1,870 research outputs found
Probing Interstellar Dust with Infrared Echoes from the Cas A Supernova
We present the analysis of an IRS 5-38 {\mu}m spectrum and MIPS photometric
measurements of an infrared echo near the Cassiopeia A supernova remnant
observed with the Spitzer Space Telescope. We have modeled the recorded echo
accounting for PAHs, quantum-heated carbon and silicate grains, as well as
thermal carbon and silicate particles. Using the fact that optical light echo
spectroscopy has established that Cas A originated from a type IIb supernova
explosion showing an optical spectrum remarkably similar to the prototypical
type IIb SN 1993J, we use the latter to construct template data input for our
simulations. We are then able to reproduce the recorded infrared echo spectrum
by combining the emission of dust heated by the UV burst produced at the shock
breakout after the core-collapse and dust heated by optical light emitted near
the visual maximum of the supernova light curve, where the UV burst and optical
light curve characteristics are based on SN 1993J. We find a mean density of
\sim680 H cm^{-3} for the echo region, with a size of a few light years across.
We also find evidence of dust processing in the form of a lack of small PAHs
with less than \sim300 carbon atoms, consistent with a scenario of PAHs
destruction by the UV burst via photodissociation at the estimated distance of
the echo region from Cas A. Furthermore, our simulations suggest that the weak
11 {\mu}m features of our recorded infrared echo spectrum are consistent with a
strong dehydrogenated state of the PAHs. This exploratory study highlights the
potential of investigating dust processing in the interstellar medium through
infrared echoes.Comment: 16 pages, 14 figures, accepted for publication in the Astrophysical
Journa
The Effect of Atmospheric Forcing Resolution on Delivery of Ocean Heat to the Antarctic Floating Ice Shelves
Oceanic melting at the base of the floating Antarctic ice shelves is now thought to be a more significant cause of mass loss for the Antarctic ice sheet than iceberg calving. In this study, a 10-km horizontal-resolution circum-Antarctic ocean–sea ice–ice shelf model [based on the Regional Ocean Modeling System (ROMS)] is used to study the delivery of ocean heat to the base of the ice shelves. The atmospheric forcing comes from the ERA-Interim reanalysis (;80-km resolution) and from simulations using the polar-optimized Weather Re- search and Forecasting Model (30-km resolution), where the upper atmosphere was relaxed to the ERA- Interim reanalysis. The modeled total basal ice shelf melt is low compared to observational estimates but increases by 14% with the higher-resolution winds and just 3% with both the higher-resolution winds and atmospheric surface temperatures. The higher-resolution winds lead to more heat being delivered to the ice shelf cavities from the adjacent ocean and an increase in the efficiency of heat transfer between the water and the ice. The higher-resolution winds also lead to changes in the heat delivered from the open ocean to the continental shelves as well as changes in the heat lost to the atmosphere over the shelves, and the sign of these changes varies regionally. Addition of the higher-resolution temperatures to the winds results in lowering, primarily during summer, the wind-driven increase in heat advected into the ice shelf cavities due to colder summer air temperatures near the coast
Relationship between the magnetic hyperfine field and the magnetic moment
Based on experimental data it is shown, for some chosen alloys and compounds
of iron, that there is no one unique relationship between the 57Fe-site
magnetic hyperfine field, Bhf, and the magnetic moment per Fe atom, m. Instead,
the Bhf-m plot consists of several branches, each of them being characteristic
of a given alloy or compound. Consequently, the effective proportionality
constant (hyperfine coupling constant) depends on the alloy system or compound,
and for a given alloy system or compound it depends on the composition or even
on the lattice site. Consequently, the scaling of Bhf into the underlying m
cannot be done a priopri
Extremely Red Objects in The Lockman Hole
We investigate Extremely Red Objects (EROs) using near- and mid-infrared
observations in five passbands (3.6 to 24 micron) obtained from the Spitzer
Space Telescope, and deep ground-based R and K imaging. The great sensitivity
of the IRAC camera allows us to detect 64 EROs in only 12 minutes of IRAC
exposure time, by means of an R-[3.6] color cut (analogous to the traditional
red R-K cut). A pure infrared K-[3.6] red cut detects a somewhat different
population and may be more effective at selecting z > 1.3 EROs. We find 17% of
all galaxies detected by IRAC at 3.6 or 4.5 micron to be EROs. These
percentages rise to about 40% at 5.8 micron, and about 60% at 8.0 micron. We
utilize the spectral bump at 1.6 micron to divide the EROs into broad redshift
slices using only near-infrared colors (2.2/3.6/4.5 micron). We conclude that
two-thirds of all EROs lie at redshift z > 1.3. Detections at 24 micron imply
that at least 11% of 0.6 1.3 EROs are
dusty star-forming galaxies.Comment: to appear in the special Spitzer issue of the ApJ
IRAC Imaging of Lockman Hole
IRAC imaging of a 4'7x4'7 area in the Lockman Hole detected over 400 galaxies
in the IRAC 3.6 micron and 4.5 micron bands, 120 in the 5.8 micron, and 80 in
the 8 micron bandin 30 minutes of observing time. Color-color diagrams suggest
that about half of these galaxies are at redshifts 0.6<z<1.3 with about a
