798 research outputs found
High-efficiency Dc to Dc Converter-regulators
High efficiency dc to dc converter regulators for spacecraft power suppl
A Low-power High-efficiency Dc to Dc Converter
Low power high efficiency dc to dc converter- regulator for electronic system
Modeling the HD32297 Debris Disk with Far-IR Herschel Data
HD32297 is a young A-star (~30 Myr) 112 pc away with a bright edge-on debris
disk that has been resolved in scattered light. We observed the HD32297 debris
disk in the far-infrared and sub-millimeter with the Herschel Space Observatory
PACS and SPIRE instruments, populating the spectral energy distribution (SED)
from 63 to 500{\mu}m. We aimed to determine the composition of dust grains in
the HD32297 disk through SED modeling, using geometrical constraints from the
resolved imaging to break degeneracies inherent in SED modeling. We found the
best fitting SED model has 2 components: an outer ring centered around 110 AU,
seen in the scattered light images, and an inner disk near the habitable zone
of the star. The outer disk appears to be composed of grains > 2{\mu}m
consisting of silicates, carbonaceous material, and water ice with an abundance
ratio of 1:2:3 respectively and 90% porosity. These grains appear consistent
with cometary grains, implying the underlying planetesimal population is
dominated by comet-like bodies. We also discuss the 3.7{\sigma} detection of [C
II] emission at 158{\mu}m with the Herschel PACS Spectrometer, making HD32297
one of only a handful of debris disks with circumstellar gas detected.Comment: 11 pages, 4 figures, accepted for publication in The Astrophysical
Journa
Optical Coronagraphic Spectroscopy of AU Mic: Evidence of Time Variable Colors?
We present coronagraphic long slit spectra of AU Mic's debris disk taken with
the STIS instrument aboard the Hubble Space Telescope (HST). Our spectra are
the first spatially resolved, scattered light spectra of the system's disk,
which we detect at projected distances between approximately 10 and 45 AU. Our
spectra cover a wavelength range between 5200 and 10200 angstroms. We find that
the color of AU Mic's debris disk is bluest at small (12-35 AU) projected
separations. These results both confirm and quantify the findings qualitatively
noted by Krist et al. (2005), and are different than IR observations that
suggested a uniform blue or gray color as a function of projected separation in
this region of the disk. Unlike previous literature that reported the color of
AU Mic's disk became increasingly more blue as a function of projected
separation beyond approximately 30 AU, we find the disk's optical color between
35-45 AU to be uniformly blue on the southeast side of the disk and
decreasingly blue on the northwest side. We note that this apparent change in
disk color at larger projected separations coincides with several fast, outward
moving "features" that are passing through this region of the southeast side of
the disk. We speculate that these phenomenon might be related, and that the
fast moving features could be changing the localized distribution of sub-micron
sized grains as they pass by, thereby reducing the blue color of the disk in
the process. We encourage follow-up optical spectroscopic observations of the
AU Mic to both confirm this result, and search for further modifications of the
disk color caused by additional fast moving features propagating through the
disk.Comment: Accepted by AJ, 13 pages, 8 figures, 1 tabl
Radial Surface Density Profiles of Gas and Dust in the Debris Disk around 49 Ceti
We present ~0.4 resolution images of CO(3-2) and associated continuum
emission from the gas-bearing debris disk around the nearby A star 49 Ceti,
observed with the Atacama Large Millimeter/Submillimeter Array (ALMA). We
analyze the ALMA visibilities in tandem with the broad-band spectral energy
distribution to measure the radial surface density profiles of dust and gas
emission from the system. The dust surface density decreases with radius
between ~100 and 310 au, with a marginally significant enhancement of surface
density at a radius of ~110 au. The SED requires an inner disk of small grains
in addition to the outer disk of larger grains resolved by ALMA. The gas disk
exhibits a surface density profile that increases with radius, contrary to most
previous spatially resolved observations of circumstellar gas disks. While ~80%
of the CO flux is well described by an axisymmetric power-law disk in Keplerian
rotation about the central star, residuals at ~20% of the peak flux exhibit a
departure from axisymmetry suggestive of spiral arms or a warp in the gas disk.
