10,251 research outputs found
Spectral Energy Distributions of Passive T Tauri Disks: Inclination
We compute spectral energy distributions (SEDs) for passive T Tauri disks
viewed at arbitrary inclinations. Semi-analytic models of disks in radiative
and hydrostatic equilibrium are employed. Over viewing angles for which the
flared disk does not occult the central star, the SED varies negligibly with
inclination. For such aspects, the SED shortward of ~80 microns is particularly
insensitive to orientation, since short wavelength disk emission is dominated
by superheated surface layers which are optically thin. The SED of a nearly
edge-on disk is that of a class I source. The outer disk occults inner disk
regions, and emission shortward of ~30 microns is dramatically extinguished.
Spectral features from dust grains may appear in absorption. However,
millimeter wavelength fluxes decrease by at most a factor of 2 from face-on to
edge-on orientations.
We present illustrative applications of our SED models. The class I source
04108+2803B is considered a T Tauri star hidden from view by an inclined
circumstellar disk. Fits to its observed SED yield model-dependent values for
the disk mass of ~0.015 solar masses and a disk inclination of ~65 degrees
relative to face-on. The class II source GM Aur represents a T Tauri star
unobscured by its circumstellar disk. Fitted parameters include a disk mass of
\~0.050 solar masses and an inclination of ~60 degrees.Comment: Accepted to ApJ, 20 pages, 7 figures, aaspp4.st
Keck Pencil-Beam Survey for Faint Kuiper Belt Objects
We present the results of a pencil-beam survey of the Kuiper Belt using the
Keck 10-m telescope. A single 0.01 square degree field is imaged 29 times for a
total integration time of 4.8 hr. Combining exposures in software allows the
detection of Kuiper Belt Objects (KBOs) having visual magnitude V < 27.9. Two
new KBOs are discovered. One object having V = 25.5 lies at a probable
heliocentric distance d = 33 AU. The second object at V = 27.2 is located at d
= 44 AU. Both KBOs have diameters of about 50 km, assuming comet-like albedos
of 4%.
Data from all surveys are pooled to construct the luminosity function from
red magnitude R = 20 to 27. The cumulative number of objects per square degree,
N (< R), is fitted to a power law of the form log_(10) N = 0.52 (R - 23.5).
Differences between power laws reported in the literature are due mainly to
which survey data are incorporated, and not to the method of fitting. The
luminosity function is consistent with a power-law size distribution for
objects having diameters s = 50 to 500 km; dn ~ s^(-q) ds, where the
differential size index q = 3.6 +/- 0.1. The distribution is such that the
smallest objects possess most of the surface area, but the largest bodies
contain the bulk of the mass. Though our inferred size index nearly matches
that derived by Dohnanyi (1969), it is unknown whether catastrophic collisions
are responsible for shaping the size distribution. Implications of the absence
of detections of classical KBOs beyond 50 AU are discussed.Comment: Accepted to AJ. Final proof-edited version: references added,
discussion of G98 revised in sections 4.3 and 5.
High Rayleigh number convection with double diffusive fingers
An electrodeposition cell is used to sustain a destabilizing concentration
difference of copper ions in aqueous solution between the top and bottom
boundaries of the cell. The resulting convecting motion is analogous to
Rayleigh-B\'enard convection at high Prandtl numbers. In addition, a
stabilizing temperature gradient is imposed across the cell. Even for thermal
buoyancy two orders of magnitude smaller than chemical buoyancy, the presence
of the weak stabilizing gradient has a profound effect on the convection
pattern. Double diffusive fingers appear in all cases. The size of these
fingers and the flow velocities are independent of the height of the cell, but
they depend on the ion concentration difference between top and bottom
boundaries as well as on the imposed temperature gradient. The scaling of the
mass transport is compatible with previous results on double diffusive
convection
Vertical Shearing Instabilities in Radially Shearing Disks: The Dustiest Layers of the Protoplanetary Nebula
Gravitational instability of a vertically thin, dusty sheet near the midplane
of a protoplanetary disk has long been proposed as a way of forming
planetesimals. Before Roche densities can be achieved, however, the dust-rich
layer, sandwiched from above and below by more slowly rotating dust-poor gas,
threatens to overturn and mix by the Kelvin-Helmholtz instability (KHI).