quarter at higher redshifts (z>1.3). We also detect IRAC counterparts for 6 of
the 7 SCUBA sources and all 9 XMM sources in this area. The detection of the
counterparts of the SCUBA sources and galaxies at z>1.3 demonstrates the
ability of IRAC to probe the universe at very high redshifts.Comment: 11 pages, 2 figures. accepted by ApJS, Spizter Special Issu
24 MICRON PROPERTIES OF X-RAY-SELECTED ACTIVE GALACTIC NUCLEI
We examine the 24 μm to X-ray color of 157 X-ray-selected active galactic nuclei (AGNs) as a function of X-ray obscuration and optical classification in the Chandra Deep Field-South. The sample consists of the Chandra hard-band detections with 2-8 keV flux above 10-15 ergs s-1 cm-2. A deep 24 μm mosaic obtained with Spitzer provides mid-infrared fluxes for the sample. Since obscured AGNs locally have higher 24 μm/2-8 keV flux ratios than unobscured AGNs, and since X-ray background models predict a large population of obscured AGNs, we expect to find many X-ray-hard, IR-bright AGNs. Instead, we find that the 24 μm to X-ray flux ratio does not depend on X-ray hardness in the full sample, nor does it differ between narrow- and broad-line AGNs. We identify five nearly Compton-thick AGNs and find they have similar 24 μm to X-ray flux ratios compared to the full sample. We consider AGNs in the narrow redshift spikes at z ~ 0.7; for these AGNs, there is some evidence that the flux ratio increases with X-ray hardness. The redshift slice also shows an odd trend that is also prominent in the full sample: a group of X-ray-hard AGNs with very low 24 μm to X-ray flux ratios
Annual variation in the levels of transcripts of sex-specific genes in the mantle of the common mussel, Mytilus edulis
Mytilus species are used as sentinels for the assessment of environmental health but sex or stage in the reproduction cycle is rarely considered even though both parameters are likely to influence responses to pollution. We have validated the use of a qPCR assay for sex identification and related the levels of transcripts to the reproductive cycle. A temporal study of mantle of Mytilus edulis found transcripts of male-specific vitelline coat lysin (VCL) and female-specific vitelline envelope receptor for lysin (VERL) could identify sex over a complete year. The levels of VCL/VERL were proportional to the numbers of sperm/ova and are indicative of the stage of the reproductive cycle. Maximal levels of VCL and VERL were found in February 2009 declining to minima between July - August before increasing and re-attaining a peak in February 2010. Water temperature may influence these transitions since they coincide with minimal water temperature in February and maximal temperature in August. An identical pattern of variation was found for a cryptic female-specific transcript (H5) but a very different pattern was observed for oestrogen receptor 2 (ER2). ER2 varied in a sex-specific way with male > female for most of the cycle, with a female maxima in July and a male maxima in December. Using artificially spawned animals, the transcripts for VCL, VERL and H5 were shown to be present in gametes and thus their disappearance from mantle is indicative of spawning. VCL and VERL are present at equivalent levels in February and July-August but during gametogenesis (August to January) and spawning (March to June) VCL is present at lower relative amounts than VERL. This may indicate sex-specific control mechanisms for these processes and highlight a potential pressure point leading to reduced reproductive output if environmental factors cause asynchrony to gamete maturation or release
Infrared Echoes near the Supernova Remnant Cassiopeia A
Two images of Cassiopeia A obtained at 24 micrometer with the Spitzer Space
Telescope over a one year time interval show moving structures outside the
shell of the supernova remnant to a distance of more than 20 arcmin. Individual
features exhibit apparent motions of 10 to 20 arcsec per year, independently
confirmed by near-infrared observations. The observed tangential velocities are
at roughly the speed of light. It is likely that the moving structures are
infrared echoes, in which interstellar dust is heated by the explosion and by
flares from the compact object near the center of the remnant.Comment: To be published in Science on June 10, 2005. 11 pages, 4 figures.
Additional information available at
http://www.spitzer.caltech.edu/Media/releases/ssc2005-14/index.shtm
Far Infrared Source Counts at 70 and 160 microns in Spitzer Deep Surveys
We derive galaxy source counts at 70 and 160 microns using the Multiband
Imaging Photometer for Spitzer (MIPS) to map the Chandra Deep Field South
(CDFS) and other fields. At 70 microns, our observations extend upwards about 2
orders of magnitude in flux density from a threshold of 15 mJy, and at 160
microns they extend about an order of magnitude upward from 50 mJy. The counts
are consistent with previous observations on the bright end. Significant
evolution is detected at the faint end of the counts in both bands, by factors
of 2-3 over no-evolution models. This evolution agrees well with models that
indicate most ofthe faint galaxies lie at redshifts between 0.7 and 0.9. The
new Spitzer data already resolve about 23% of the Cosmic Far Infrared
Background at 70 microns and about 7% at 160 microns.Comment: Small modifications to match printed version. Models in Differential
Counts plots were changed. MIPS Source Counts are available at:
http://lully.as.arizona.edu/GTODeep/Counts/ . Accepted for Publication in
ApJS Special Issue on Spitze
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