When viewed in optical starlight scattered by dust, the nearly edge-on debris disk surrounding the A5V star beta Pictoris (distance 19.3 pc; ref. 1) extends farther than 1,450 au from the star. Its large-scale complexity has been well characterized, but the detailed structure of the disk's central approximately 200-au region has remained elusive. This region is of special interest, because planets may have formed there during the star's 10-20-million-year lifetime, perhaps resulting in both the observed tilt of 4.6 degrees relative to the large-scale main disk and the partial clearing of the innermost dust. A peculiarity of the central disk (also possibly related to the presence of planets) is the asymmetry in the brightness of the 'wings', in which the southwestern wing is brighter and more extended at 12 microm than the northeastern wing. Here we present thermal infrared images of the central disk that imply that the brightness asymmetry results from the presence of a bright clump composed of particles that may differ in size from dust elsewhere in the disk. We suggest that this clump results from the collisional grinding of resonantly trapped planetesimals or the cataclysmic break-up of a planetesimal.

A computer-automated cw sodium guidestar FASOR (Frequency Addition Source of Optical Radiation) producing a single frequency 589-nm beam with up to 50 W for mesospheric beacon generation has been integrated with the 3.5-m telescope at the Starfire Optical Range, Kirtland AFB, New Mexico. Radiance tests have produced a peak guidestar V 1 magnitude = 5.1 (~7000 photons/s/cm 2 at zenith) for 30 W of circularly polarized pump power in November 2005. Estimated theoretical maximum guidestar radiance is about 3 times greater than measured values indicating saturation due to atoms possibly becoming trapped in F' = 1 and/or atomic recoil. From sky tests over 3.5 years, we have tracked the annual variation of the sodium column density by measuring the return flux as a function of fasor power and determining the slope at zero power. The maximum occurs on October 30 and the minimum on May 30, with corresponding predicted returns of 8000 (V 1 = 4.8) and 3000 (V 1 = 5.8) ph/s/cm 2 with 50 W of fasor power and circular polarization. The effect of the Earth's magnetic field on the radiance of the sodium laser guidestar (LGS) from various azimuths and elevations has been measured. The peak return flux over our observatory occurs at [az=198°; el=+71°], compared with the direction of the magnetic field lines at [190°; +62°], and it can vary by a factor of 3 over the sky above el = 30°. First results for non-optimized sodium LGS adaptive optics (AO) closed-loop operation have been obtained using binary stars. Strehl ratios of 0.03 have been measured at 850 nm and a 0.14 arc second binary star has been resolved during first closed loop observations. Guidestar characteristics, including radiance, size, and Rayleigh backscatter, the sodium LGS wavefront sensor (WFS) AO system, and recent closed-loop results on binary stars are presented.

This paper summarises a numerical investigation of phase mixing in time-independent Hamiltonian systems that admit a coexistence of regular and chaotic phase space regions, allowing also for low amplitude perturbations idealised as periodic driving, friction, and/or white and colored noise. The evolution of initially localised ensembles of orbits was probed through lower order moments and coarse-grained distribution functions. In the absence of time-dependent perturbations, regular ensembles disperse initially as a power law in time and only exhibit a coarse-grained approach towards an invariant equilibrium over comparatively long times. Chaotic ensembles generally diverge exponentially fast on a time scale related to a typical finite time Lyapunov exponent, but can exhibit complex behaviour if they are impacted by the effects of cantori or the Arnold web. Viewed over somewhat longer times, chaotic ensembles typical converge exponentially towards an invariant or near-invariant equilibrium. This, however, need not correspond to a true equilibrium, which may only be approached over very long time scales. Time-dependent perturbations can dramatically increase the efficiency of phase mixing, both by accelerating the approach towards a near-equilibrium and by facilitating diffusion through cantori or along the Arnold web so as to accelerate the approach towards a true equilibrium. The efficacy of such perturbations typically scales logarithmically in amplitude, but is comparatively insensitive to most other details, a conclusion which reinforces the interpretation that the perturbations act via a resonant coupling. PACS number(s): 05.60.+w, 05.40.+j, 51.10.+y, 98.10.+z

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