The radial extent of the gas disk (~220 au) is smaller than that of the dust
disk (~300 au), consistent with recent observations of other gas-bearing debris
disks. While there are so far only three broad debris disks with well
characterized radial dust profiles at millimeter wavelengths, 49 Ceti's disk
shows a markedly different structure from two radially resolved gas-poor debris
disks, implying that the physical processes generating and sculpting the gas
and dust are fundamentally different.Comment: 20 pages, 8 figures, accepted for publication in ApJ March 31, 2017
(submitted Nov 2016
The Carbon-Rich Gas in the Beta Pictoris Circumstellar Disk
The edge-on disk surrounding the nearby young star Beta Pictoris is the
archetype of the "debris disks", which are composed of dust and gas produced by
collisions and evaporation of planetesimals, analogues of Solar System comets
and asteroids. These disks provide a window on the formation and early
evolution of terrestrial planets. Previous observations of Beta Pic concluded
that the disk gas has roughly solar abundances of elements [1], but this poses
a problem because such gas should be rapidly blown away from the star, contrary
to observations of a stable gas disk in Keplerian rotation [1, 2]. Here we
report the detection of singly and doubly ionized carbon (CII, CIII) and
neutral atomic oxygen (OI) gas in the Beta Pic disk; measurement of these
abundant volatile species permits a much more complete gas inventory. Carbon is
extremely overabundant relative to every other measured element. This appears
to solve the problem of the stable gas disk, since the carbon overabundance
should keep the gas disk in Keplerian rotation [3]. New questions arise,
however, since the overabundance may indicate the gas is produced from material
more carbon-rich than the expected Solar System analogues.Comment: Accepted for publication in Nature. PDF document, 12 pages.
Supplementary information may be found at
http://www.dtm.ciw.edu/akir/Documents/roberge_supp.pdf *** Version 2 :
Removed extraneous publication information, per instructions from the Nature
editor. No other changes mad
Herschel PACS Observations and Modeling of Debris Disks in the Tucana-Horologium Association
We present Herschel PACS photometry of seventeen B- to M-type stars in the 30
Myr-old Tucana-Horologium Association. This work is part of the Herschel Open
Time Key Programme "Gas in Protoplanetary Systems" (GASPS). Six of the
seventeen targets were found to have infrared excesses significantly greater
than the expected stellar IR fluxes, including a previously unknown disk around
HD30051. These six debris disks were fitted with single-temperature blackbody
models to estimate the temperatures and abundances of the dust in the systems.
For the five stars that show excess emission in the Herschel PACS photometry
and also have Spitzer IRS spectra, we fit the data with models of optically
thin debris disks with realistic grain properties in order to better estimate
the disk parameters. The model is determined by a set of six parameters:
surface density index, grain size distribution index, minimum and maximum grain
sizes, and the inner and outer radii of the disk. The best fitting parameters
give us constraints on the geometry of the dust in these systems, as well as
lower limits to the total dust masses. The HD105 disk was further constrained
by fitting marginally resolved PACS 70 micron imaging.Comment: 15 pages, 7 figures, Accepted to Ap
Tensor interaction constraints from beta decay recoil spin asymmetry of trapped atoms
We have measured the angular distribution of recoiling daughter nuclei
emitted from the Gamow-Teller decay of spin-polarized Rb. The
asymmetry of this distribution vanishes to lowest order in the Standard Model
(SM) in pure Gamow-Teller decays, producing an observable very sensitive to new
interactions. We measure the non-SM contribution to the asymmetry to be
= 0.015 0.029 (stat) 0.019 (syst), consistent with the SM
prediction. We constrain higher-order SM corrections using the measured
momentum dependence of the asymmetry, and their remaining uncertainty dominates
the systematic error. Future progress in determining the weak magnetism term
theoretically or experimentally would reduce the final errors. We describe the
resulting constraints on fundamental 4-Fermi tensor interactions.Comment: 11 pages, 13 figures; v2 published in Phys. Rev. C, with referee
clarifications and figures improved for black-and-whit
Gold alignment & internal dissipation
The measures of mechanical alignment were obtained for both prolate and
oblate grains when their temperature is comparable with grain kinetic energy
devided by k, the Boltzmann constant. For such grains, the alignment of angular
momentum, J, with the axis of maximal inertia, a, is only partial. This
substantially alters the alignment as compared with the results in Lazarian
(1995) and Roberge, Hanany & Messinger (1996) obtained on the assumption of
perfect alignment.
We also describe the Gold alignment when the Barnett dissipation is
suppressed and derive an analytical expression which relates the measure of
alignment with parameters of grain nonsphericity and the direction of the gas -
grain drift. This solution provides the lower limit for the alignment measure,
while the upper limit is given by the analytics derived in Lazarian (1994).
Using results of a recent study of incomplete internal relaxation in Lazarian &
Roberge (1996), we find measures of alignment for the whole range of ratios of
grain rotational energy to k over T_s, where T_s is the grain temperature. To
describe alignment for mildly supersonic drifts, we suggest an analytical
approach which provides good correspondence with the results of direct
numerical simulations in Roberge, Hanany & Messinger (1995). We also extend our
approach to account for the simultaneous action of the Gold and
Davis-Greenstein mechanisms.Comment: 25 pages, 14 figures. Accepted to Ap
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