1,593 research outputs found
High-efficiency Dc to Dc Converter-regulators
High efficiency dc to dc converter regulators for spacecraft power suppl
Alien Registration- Roberge, Mary A. (Sanford, York County)
https://digitalmaine.com/alien_docs/4158/thumbnail.jp
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
Volatile-Rich Circumstellar Gas in the Unusual 49 Ceti Debris Disk
We present Hubble Space Telescope STIS far-UV spectra of the edge-on disk
around 49 Ceti, one of the very few debris disks showing sub-mm CO emission.
Many atomic absorption lines are present in the spectra, most of which arise
from circumstellar gas lying along the line-of-sight to the central star. We
determined the line-of-sight CI column density, estimated the total carbon
column density, and set limits on the OI column density. Surprisingly, no
line-of-sight CO absorption was seen. We discuss possible explanations for this
non-detection, and present preliminary estimates of the carbon abundances in
the line-of-sight gas. The C/Fe ratio is much greater than the solar value,
suggesting that 49 Cet harbors a volatile-rich gas disk similar to that of Beta
Pictoris.Comment: Accepted for publication in ApJ Letters. 5 pages, 4 figure
Cosmic rays and grain alignment
The recent detection of interstellar polarization in the solid CO feature
near 4.67 micron shows that CO-mantled grains can be aligned in cold molecular
clouds. These observations conflict with a theory of grain alignment which
attributes the polarization in molecular clouds to the effects of cosmic rays:
according to this theory, oblate spheroidal grains with H_2O and CO_2-dominated
ice mantles are spun up to suprathermal energies by molecular evaporation from
cosmic ray impact sites but spin up does not occur for CO-mantled grains.
Motivated by this conflict, we reexamine the effects of cosmic rays on the
alignment of icy grains. We show that the systematic torques produced by cosmic
rays are insufficient to cause suprathermal spin. In principle, the random
torques due to cosmic rays can enhance the efficiency of Davis-Greenstein
alignment by raising the grain rotational temperature. However, a significant
enhancement would require cosmic ray fluxes 6--7 orders of magnitude larger
than the flux in a typical cold cloud.Comment: 14 pages, 1 figure. Accepted to MNRA
The Large Ultraviolet/Optical/Infrared Surveyor
Astronomy crossed a threshold three decades ago with the discovery of planets around other stars. Compared to scientists' previous expectations set by the Solar System, exoplanets are wonderfully abundant and varied. Indirect planet discovery techniques have shown that small rocky planets residing in stellar habitable zones, where such planets may have liquid water on their surfaces, are not rare. This revelation drives us to ask more ambitious and fundamental questions, that fascinate scientists and the public alike: are there other truly Earth-like planets out there and do any of them harbour life? Today, exoplanets are largely small black shadows' to us, with measurements of orbits, sizes and masses (all three in the best cases).The upcoming James Webb Space Telescope and future 30-m-class ground-based telescopes will characterize the atmospheres of habitable planet candidates orbit in glow-mass M dwarf stars. However, deeply probing atmospheres of the exoplanets most similar to the Earth, those around Sun-like stars, remains out of reach for currently planned observatories. Bringing them within our grasp is a primary motivation for the Large UV/Optical/Infrared Surveyor(LUVOIR) mission concept, currently the focus of a three-year NASA study
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
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