1,163 research outputs found
Improved Si:As BIBIB (Back-Illuminated Blocked-Impurity-Band) hybrid arrays
Results of a program to increase the short wavelength (less than 10 microns) detective quantum efficiency, eta/beta, of Si:As Impurity Band Conduction arrays are presented. The arrays are epitaxially grown Back-Illuminated Blocked (BIB) Impurity-Band (BIBIB) 10x50 detectors bonded to switched-FET multiplexers. It is shown that the 4.7 microns detective quantum efficiency increases proportionately with the thickness of the infrared active layer. A BIB array with a thick active layer, designed for low dark current, exhibits eta/beta = 7 to 9 percent at 4.7 microns for applied bias voltages between 3 and 5 V. The product of quantum efficiency and photoelectric gain, etaG, increases from 0.3 to 2.5 as the voltage increases from 3 to 5 V. Over this voltage range, the dark current increases from 8 to 120 e(-)s(-1) at a device temperature of 4.2 K and is under 70 e(-)s(-1) for all voltages at 2 K. Because of device gain, the effective dark current (equivalent photon rate) is less than 3 e(-)s(-1) under all operating conditions. The effective read noise (equivalent photon noise) is found to be less than 12 electrons under all operating conditions and for integration times between 0.05 and 100 seconds
H_2 morphology of young planetary nebulae
The distributions of H_2 1-0 S(l) emission in the young planetary nebulae BD +30°3639 and NGC 7027 show striking similarities: both have limb-brightened arcs of H_2 emission with radii that are about twice those of their H II regions. The extended H_2 emission in both nebulae is attributed to a photodissociation region. This implies that the neutral envelopes of these young planetaries extend well beyond the edge of the H II region, in contrast to older nebulae where the ionized and molecular gas are more nearly coextensive. The contrast between young and old planetaries can only be explained if the molecular envelope is inhomogeneous. We endorse a scenario for the evolution of a planetary nebula in which a photodissociation front propagates through the clumpy molecular envelope, leaving the ionized core embedded in an envelope of partially ionized atomic gas and dense molecular knots. In an evolved planetary, the H II region has expanded to engulf some of the dense molecular knots, which can be identified with bright [O I] and H_2 1-0 S(l) condensations, while the remnant of the photodissociated envelope may be detected as a faint optical halo
A spatially resolved photodissociation region in the planetary nebula NGC 7027
High spatial resolution, narrow band, infrared line images and CO (1—0) mm interferometer data are presented for NGC 7027. These data trace emission from the central H II region (Brɑ), the intermediate photodissociation region [H_2 1—0S(1) and 3.3 µm dust feature], and the molecular circumstellar envelope [CO (1—0)]. The H II region lies in a cavity in the CO envelope, and consists of a smooth elliptical shell. A striking change of morphology is seen in the H_2 emission and the dust feature. The H_2 1—0 S(l) emission is composed of two components: (1) an incomplete elliptical ring of knots which bounds the ionized gas; (2) a remarkable thin shell which loops around the H II region with fourfold symmetry. The dust emission is similar to that from the ionized gas, but is displaced further from the center, and extends at low surface brightness into four “ears” which fill in the bays delineated by the outermost loops of H_2 emission. No 3.3 µm emission is detectable beyond the outer H_2 shell. The outer loops of H_2 emission and the 3.3 µm emission occupy the region between the edge of the H II region and the inner edge of the molecular gas. It is natural to ascribe the morphology of NGC 7027 to a photodissociation region which separates the ionized and molecular gas. If this is correct then the exterior H_2 loops are due to molecular gas heated by the far-UV emission escaping from the H II region, and delineate a photodissociation front. The H_2 and CO kinematics rule out shock excitation of the H_2 emission and favor UV excitation
The relativistic shift of narrow spectral features from black-hole accretion discs
Transient spectral features have been discovered in the X-ray spectra of
Active Galactic Nuclei, mostly in the 5--7 keV energy range. Several
interpretations were proposed for the origin of these features. We examined a
model of Doppler boosted blue horns of the iron line originating from a spot in
a black hole accretion disc, taking into account different approximations of
general relativistic light rays and the resulting shift of energy of photons.