Whether such a threat is real has never been demonstrated: the Richardson
criterion for the KHI is derived for 2-D Cartesian shear flow and does not
account for rotational forces. Here we present 3-D numerical simulations of
gas-dust mixtures in a shearing box, accounting for the full suite of
disk-related forces: the Coriolis and centrifugal forces, and radial tidal
gravity. Dust particles are assumed small enough to be perfectly entrained in
gas; the two fluids share the same velocity field but obey separate continuity
equations. We find that the Richardson number Ri does not alone determine
stability. The critical value of Ri below which the dust layer overturns and
mixes depends on the height-integrated metallicity Z (surface density ratio of
dust to gas). Nevertheless, for Z between one and five times solar, the
critical Ri is nearly constant at 0.1. Keplerian radial shear stabilizes those
modes that would otherwise disrupt the layer at large Ri. If Z is at least 5
times greater than the solar value of 0.01, then midplane dust densities can
approach Roche densities. Such an environment might be expected to produce gas
giant planets having similarly super-solar metallicities.Comment: ApJ, in press. Connections made to baroclinic instability. Movies
available at http://astro.berkeley.edu/~echiang/im/im.htm
Phase-sensitive quantum effects in Andreev conductance of the SNS system of metals with macroscopic phase breaking length
The dissipative component of electron transport through the doubly connected
SNS Andreev interferometer indium (S)-aluminium (N)-indium (S) has been
studied. Within helium temperature range, the conductance of the individual
sections of the interferometer exhibits phase-sensitive oscillations of
quantum-interference nature. In the non-domain (normal) state of indium
narrowing adjacent to NS interface, the nonresonance oscillations have been
observed, with the period inversely proportional to the area of the
interferometer orifice. In the domain intermediate state of the narrowing, the
magneto-temperature resistive oscillations appeared, with the period determined
by the coherence length in the magnetic field equal to the critical one. The
oscillating component of resonance form has been observed in the conductance of
the macroscopic N-aluminium part of the system. The phase of the oscillations
appears to be shifted by compared to that of nonresonance oscillations.
We offer an explanation in terms of the contribution into Josephson current
from the coherent quasiparticles with energies of order of the Thouless energy.
The behavior of dissipative transport with temperature has been studied in a
clean normal metal in the vicinity of a single point NS contact.Comment: 9 pages, 7 figures, to be published in Low Temp. Phys., v. 29, No.
12, 200
Visualization of Coherent Destruction of Tunneling in an Optical Double Well System
We report on a direct visualization of coherent destruction of tunneling
(CDT) of light waves in a double well system which provides an optical analog
of quantum CDT as originally proposed by Grossmann, Dittrich, Jung, and Hanggi
[Phys. Rev. Lett. {\bf 67}, 516 (1991)]. The driven double well, realized by
two periodically-curved waveguides in an Er:Yb-doped glass, is designed so that
spatial light propagation exactly mimics the coherent space-time dynamics of
matter waves in a driven double-well potential governed by the Schr\"{o}dinger
equation. The fluorescence of Er ions is exploited to image the spatial
evolution of light in the two wells, clearly demonstrating suppression of light
tunneling for special ratios between frequency and amplitude of the driving
field.Comment: final versio
Spectral Energy Distributions of T Tauri Stars With Passive Circumstellar Disks
We derive hydrostatic, radiative equilibrium models for passive disks
surrounding T Tauri stars. Each disk is encased by an optically thin layer of
superheated dust grains. This layer re-emits directly to space about half the
stellar energy it absorbs. The other half is emitted inward and regulates the
interior temperature of the disk. The heated disk flares. As a consequence, it
absorbs more stellar radiation, especially at large radii, than a flat disk
would. The portion of the spectral energy distribution contributed by the disk
is fairly flat throughout the thermal infrared. At fixed frequency, the
contribution from the surface layer exceeds that from the interior by about a
factor 3 and is emitted at more than an order of magnitude greater radius.
