3,259 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
Psychological principles of successful aging technologies: A mini-review
Based on resource-oriented conceptions of successful life-span development, we propose three principles for evaluating assistive technology: (a) net resource release; (b) person specificity, and (c) proximal versus distal frames of evaluation. We discuss how these general principles can aid the design and evaluation of assistive technology in adulthood and old age, and propose two technological strategies, one targeting sensorimotor and the other cognitive functioning. The sensorimotor strategy aims at releasing cognitive resources such as attention and working memory by reducing the cognitive demands of sensory or sensorimotor aspects of performance. The cognitive strategy attempts to provide adaptive and individualized cuing structures orienting the individual in time and space by providing prompts that connect properties of the environment to the individual's action goals. We argue that intelligent assistive technology continuously adjusts the balance between `environmental support' and `self-initiated processing' in person-specific and aging-sensitive ways, leading to enhanced allocation of cognitive resources. Furthermore, intelligent assistive technology may foster the generation of formerly latent cognitive resources by activating developmental reserves (plasticity). We conclude that `lifespan technology', if co-constructed by behavioral scientists, engineers, and aging individuals, offers great promise for improving both the transition from middle adulthood to old age and the degree of autonomy in old age in present and future generations. Copyright (C) 2008 S. Karger AG, Basel
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
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.
Exozodiacal Dust Workshop
The purpose of the workshop was to understand what effect circumstellar dust clouds will have on NASA's proposed Terrestrial Planet Finder (TPF) mission's ability to search for terrestrial-sized planets orbiting stars in the solar neighborhood. The workshop participants reviewed the properties of TPF, summarized what is known about the local zodiacal cloud and about exozodiacal clouds, and determined what additional knowledge must be obtained to help design TPF for maximum effectiveness within its cost constraint. Recommendations were made for ways to obtain that additional knowledge, at minimum cost. The workshop brought together approximately 70 scientists, from four different countries. The active participants included astronomers involved in the study of the local zodiacal cloud, in the formation of stars and planetary systems, and in the technologies and techniques of ground- and space-based infrared interferometry. During the course of the meeting, 15 invited talks and 20 contributed poster papers were presented, and there were four working sessions. This is a collection of the invited talks, contributed poster papers, and summaries of the working sessions
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
New Debris Disk Candidates Around 49 Nearby Stars
We present 49 new candidate debris disks that were detected around nearby stars with the Spitzer Space Telescope using the Multiband Imaging Photometer (MIPS) at 24 μm (MIPS24) and 70 μm (MIPS70). The survey sample was composed of stars within 25 pc of the Sun that were not previously observed by any other MIPS survey. Only stars with V < 9 were selected, corresponding to spectral types earlier than M0. MIPS24 integration times were chosen to detect the stellar photosphere at 10σ levels or better. MIPS70 observations were designed to detect excess infrared emission from any star in the MIPS70 sample with a disk as luminous at that around epsilon Eridani. The resulting sample included over 436 nearby stars that were observed with both MIPS24 and MIPS70, plus an additional 198 observed only with MIPS24. Debris disk candidates were defined as targets where excess emission was detected at 3σ levels or greater, and the ratio of observed flux density to expected photosphere emission was three standard deviations or more above the mean value for the sample. The detection rate implied by the resulting 29 MIPS24 candidates is 4.6%. A detection rate of 4.8% is implied by 21 MIPS70 candidates. The distribution of spectral types for stars identified as candidates resembles that of the general sample and yields strong evidence that debris-disk occurrence does not decrease for K dwarfs. Modeling of non-uniform sensitivity in the sample is required to interpret quantitative estimates of the overall detection frequency and will be presented in a future work
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PSTD Simulations of Multiple Light Scattering in 3-D Macrocsopic Random Media
We report a full-vector, three-dimensional, numerical solution of Maxwell's equations for optical propagation within, and scattering by, a random medium of macroscopic dimensions. The total scattering cross-section is determined using the pseudospectral time-domain technique. Specific results reported in this Paper indicate that multiply scattered light also contains information that can be extracted by the proposed cross-correlation analysis. On a broader perspective, our results demonstrate the feasibility of accurately determining the optical characteristics of arbitrary, macroscopic random media, including geometries with continuous variations of refractive index. Specifically, our results point toward the new possibilities of tissue optics--by numerically solving Maxwell's equations, the optical properties of tissue structures can be determined unambiguously
The Debris Disk Around HR 8799
We have obtained a full suite of Spitzer observations to characterize the
debris disk around HR 8799 and to explore how its properties are related to the
recently discovered set of three massive planets orbiting the star. We
distinguish three components to the debris system: (1) warm dust (T ~150 K)
orbiting within the innermost planet; (2) a broad zone of cold dust (T ~45 K)
with a sharp inner edge, orbiting just outside the outermost planet and
presumably sculpted by it; and (3) a dramatic halo of small grains originating
in the cold dust component. The high level of dynamical activity implied by
this halo may arise due to enhanced gravitational stirring by the massive
planets. The relatively young age of HR 8799 places it in an important early
stage of development and may provide some help in understanding the interaction
of planets and planetary debris, an important process in the evolution of our
own solar system.Comment: emulateapj format, 13 pages, 10 figures, accepted to Ap
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
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