We provide a practical formula for the blue horn energy of an intrinsically
narrow line and assess its accuracy by comparing the approximation against an
exact value, predicted under the assumption of a planar accretion disc. The
most accurate approximation provides excellent agreement with the spot orbital
radius down to the marginally stable orbit of a non-rotating black hole.Comment: Accepted for publication in A&A; 8 pages, 5 figure
Reflected Shock Tunnel Noise Measurement by Focused Differential Interferometry
A series of experiments is conducted where a quantitative non-intrusive optical technique is used to investigate disturbances in the free-stream of T5, the free-piston driven reflected shock tunnel at Caltech. The optical technique, focused laser differential interferometry (FLDI), measures fluctuations in density. In the test matrix, reservoir enthalpy is varied while the reservoir pressure is held fixed. The results show the perturbations in density are not a strong function of the reservoir enthalpy. During one experiment, exceptional levels of noise were detected; this unique result is attributed to non-ideal operation of the shock tunnel. The data indicate that rms density fluctuations of less than 0.75% are achievable with attention to tunnel cleanliness. In addition, the spectral content of density fluctuation does not change throughout the test time
The 1.2 Millimeter Image of the beta Pictoris Disk
We present millimeter imaging observations in the 1200 micron continuum of
the disk around beta Pictoris. With the 25 arcsec beam, the beta Pic disk is
unresolved perpendicularly to the disk plane (< 10 arcsec), but slightly
resolved in the northeast-southwest direction (26 arcsec). Peak emission is
observed at the stellar position. A secondary maximum is found 1000 AU along
the disk plane in the southwest, which does not positionally coincide with a
similar feature reported earlier at 850 micron. Arguments are presented which
could be seen in support of the reality of these features. The observed
submm/mm emission is consistent with thermal emission from dust grains, which
are significantly larger than those generally found in the interstellar medium,
including mm-size particles, and thus more reminiscent of the dust observed in
protostellar disks. Modelling the observed scattered light in the visible and
the emission in the submm/mm provides evidence for the particles dominating the
scattering in the visible/NIR and those primarily responsible for the thermal
emission at longer wavelengths belonging to different populations.Comment: 6 pages, 3 postscript figures, accepted for publication in Astronomy
and Astrophysic
Young Stars and Protostellar Cores near NGC 2023
We investigate the young (proto)stellar population in NGC 2023 and the L 1630
molecular cloud bordering the HII region IC 434, using Spitzer IRAC and MIPS
archive data, JCMT SCUBA imaging and spectroscopy as well as targeted BIMA
observations of one of the Class 0 protostars, NGC 2023 MM1. We have performed
photometry of all IRAC and MIPS images, and used color-color diagrams to
identify and classify all young stars seen within a 22'x26' field along the
boundary between IC 434 and L 1630. For some stars, which have sufficient
optical, IR, and/or sub-millimeter data we have also used the online SED
fitting tool for a large 2D archive of axisymmetric radiative transfer models
to perform more detailed modeling of the observed SEDs. We identify 5
sub-millimeter cores in our 850 and 450 micron SCUBA images, two of which have
embedded class 0 or I protostars. Observations with BIMA are used to refine the
position and characteristics of the Class 0 source NGC 2023 MM 1. These
observations show that it is embedded in a very cold cloud core, which is
strongly enhanced in NH2D. We find that HD 37903 is the most massive member of
a cluster with 20 -- 30 PMS stars. We also find smaller groups of PMS stars
formed from the Horsehead nebula and another elephant trunk structure to the
north of the Horsehead. We refine the spectral classification of HD 37903 to B2
Ve. Our study shows that the expansion of the IC 434 HII region has triggered
star formation in some of the dense elephant trunk structures and compressed
gas inside the L 1630 molecular cloud. This pre-shock region is seen as a
sub-millimeter ridge in which stars have already formed. The cluster associated
with NGC 2023 is very young, and has a large fraction of Class I sources.Comment: 21 pages, 11 figures. Accepted for publication in A&A Replaced with
higher resolution figure
The Circumstellar Environments of High-Mass Protostellar Objects I: Submillimetre Continuum Emission
We present maps of the 850 micron and 450 micron continuum emission seen
towards a sample of 68 high-mass protostellar candidates with luminosities
ranging from 10^2.5 to 10^5 solar luminosity. Most of these candidate high-mass
stars are in the earliest stages of evolution, and have not yet developed an
ultra-compact HII region. We observe a variety of continuum emission
morphologies, from compact symmetric sources through to multiple cores embedded
in long filaments of emission. We find on average there is a 65% probability of
an IRAS point-source having a companion detection at submillimetre wavelengths.
