4,440,052 research outputs found
Infrared emission from interplanetary dust
The infrared sky is dominated on large scales by emission from interplanetary dust, which produces the zodiacal emission (ZE), and interstellar dust. These two components of the infrared background differ in angular and spectral distribution, allowing the two to be separated easily in some places. A method of determining the emission from interplanetary dust near the Earth's orbit is described, and the results are compared to predictions for realistic materials with the interplanetary size distribution measured in situ
Gamma-ray Emission from Crushed Clouds in Supernova Remnants
It is shown that the radio and gamma-ray emission observed from newly-found
"GeV-bright" supernova remnants (SNRs) can be explained by a model, in which a
shocked cloud and shock-accelerated cosmic rays (CRs) frozen in it are
simultaneously compressed by the supernova blastwave as a result of formation
of a radiative cloud shock. Simple reacceleration of pre-existing CRs is
generally sufficient to power the observed gamma-ray emission through the
decays of neutral pions produced in hadronic interactions between high-energy
protons (nuclei) and gas in the compressed-cloud layer. This model provides a
natural account of the observed synchrotron radiation in SNRs W51C, W44 and IC
443 with flat radio spectral index, which can be ascribed to a combination of
secondary and reaccelerated electrons and positrons.Comment: 5 pages, 2 figures, minor modifications made in Introduction and
Discussion, accepted for publication in ApJ
Determining dust temperatures and masses in the Herschel era: The importance of observations longward of 200 micron
Context. The properties of the dust grains (e.g., temperature and mass) can be derived from fitting far-IR SEDs (≥100 μm). Only with SPIRE on Herschel has it been possible to get high spatial resolution at 200 to 500 μm that is beyond the peak (~160 μm) of dust emission in most galaxies.
Aims. We investigate the differences in the fitted dust temperatures and masses determined using only 200 μm data (new SPIRE observations) to determine how important having >200 μm data is for deriving these dust properties.
Methods. We fit the 100 to 350 μm observations of the Large Magellanic Cloud (LMC) point-by-point with a model that consists of a single temperature and fixed emissivity law. The data used are existing observations at 100 and 160 μm (from IRAS and Spitzer) and new SPIRE observations of 1/4 of the LMC observed for the HERITAGE key project as part of the Herschel science demonstration phase.
Results. The dust temperatures and masses computed using only 100 and 160 μm data can differ by up to 10% and 36%, respectively, from those that also include the SPIRE 250 & 350 μm data. We find that an emissivity law proportional to λ^(−1.5) minimizes the 100–350 μm fractional residuals. We find that the emission at 500 μm is ~10% higher than expected from extrapolating the fits made at shorter wavelengths. We find the fractional 500 μm excess is weakly anti-correlated with MIPS 24 μm flux and the total gas surface density. This argues against a flux calibration
error as the origin of the 500 μm excess. Our results do not allow us to distinguish between a systematic variation in the wavelength dependent emissivity law or a population of very cold dust only detectable at λ ≥ 500 μm for the origin of the 500 μm excess
Cold dust clumps in dynamically hot gas
Aims. We present clumps of dust emission from Herschel observations of the Large Magellanic Cloud (LMC) and their physical and statistical
properties. We catalog cloud features seen in the dust emission from Herschel observations of the LMC, the Magellanic type irregular galaxy
closest to the Milky Way, and compare these features with Hi catalogs from the ATCA+Parkes Hi survey.
Methods. Using an automated cloud-finding algorithm, we identify clouds and clumps of dust emission and examine the cumulative mass distribution
of the detected dust clouds. The mass of cold dust is determined from physical parameters that we derive by performing spectral energy distribution
fits to 250, 350, and 500 μm emission from SPIRE observations using dust grain size distributions for graphite/silicate in low-metallicity
extragalactic environments.
Results. The dust cloud mass spectrum follows a power law distribution with an exponent of γ = −1.8 for clumps larger than 4 × 10^2 M_⊙ and is
similar to the Hi mass distribution. This is expected from the theory of ISM structure in the vicinity of star formation
LIGO's "Science Reach"
Technical discussions of the Laser Interferometer Gravitational Wave
Observatory (LIGO) sensitivity often focus on its effective sensitivity to
gravitational waves in a given band; nevertheless, the goal of the LIGO Project
is to ``do science.'' Exploiting this new observational perspective to explore
the Universe is a long-term goal, toward which LIGO's initial instrumentation
is but a first step. Nevertheless, the first generation LIGO instrumentation is
sensitive enough that even non-detection --- in the form of an upper limit ---
is also informative. In this brief article I describe in quantitative terms
some of the science we can hope to do with first and future generation LIGO
instrumentation: it short, the ``science reach'' of the detector we are
building and the ones we hope to build.Comment: 13 pages, including 1 inlined figure
Survey of cometary CO2, CO, and particulate emissions using the Spitzer Space Telescope: Smog check for comets
We surveyed 23 comets using the Infrared Array Camera on the Spitzer Space
Telescope in wide filters centered at 3.6 and 4.5 microns. Emission in the 3.6
micron filter arises from sunlight scattered by dust grains; these images
generally have a coma near the nucleus and a tail in the antisolar direction
due to dust grains swept back by solar radiation pressure. The 4.5 micron
filter contains the same dust grains, as well as strong emission lines from CO2
and CO gas; these show distinct morphologies, in which cases we infer they are
dominated by gas. Based on the ratio of 4.5 to 3.6 micron brightness, we
classify the survey comets as CO2+CO "rich" and "poor." This classification is
correlated with previous classifications by A'Hearn based on carbon-chain
molecule abundance, in the sense that comets classified as "depleted" in
carbon-chain molecules are also "poor" in CO2+CO. The gas emission in the IRAC
4.5 micron images is characterized by a smooth morphology, typically a fan in
the sunward hemisphere with a radial profile that varies approximately as the
inverse of projected distance from the nucleus, as would apply for constant
production and free expansion. There are very significant radial and azimuthal
enhancements in many of the comets, and these are often distinct between the
gas and dust, indicating that ejection of solid material may be driven either
by H2O or CO2. Notable features in the images include the following. There is a
prominent loop of gas emission from 103P/Hartley 2, possible due to an outburst
of CO2 before the Spitzer image. Prominent, double jets are present in the
image of 88P/Howell. A prominent single jet is evident for 3 comets. Spirals
are apparent in 29P and C/2006 W3; we measure a rotation rate of 21 hr for the
latter comet. Arcs (possibly parts of a spiral) are apparent in the images of
10P/Tempel 2, and 2P/Encke.Comment: accepted for publication in Icaru
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