876 research outputs found
Microstructural investigations of ternary Cu/Zn/Al oxide catalysts for methanol steam reforming
Mid-infrared Imaging of a Circumstellar Disk Around HR 4796: Mapping the Debris of Planetary Formation
We report the discovery of a circumstellar disk around the young A0 star, HR
4796, in thermal infrared imaging carried out at the W.M. Keck Observatory. By
fitting a model of the emission from a flat dusty disk to an image at
lambda=20.8 microns, we derive a disk inclination, i = 72 +6/-9 deg from face
on, with the long axis of emission at PA 28 +/-6 deg. The intensity of emission
does not decrease with radius as expected for circumstellar disks but increases
outward from the star, peaking near both ends of the elongated structure. We
simulate this appearance by varying the inner radius in our model and find an
inner hole in the disk with radius R_in = 55+/-15 AU. This value corresponds to
the radial distance of our own Kuiper belt and may suggest a source of dust in
the collision of cometesimals. By contrast with the appearance at 20.8 microns,
excess emission at lambda = 12.5 microns is faint and concentrated at the
stellar position. Similar emission is also detected at 20.8 microns in residual
subtraction of the best-fit model from the image. The intensity and ratio of
flux densities at the two wavelengths could be accounted for by a tenuous dust
component that is confined within a few AU of the star with mean temperature of
a few hundred degrees K, similar to that of zodiacal dust in our own solar
system. The morphology of dust emission from HR 4796 (age 10 Myr) suggests that
its disk is in a transitional planet-forming stage, between that of massive
gaseous proto-stellar disks and more tenuous debris disks such as the one
detected around Vega.Comment: 9 pages, 4 figures as LaTex manuscript and postscript files in
gzipped tar file. Accepted for publication in Astrophysical Journal Letters.
http://upenn5.hep.upenn.edu/~davidk/hr4796.htm
The Mid-Infrared Instrument for the James Webb Space Telescope, VIII: The MIRI Focal Plane System
We describe the layout and unique features of the focal plane system for
MIRI. We begin with the detector array and its readout integrated circuit
(combining the amplifier unit cells and the multiplexer), the electronics, and
the steps by which the data collection is controlled and the output signals are
digitized and delivered to the JWST spacecraft electronics system. We then
discuss the operation of this MIRI data system, including detector readout
patterns, operation of subarrays, and data formats. Finally, we summarize the
performance of the system, including remaining anomalies that need to be
corrected in the data pipeline
The Compact Nucleus of the Deep Silicate Absorption Galaxy NGC 4418
High resolution, Hubble Space Telescope (HST) near-infrared and Keck
mid-infrared images of the heavily extinguished, infrared luminous galaxy NGC
4418 are presented. These data make it possible to observe the imbedded
near-infrared structure on scales of 10-20 pc, and to constrain the size of the
mid-infrared emitting region. The 1.1-2.2 um data of NGC 4418 show no clear
evidence of nuclear star clusters or of a reddened active galactic nucleus.
Instead, the nucleus of the galaxy consists of a ~100-200 pc linear structure
with fainter structures extending radially outward. The near-infrared colors of
the linear feature are consistent with a 10-300 Myr starburst suffering
moderate levels (few magnitudes) of visual extinction. At 7.9-24.5 um, NGC 4418
has estimated size upper limits in the range of 30-80 pc. These dimensions are
consistent with the highest resolution radio observations obtained to date of
NGC 4418, as well as the size of 50-70 pc expected for a blackbody with a
temperature derived from the 25 um, 60 um, and 100 um flux densities of the
galaxy. Further, a spectral energy distribution constructed from the
multi-wavelength mid-infrared observations show the strong silicate absorption
feature at 10 um, consistent with previous mid-infrared observations of NGC
4418. An infrared surface brightness of 2.1x10^13 L_sun kpc^-2 is derived for
NGC 4418. Such a value, though consistent with the surface brightness of warm
ultraluminous infrared galaxies (ULIGs: L_IR [8-1000 um] >~ 10^12 L_sun) such
as IRAS 05189-2524 and IRAS 08572+3915, is not large enough to distinguish NGC
4418 as a galaxy powered by an Active Galactic Nucleus (AGN), as opposed to a
lower surface brightness starburst.Comment: LaTex, 7 pages, including 2 jpg figures and 3 postscript figures, AJ,
in press (May, 2003
Mid-Infrared Observations of Class I/Flat-Spectrum Systems in Six Nearby Molecular Clouds
We have obtained new mid-infrared observations of 65 Class I/Flat-Spectrum
(F.S.) objects in the Perseus, Taurus, Chamaeleon I/II, Rho Ophiuchi, and
Serpens dark clouds. We detected 45/48 (94%) of the single sources, 16/16
(100%) of the primary components, and 12/16 (75%) of the secondary/triple
components of the binary/multiple objects surveyed. The composite spectral
energy distributions (SEDs) for all of our sample sources are either Class I or
F.S., and, in 15/16 multiple systems, at least one of the individual components
displays a Class I or F.S. spectral index. However, the occurrence of mixed
pairings, such as F.S. with Class I, F.S. with Class II, and, in one case, F.S.