The ratio of integrated flux to peak flux for our detections shows no strong
dependence on distance, suggesting the emission we have observed is primarily
from scale-free envelopes with power-law density structures. Assuming a near
kinematic distance projection, the clumps we detect vary in mass from ~1 to
over 1000 solar mass, with a mean clump mass of 330 solar mass, column density
of 9x10^23 cm^-2 and diameter of ~0.6 pc. The high luminosity and low mass of
the smallest clumps suggests they are accompanied by a minimal number of
stellar companions, while the most massive clumps may be examples of young
protogroups and protoclusters. We measure the spectral index of the dust
emission (alpha) and the spectral index of the dust grain opacity (beta)
towards each object, finding clumps with morphologies suggestive of strong
temperature gradients, and of grain growth in their dense inner regions. We
find a mean value for beta of 0.9, significantly smaller than observed towards
UCHII regions.Comment: Accepted for publication in A&A (22 pages, 14 figures) Fixed missing
pages in Tables 2 and
Accretion Disks and Dynamos: Toward a Unified Mean Field Theory
Conversion of gravitational energy into radiation in accretion discs and the
origin of large scale magnetic fields in astrophysical rotators have often been
distinct topics of research. In semi-analytic work on both problems it has been
useful to presume large scale symmetries, necessarily resulting in mean field
theories. MHD turbulence makes the underlying systems locally asymmetric and
nonlinear. Synergy between theory and simulations should aim for the
development of practical mean field models that capture essential physics and
can be used for observational modeling. Mean field dynamo (MFD) theory and
alpha-viscosity accretion theory exemplify such ongoing pursuits. 21st century
MFD theory has more nonlinear predictive power compared to 20th century MFD
theory, whereas accretion theory is still in a 20th century state. In fact,
insights from MFD theory are applicable to accretion theory and the two are
artificially separated pieces of what should be a single theory. I discuss
pieces of progress that provide clues toward a unified theory. A key concept is
that large scale magnetic fields can be sustained via local or global magnetic
helicity fluxes or via relaxation of small scale magnetic fluctuations, without
the kinetic helicity driver of 20th century textbooks. These concepts may help
explain the formation of large scale fields that supply non-local angular
momentum transport via coronae and jets in a unified theory of accretion and
dynamos. In diagnosing the role of helicities and helicity fluxes in disk
simulations, each disk hemisphere should be studied separately to avoid being
misled by cancelation that occurs as a result of reflection asymmetry. The
fraction of helical field energy in disks is expected to be small compared to
the total field in each hemisphere as a result of shear, but can still be
essential for large scale dynamo action.Comment: For the Proceedings of the Third International Conference and
Advanced School "Turbulent Mixing and Beyond," TMB-2011 held on 21 - 28
August 2011 at the Abdus Salam International Centre for Theoretical Physics,
Trieste, http://users.ictp.it/~tmb/index2011.html Italy, To Appear in Physica
Scripta (corrected small items to match version in print
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