Spectral features from dust grains in the superheated layer appear in emission
if the disk is viewed nearly face-on.Comment: 29 LaTeX pages w/ 10 eps. figures, aaspp4.sty, final version with few
minor stylistic alterations and 1 content change (section 4.1.1 on GM Aur and
non-zero inclination
Fomalhaut's Debris Disk and Planet: Constraining the Mass of Formalhaut B from Disk Morphology
Following the optical imaging of exoplanet candidate Fomalhaut b (Fom b), we present a numerical model of how Fomalhaut's debris disk is gravitationally shaped by a single interior planet. The model is simple, adaptable to other debris disks, and can be extended to accommodate multiple planets. If Fom b is the dominant perturber of the belt, then to produce the observed disk morphology it must have a mass M(sub pl) 101.5AU, and an orbital eccentricity e(sub pl) = 0.11 - 0.13. These conclusions are independent of Fom b's photometry. To not disrupt the disk, a greater mass for Fom b demands a smaller orbit farther removed from the disk; thus, future astrometric measurement of Fom b's orbit, combined with our model of planet-disk interaction, can be used to determine the mass more precisely. The inner edge of the debris disk at a approximately equals 133AU lies at the periphery of Fom b's chaotic zone, and the mean disk eccentricity of e approximately equals 0.11 is secularly forced by the planet, supporting predictions made prior to the discovery of Fom b. However, previous mass constraints based on disk morphology rely on several oversimplifications. We explain why our constraint is more reliable. It is based on a global model of the disk that is not restricted to the planet's chaotic zone boundary. Moreover, we screen disk parent bodies for dynamical stability over the system age of approximately 100 Myr, and model them separately from their dust grain progeny; the latter's orbits are strongly affected by radiation pressure and their lifetimes are limited to approximately 0.1 Myr by destructive grain-grain collisions. The single planet model predicts that planet and disk orbits be apsidally aligned. Fomalhaut b's nominal space velocity does not bear this out, but the astrometric uncertainties are difficult to quantify. Even if the apsidal misalignment proves real, our calculated upper mass limit of 3 M(sub J) still holds. Parent bodies are evacuated from mean-motion resonances with Fom b; these empty resonances are akin to the Kirkwood gaps opened by Jupiter. The belt contains at least 3M(sub Earth) of solids that are grinding down to dust, their velocity dispersions stirred so strongly by Fom b that collisions are destructive. Such a large mass in solids is consistent with Fom b having formed in situ
Shifts of the nuclear resonance in the vortex lattice in YBaCuO
The NMR and NQR spectra of Cu in the CuO plane of
YBaCuO in the superconducting state are discussed in terms of the
phenomenological theory of Ginzburg-Landau type extended to lower temperatures.
We show that the observed spectra, Kumagai {\em et al.}, PRB {\bf 63}, 144502
(2001), can be explained by a standard theory of the Bernoulli potential with
the charge transfer between CuO planes and CuO chains assumed.Comment: 11 pages 7 figure
Apse Alignment of Narrow Eccentric Planetary Rings
The boundaries of the Uranian ∈, α, and β rings can be fitted by Keplerian ellipses. The pair of ellipses that outline a given ring share a common line of apsides. Apse alignment is surprising because the quadrupole moment of Uranus induces differential precession. We propose that rigid precession is maintained by a balance of forces due to ring self-gravity, planetary oblateness, and interparticle collisions. Collisional impulses play an especially dramatic role near ring edges. Pressure-induced accelerations are maximal near edges because there (1) velocity dispersions are enhanced by resonant satellite perturbations and (2) the surface density declines steeply. Remarkably, collisional forces felt by material in the last ~100 m of a ~10 km wide ring can increase equilibrium masses up to a factor of ~100. New ring surface densities are derived that accord with Voyager radio measurements. In contrast to previous models, collisionally modified self-gravity appears to allow for both negative and positive eccentricity gradients; why all narrow planetary rings exhibit positive eccentricity gradients remains an open question
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