2,529 research outputs found
Measuring the Interplanetary Medium with a Solar Sail
NASA has been considering a solar sail that would accelerate a craft to a
high velocity (~ 14 AU/yr) by the time it reached 5 AU. Then the sail would be
dropped and the craft would coast alone to deep space. We propose that the sail
be retained longer. Then the density of the interplanetary medium could be
determined by measuring the drag force on the huge sail using radiometric
navigational data. Such an experiment would yield an independent, new type of
measurement of the interplanetary medium and should be pursued.Comment: 12 page
A Spitzer/IRAC Search for Substellar Companions of the Debris Disk Star epsilon Eridani
We have used the InfraRed Array Camera (IRAC) onboard the Spitzer Space
telescope to search for low mass companions of the nearby debris disk star
epsilon Eridani. The star was observed in two epochs 39 days apart, with
different focal plane rotation to allow the subtraction of the instrumental
Point Spread Function, achieving a maximum sensitivity of 0.01 MJy/sr at 3.6
and 4.5 um, and 0.05 MJy/sr at 5.8 and 8.0 um. This sensitivity is not
sufficient to directly detect scattered or thermal radiation from the epsilon
Eridani debris disk. It is however sufficient to allow the detection of Jovian
planets with mass as low as 1 MJ in the IRAC 4.5 um band. In this band, we
detected over 460 sources within the 5.70 arcmin field of view of our images.
To test if any of these sources could be a low mass companion to epsilon
Eridani, we have compared their colors and magnitudes with models and
photometry of low mass objects. Of the sources detected in at least two IRAC
bands, none fall into the range of mid-IR color and luminosity expected for
cool, 1 Gyr substellar and planetary mass companions of epsilon Eridani, as
determined by both models and observations of field M, L and T dwarf. We
identify three new sources which have detections at 4.5 um only, the lower
limit placed on their [3.6]-[4.5] color consistent with models of planetary
mass objects. Their nature cannot be established with the currently available
data and a new observation at a later epoch will be needed to measure their
proper motion, in order to determine if they are physically associated to
epsilon Eridani.Comment: 36 pages, to be published on The Astrophysical Journal, vol. 647,
August 200
Radial Distribution of Dust Grains Around HR 4796A
We present high-dynamic-range images of circumstellar dust around HR 4796A
that were obtained with MIRLIN at the Keck II telescope at lambda = 7.9, 10.3,
12.5 and 24.5 um. We also present a new continuum measurement at 350 um
obtained at the Caltech Submillimeter Observatory. Emission is resolved in Keck
images at 12.5 and 24.5 um with PSF FWHM's of 0.37" and 0.55", respectively,
and confirms the presence of an outer ring centered at 70 AU. Unresolved excess
infrared emission is also detected at the stellar position and must originate
well within 13 AU of the star. A model of dust emission fit to flux densities
at 12.5, 20.8, and 24.5 um indicates dust grains are located 4(+3/-2) AU from
the star with effective size, 28+/-6 um, and an associated temperature of
260+/-40 K.
We simulate all extant data with a simple model of exozodiacal dust and an
outer exo-Kuiper ring. A two-component outer ring is necessary to fit both Keck
thermal infrared and HST scattered-light images. Bayesian parameter estimates
yield a total cross-sectional area of 0.055 AU^2 for grains roughly 4 AU from
the star and an outer-dust disk composed of a narrow large-grain ring embedded
within a wider ring of smaller grains. The narrow ring is 14+/-1 AU wide with
inner radius 66+/-1 AU and total cross-sectional area 245 AU^2. The outer ring
is 80+/-15 AU wide with inner radius 45+/-5 AU and total cross-sectional area
90 AU^2. Dust grains in the narrow ring are about 10 times larger and have
lower albedos than those in the wider ring. These properties are consistent
with a picture in which radiation pressure dominates the dispersal of an
exo-Kuiper belt.Comment: Accepted by Astrophysical Journal (Part1) on September 9, 2004. 13
pages, 10 figures, 2 table
Penetration depth of low-coherence enhanced backscattered light in sub-diffusion regime
The mechanisms of photon propagation in random media in the diffusive
multiple scattering regime have been previously studied using diffusion
approximation. However, similar understanding in the low-order (sub-diffusion)
scattering regime is not complete due to difficulties in tracking photons that
undergo very few scatterings events. Recent developments in low-coherence
enhanced backscattering (LEBS) overcome these difficulties and enable probing
photons that travel very short distances and undergo only a few scattering
events. In LEBS, enhanced backscattering is observed under illumination with
spatial coherence length L_sc less than the scattering mean free path l_s. In
order to understand the mechanisms of photon propagation in LEBS in the
subdiffusion regime, it is imperative to develop analytical and numerical
models that describe the statistical properties of photon trajectories. Here we
derive the probability distribution of penetration depth of LEBS photons and
report Monte Carlo numerical simulations to support our analytical results. Our
results demonstrate that, surprisingly, the transport of photons that undergo
low-order scattering events has only weak dependence on the optical properties
of the medium (l_s and anisotropy factor g) and strong dependence on the
spatial coherence length of illumination, L_sc, relative to those in the
diffusion regime. More importantly, these low order scattering photons
typically penetrate less than l_s into the medium due to low spatial coherence
length of illumination and their penetration depth is proportional to the
one-third power of the coherence volume (i.e. [l_s \pi L_sc^2 ]^1/3).Comment: 32 pages(including 7 figures), modified version to appear in Phys.
