from three XMM-Newton observations of the M31 low mass X-ray binary XMMU J004314.4+410726.3 (Bo 158), spaced over 3 days in 2004, July. Bo 158 was the first dipping LMXB to be discovered in M31. Periodic intensity dips were previously seen to occur on a 2.78-hr period, due to absorption in material that is raised out of the plane of the accretion disc. The report of these observations stated that the dip depth was anti-correlated with source intensity. However, our new observations do not favour a strict intensity dependance, but rather suggest that the dip variation is due to precession of the accretion disc. This is to be expected in LMXBs with a mass ratio <~ 0.3 (period <~ 4 hr), as the disc reaches the 3:1 resonance with the binary companion, causing elongation and precession of the disc. A smoothed particle hydrodynamics simulation of the disc in this system shows retrograde rotation of a disc warp on a period of ~11 P_orb, and prograde disc precession on a period of ~29 P_orb. This is consistent with the observed variation in the depth of the dips. We find that the dipping behaviour is most likely to be modified by the disc precession, hence we predict that the dipping behaviour repeats on a 81+/-3 hr cycle.Comment: 9 pages, 6 figures, accepted for publication by MNRAS, changed conten

Renormalised four-point coupling constant in the three-dimensional O(N) model with N=0

We simulate self-avoiding walks on a cubic lattice and determine the second virial coefficient for walks of different lengths. This allows us to determine the critical value of the renormalized four-point coupling constant in the three-dimensional N-vector universality class for N=0. We obtain g* = 1.4005(5), where g is normalized so that the three-dimensional field-theoretical beta-function behaves as \beta(g) = - g + g^2 for small g. As a byproduct, we also obtain precise estimates of the interpenetration ratio Psi*, Psi* = 0.24685(11), and of the exponent \nu, \nu = 0.5876(2).Comment: 16 page