Coronagraphic Low Order Wavefront Sensor: Principle and Application to a Phase-Induced Amplitude Coronagraph

High contrast coronagraphic imaging of the immediate surrounding of stars requires exquisite control of low-order wavefront aberrations, such as tip-tilt (pointing) and focus. We propose an accurate, efficient and easy to implement technique to measure such aberrations in coronagraphs which use a focal plane mask to block starlight. The Coronagraphic Low Order Wavefront Sensor (CLOWFS) produces a defocused image of a reflective focal plane ring to measure low order aberrations. Even for small levels of wavefront aberration, the proposed scheme produces large intensity signals which can be easily measured, and therefore does not require highly accurate calibration of either the detector or optical elements. The CLOWFS achieves nearly optimal sensitivity and is immune from non-common path errors. This technique is especially well suited for high performance low inner working angle (IWA) coronagraphs. On phase-induced amplitude apodization (PIAA) type coronagraphs, it can unambiguously recover aberrations which originate from either side of the beam shaping introduced by the PIAA optics. We show that the proposed CLOWFS can measure sub-milliarcsecond telescope pointing errors several orders of magnitude faster than would be possible in the coronagraphic science focal plane alone, and can also accurately calibrate residual coronagraphic leaks due to residual low order aberrations. We have demonstrated 1e-3 lambda/D pointing stability in a laboratory demonstration of the CLOWFS on a PIAA type coronagraph

Collisional Grooming Models of the Kuiper Belt Dust Cloud

We modeled the 3-D structure of the Kuiper Belt dust cloud at four different dust production rates, incorporating both planet-dust interactions and grain-grain collisions using the collisional grooming algorithm. Simulated images of a model with a face-on optical depth of ~10^-4 primarily show an azimuthally-symmetric ring at 40-47 AU in submillimeter and infrared wavelengths; this ring is associated with the cold classical Kuiper Belt. For models with lower optical depths (10^-6 and 10^-7), synthetic infrared images show that the ring widens and a gap opens in the ring at the location of of Neptune; this feature is caused by trapping of dust grains in Neptune's mean motion resonances. At low optical depths, a secondary ring also appears associated with the hole cleared in the center of the disk by Saturn. Our simulations, which incorporate 25 different grain sizes, illustrate that grain-grain collisions are important in sculpting today's Kuiper Belt dust, and probably other aspects of the Solar System dust complex; collisions erase all signs of azimuthal asymmetry from the submillimeter image of the disk at every dust level we considered. The model images switch from being dominated by resonantly-trapped small grains ("transport dominated") to being dominated by the birth ring ("collision dominated") when the optical depth reaches a critical value of tau ~ v/c, where v is the local Keplerian speed.Comment: 31 pages, including 9 figure

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

Long-Term Dynamics and the Orbital Inclinations of the Classical Kuiper Belt Objects

We numerically integrated the orbits of 1458 particles in the region of the classical Kuiper Belt (41 AU < a < 47 AU) to explore the role of dynamical instabilities in sculpting the inclination distribution of the classical Kuiper Belt Objects (KBOs). We find that the selective removal of low-inclination objects by overlapping secular resonances (nu_17 and nu_18) acts to raise the mean inclination of the surviving population of particles over 4 billion years of interactions with Jupiter, Saturn, Uranus and Neptune, though these long-term dynamical effects do not themselves appear to explain the discovery of KBOs with inclinations near 30 degrees. Our integrations also imply that after 3 billion years of interaction with the massive planets, high inclination KBOs more efficiently supply Neptune-encountering objects, the likely progenitors of short-period comets, Centaurs, and scattered KBOs. The secular resonances at low inclinations may indirectly cause this effect by weeding out objects unprotected by mean motion resonances during the first 3 billion years.Comment: 23 pages, including 10 figures. Accepted for publication in A

Apocenter glow in eccentric debris disks: implications for Fomalhaut and epsilon Eridani

Debris disks often take the form of eccentric rings with azimuthal asymmetries in surface brightness. Such disks are often described as showing "pericenter glow", an enhancement of the disk brightness in regions nearest the central star. At long wavelengths, however, the disk apocenters should appear brighter than their pericenters: in the long wavelength limit, we find the apocenter/pericenter flux ratio scales as 1+e for disk eccentricity e. We produce new models of this "apocenter glow" to explore its causes and wavelength dependence and study its potential as a probe of dust grain properties. Based on our models, we argue that several far-infrared and (sub)millimeter images of the Fomalhaut and epsilon Eridani debris rings obtained with Herschel, JCMT, SHARC II, ALMA, and ATCA should be reinterpreted as suggestions or examples of apocenter glow. This reinterpretation yields new constraints on the disks' dust grain properties and size distributions.Comment: 20 pages, 7 figures; accepted to Ap

A Search for Resonant Structures in the Zodiacal Cloud with COBE DIRBE: The Mars Wake and Jupiter's Trojan Clouds

We searched the COBE DIRBE Sky and Zodi Atlas for a wake of dust trailing Mars and for Trojan dust near Jupiter's L5 Lagrange point. We compare the DIRBE images to a model Mars wake based on the empirical model of the Earth's wake as seen by the DIRBE and place a 3-sigma upper limit on the fractional overdensity of particles in the Mars wake of 18% of the fractional overdensity trailing the Earth. We place a 3-sigma upper limit on the effective emitting area of large (10-100 micron diameter) particles trapped at Jupiter's L5 Lagrange point of 6 x 10^17 cm^2, assuming that these large dust grains are distributed in space like the Trojan asteroids. We would have detected the Mars wake if the surface area of dust in the wake scaled simply as the mass of the planet times the Poynting-Robertson time scale.Comment: Sixteen pages, and figures 1, 2, 3a, 3b, 4, 5, 6, and 7. Accepted for publication in Icaru