Rev.
Force balance and membrane shedding at the Red Blood Cell surface
During the aging of the red-blood cell, or under conditions of extreme
echinocytosis, membrane is shed from the cell plasma membrane in the form of
nano-vesicles. We propose that this process is the result of the
self-adaptation of the membrane surface area to the elastic stress imposed by
the spectrin cytoskeleton, via the local buckling of membrane under increasing
cytoskeleton stiffness. This model introduces the concept of force balance as a
regulatory process at the cell membrane, and quantitatively reproduces the rate
of area loss in aging red-blood cells.Comment: 4 pages, 3 figure
Discovery of Two T Dwarf Companions with the Spitzer Space Telescope
We report the discovery of T dwarf companions to the nearby stars HN Peg
(G0V, 18.4 pc, ~0.3 Gyr) and HD 3651 (K0V, 11.1 pc, ~7 Gyr). During an ongoing
survey of 5'x5' fields surrounding stars in the solar neighborhood with IRAC
aboard the Spitzer Space Telescope, we identified these companions as candidate
T dwarfs based on their mid-IR colors. Using near-IR spectra obtained with SpeX
at the NASA IRTF, we confirm the presence of methane absorption that
characterizes T dwarfs and measure spectral types of T2.5+/-0.5 and T7.5+/-0.5
for HN Peg B and HD 3651 B, respectively. By comparing our Spitzer data to
images from 2MASS obtained several years earlier, we find that the proper
motions of HN Peg B and HD 3651 B are consistent with those of the primaries,
confirming their companionship. HN Peg B and HD 3651 B have angular separations
of 43.2" and 42.9" from their primaries, which correspond to projected physical
separations of 795 and 476 AU, respectively. A comparison of their luminosities
to the values predicted by theoretical evolutionary models implies masses of
0.021+/-0.009 and 0.051+/-0.014 Msun for HN Peg B and HD 3651 B. In addition,
the models imply an effective temperature for HN Peg B that is significantly
lower than the values derived for other T dwarfs at similar spectral types,
which is the same behavior reported by Metchev & Hillenbrand for the young
late-L dwarf HD 203030 B. Thus, the temperature of the L/T transition appears
to depend on surface gravity. Meanwhile, HD 3651 B is the first substellar
companion directly imaged around a star that is known to harbor a close-in
planet from RV surveys. The discovery of this companion supports the notion
that the high eccentricities of close-in planets like the one near HD 3651 may
be the result of perturbations by low-mass companions at wide separations.Comment: Astrophysical Journal, in pres
Investigation of Zero-Sound Dispersion Equation in the Complex Plane of Frequency
The known solutions to the zero-sound dispersion equation are considered as
placed on the physical and unphysical sheets in the complex plane of frequency.Comment: 15 pages, 7 figure
Multi-Epoch Observations of HD69830: High Resolution Spectroscopy and Limits to Variability
The main-sequence solar-type star HD69830 has an unusually large amount of
dusty debris orbiting close to three planets found via the radial velocity
technique. In order to explore the dynamical interaction between the dust and
planets, we have performed multi-epoch photometry and spectroscopy of the
system over several orbits of the outer dust. We find no evidence for changes
in either the dust amount or its composition, with upper limits of 5-7% (1
per spectral element) on the variability of the {\it dust spectrum}
over 1 year, 3.3% (1 ) on the broad-band disk emission over 4 years,
and 33% (1 ) on the broad-band disk emission over 24 years. Detailed
modeling of the spectrum of the emitting dust indicates that the dust is
located outside of the orbits of the three planets and has a composition
similar to main-belt, C-type asteroids asteroids in our solar system.
Additionally, we find no evidence for a wide variety of gas species associated
with the dust. Our new higher SNR spectra do not confirm our previously claimed
detection of HO ice leading to a firm conclusion that the debris can be
associated with the break-up of one or more C-type asteroids formed in the dry,
inner regions of the protoplanetary disk of the HD69830 system. The modeling of
the spectral energy distribution and high spatial resolution observations in
the mid-infrared are consistent with a 1 AU location for the emitting
material
Ten-Micron Observations of Nearby Young Stars
(abridged) We present new 10-micron photometry of 21 nearby young stars
obtained at the Palomar 5-meter and at the Keck I 10-meter telescopes as part
of a program to search for dust in the habitable zone of young stars. Thirteen
of the stars are in the F-K spectral type range ("solar analogs"), 4 have B or
A spectral types, and 4 have spectral type M. We confirm existing IRAS
12-micron and ground-based 10-micron photometry for 10 of the stars, and
present new insight into this spectral regime for the rest. Excess emission at
10 micron is not found in any of the young solar analogs, except for a possible
2.4-sigma detection in the G5V star HD 88638. The G2V star HD 107146, which
does not display a 10-micron excess, is identified as a new Vega-like
candidate, based on our 10-micron photospheric detection, combined with
previously unidentified 60-micron and 100-micron IRAS excesses. Among the
early-type stars, a 10-micron excess is detected only in HD 109573A (HR 4796A),
confirming prior observations; among the M dwarfs, excesses are confirmed in AA
Tau, CD -40 8434, and Hen 3-600A. A previously suggested N band excess in the
M3 dwarf CD -33 7795 is shown to be consistent with photospheric emission.Comment: 40 pages, 4 figures, 5 tables. To appear in the January 1, 2004 issue
of Ap
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