with Class III, is surprisingly frequent. Such behaviour is not consistent with
that of multiple systems among T Tauri stars (TTS), where the companion of a
classical TTS also tends to be a classical TTS, although other mixed pairings
have been previously observed among Class II objects. Based on an analysis of
the spectral indices of the individual binary components, there appears to be a
higher proportion of mixed Class I/F.S. systems (65-80%) than that of mixed
Classical/Weak-Lined TTS (25-40%), demonstrating that the envelopes of Class I/
F.S. systems are rapidly evolving during this evolutionary phase. We report the
discovery of a steep spectral index secondary companion to ISO-ChaI 97,
detected for the first time via our mid-infrared observations. In our previous
near- infrared imaging survey of binary/multiple Class I/F.S. sources, ISO-ChaI
97 appeared to be single. With a spectral index of Alpha >= 3.9, the secondary
component of this system is a member of a rare class of very steep spectral
index objects, those with Alpha > 3. Only three such objects have previously
been reported, all of which are either Class 0 or Class I.Comment: 31 pages, 4 figures, 6 table
High Spatial Resolution Imaging of NGC 1068 in the Mid Infrared
Mid-infrared observations of the central source of NGC 1068 have been
obtained with a spatial resolution in the deconvolved image of 0.1" (~ 7pc).
The central source is extended by ~1" in the north-south direction but appears
unresolved in the east-west direction over most of its length. About two-thirds
of its flux can be ascribed to a core structure which is itself elongated
north-south and does not show a distinct unresolved compact source. The source
is strongly asymmetric, extending significantly further to the north than to
the south. The morphology of the mid-infrared emission appears similar to that
of the radio jet, and has features which correlate with the images in [O III].
Its 12.5-24.5 micron color temperature ranges from 215 to 260 K and does not
decrease smoothly with distance from the core. Silicate absorption is strongest
in the core and to the south and is small in the north.
The core, apparently containing two-thirds of the bolometric luminosity of
the inner 4" diameter area, may be explained by a thick, dusty torus near the
central AGN viewed at an angle of ~65 deg to its plane. There are, however,
detailed difficulties with existing models, especially the narrow east-west
width of the thin extended mid-infrared "tongue" to the north of the core. We
interpret the tongue as re-processed visual and ultraviolet radiation that is
strongly beamed and that originates in the AGN.Comment: 42 pages, 2 tables, 9 figures; Accepted for publication in A
Structural characterization of vanadium oxide catalysts supported on nanostructured silica SBA-15 using X-ray absorption spectroscopy
The local structure of vanadium oxide supported on nanostructured SiO2 (VxOy/SBA-15) was investigated by in situ X-ray absorption spectroscopy (XAS). Because the number of potential parameters in XAS data analysis often exceeds the number of "independent" parameters, evaluating the reliability and significance of a particular fitting procedure is mandatory. The number of independent parameters (Nyquist) may not be sufficient. Hence, in addition to the number of independent parameters, a novel approach to evaluate the significance of structural fitting parameters in XAS data analysis is introduced. Three samples with different V loadings (i.e. 2.7 wt %, 5.4 wt %, and 10.8 wt %) were employed. Thermal treatment in air at 623 K resulted in characteristic structural changes of the V oxide species. Independent of the V loading, the local structure around V centers in dehydrated VxOy/SBA-15 corresponded to an ordered arrangement of adjacent V2O7 units. Moreover, the V2O7 units were found to persist under selective oxidation reaction conditions
Lifetime of 19Ne*(4.03 MeV)
The Doppler-shift attenuation method was applied to measure the lifetime of
the 4.03 MeV state in 19Ne. Utilizing a 3He-implanted Au foil as a target, the
state was populated using the 20Ne(3He,alpha)19Ne reaction in inverse
kinematics at a 20Ne beam energy of 34 MeV. De-excitation gamma rays were
detected in coincidence with alpha particles. At the 1 sigma level, the
lifetime was determined to be 11 +4, -3 fs and at the 95.45% confidence level
the lifetime is 11 +8, -7 fs.Comment: 6 pages, submitted to Phys. Rev.
Epigenetic meta-analysis across three civilian cohorts identifies NRG1 and HGS as blood-based biomarkers for post-traumatic stress disorder
Aim: Trauma exposure is a necessary, but not deterministic, contributor to post-traumatic stress disorder (PTSD). Epigenetic factors may distinguish between trauma-exposed individuals with versus without PTSD. Materials & methods: We conducted a meta-analysis of PTSD epigenome-wide association studies in trauma-exposed cohorts drawn from civilian contexts. Whole blood-derived DNA methylation levels were analyzed in 545 study participants, drawn from the three civilian cohorts participating in the PTSD working group of the Psychiatric Genomics Consortium. Results: Two CpG sites significantly associated with current PTSD in NRG1 (cg23637605) and in HGS (cg19577098). Conclusion: PTSD is associated with differential methylation, measured in blood, within HGS and NRG1 across three civilian cohorts
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