412 research outputs found
Formation of sharp eccentric rings in debris disks with gas but without planets
‘Debris disks’ around young stars (analogues of the Kuiper Belt in
our Solar System) show a variety of non-trivial structures attributed
to planetary perturbations and used to constrain the properties
of those planets. However, these analyses have largely
ignored the fact that some debris disks are found to contain small
quantities of gas, a component that all such disks should contain
at some level. Several debris disks have been measured with a
dust-to-gas ratio of about unity, at which the effect of hydrodynamics
on the structure of the disk cannot be ignored. Here
we report linear and nonlinear modelling that shows that dust–gas
interactions can produce some of the key patterns attributed to
planets. We find a robust clumping instability that organizes the
dust into narrow, eccentric rings, similar to the Fomalhaut debris
disk. The conclusion that such disks might contain planets is not
necessarily required to explain these systems
Band-Limited Coronagraphs using a halftone-dot process: II. Advances and laboratory results for arbitrary telescope apertures
The band-limited coronagraph is a nearly ideal concept that theoretically
enables perfect cancellation of all the light of an on-axis source. Over the
past years, several prototypes have been developed and tested in the
laboratory, and more emphasis is now on developing optimal technologies that
can efficiently deliver the expected high-contrast levels of such a concept.
Following the development of an early near-IR demonstrator, we present and
discuss the results of a second-generation prototype using halftone-dot
technology. We report improvement in the accuracy of the control of the local
transmission of the manufactured prototype, which was measured to be less than
1%.
This advanced H-band band-limited device demonstrated excellent contrast
levels in the laboratory, down to 10-6 at farther angular separations than 3
lambda/D over 24% spectral bandwidth. These performances outperform the ones of
our former prototype by more than an order of magnitude and confirm the
maturity of the manufacturing process.
Current and next generation high-contrast instruments can directly benefit
from such capabilities. In this context, we experimentally examine the ability
of the band-limited coronagraph to withstand various complex telescope
apertures.Comment: Accepted in ApJ - under pres
Keck Speckle Imaging of the White Dwarf G29-38: No Brown Dwarf Companion Detected
The white dwarf Giclas 29-38 has attracted much attention due to its large
infrared excess and the suggestion that excess might be due to a companion
brown dwarf. We observed this object using speckle interferometry at the Keck
telescope, obtaining diffraction-limited resolution (55 milliarcseconds) at K
band, and found it unresolved. Assuming the entire K band excess is due to a
single point-like companion, we place an upper limit on the binary separation
of 30 milliarcseconds, or 0.42 AU at the star's distance of 14.1 pc. This
result, combined with astroseismological data and other images of G29-38,
supports the hypothesis that the source of the near-infrared excess is not a
cool companion but a dust cloud.Comment: 7 pages, 2 figure
The Geometry of Resonant Signatures in Debris Disks with Planets
Using simple geometrical arguments, we paint an overview of the variety of
resonant structures a single planet with moderate eccentricity (e < ~0.6) can
create in a dynamically cold, optically thin dust disk. This overview may serve
as a key for interpreting images of perturbed debris disks and inferring the
dynamical properties of the planets responsible for the perturbations. We
compare the resonant geometries found in the solar system dust cloud with
observations of dust clouds around Vega and other stars and we discuss a new
model for the asymmetries in the Epsilon Eridani cloud.Comment: 27 pages, including 6 figures and 1 table, to appear in Ap
The Importance of Phase in Nulling Interferometry and a Three Telescope Closure-Phase Nulling Interferometer Concept
We discuss the theory of the Bracewell nulling interferometer and explicitly
demonstrate that the phase of the "white light" null fringe is the same as the
phase of the bright output from an ordinary stellar interferometer. As a
consequence a "closure phase" exists for a nulling interferometer with three or
more telescopes. We calculate the phase offset as a function of baseline length
for an Earth-like planet around the Sun at 10 pc, with a contrast ratio of
at 10 m. The magnitude of the phase due to the planet is radians, assuming the star is at the phase center of the array.
Although this is small, this phase may be observable in a three-telescope
nulling interferometer that measures the closure phase. We propose a simple
non-redundant three-telescope nulling interferometer that can perform this
measurement. This configuration is expected to have improved characteristics
compared to other nulling interferometer concepts, such as a relaxation of
pathlength tolerances, through the use of the "ratio of wavelengths" technique,
a closure phase, and better discrimination between exodiacal dust and planets
The New Class of Dusty DAZ White Dwarfs
Our mid-infrared survey of 124 white dwarfs with the Spitzer Space Telescope
and the IRAC imager has revealed an infrared excess associated with the white
dwarf WD 2115-560 naturally explained by circumstellar dust. This object is the
fourth white dwarf observed to have circumstellar dust. All four are DAZ white
dwarfs, i.e. they have both photospheric Balmer lines and photospheric metal
lines.
We discuss these four objects as a class, which we abbreviate "DAZd", where
the "d" stands for "dust". Using an optically-thick, geometrically-thin disk
model analogous to Saturn's rings, we find that the inner disk edges are at
>~0.1 to 0.2 Ro and that the outer disk edges are ~0.3 to 0.6 Ro. This model
naturally explains the accretion rates and lifetimes of the detected WD disks
and the accretion rates inferred from photospheric metal abundances.Comment: 27 pages, 7 figures, ApJ accepte
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