542 research outputs found
Full characterization and analysis of a terahertz heterodyne receiver based on a NbN hot electron bolometer
We present a complete experimental characterization of a quasioptical twin-slot antenna coupled small area (1.0×0.15 µm^2) NbN hot electron bolometer (HEB) mixer compatible with currently available solid state tunable local oscillator (LO) sources. The required LO power absorbed in the HEB is analyzed in detail and equals only 25 nW. Due to the small HEB volume and wide antenna bandwidth, an unwanted direct detection effect is observed which decreases the apparent sensitivity. Correcting for this effect results in a receiver noise temperature of 700 K at 1.46 THz. The intermediate frequency (IF) gain bandwidth is 2.3 GHz and the IF noise bandwidth is 4 GHz. The single channel receiver stability is limited to 0.2–0.3 s in a 50 MHz bandwidth
A Fourier optics approach to evaluate the astrometric performance of MICADO
We present our investigation into the impact of wavefront errors on high
accuracy astrometry using Fourier Optics. MICADO, the upcoming near-IR imaging
instrument for the Extremely Large Telescope, will offer capabilities for
relative astrometry with an accuracy of 50 micro arcseconds ({\mu}as). Due to
the large size of the point spread function (PSF) compared to the astrometric
requirement, the detailed shape and position of the PSF on the detector must be
well understood. Furthermore, because the atmospheric dispersion corrector of
MICADO is a moving component within an otherwise mostly static instrument, it
might not be sufficient to perform a simple pre-observation calibration.
Therefore, we have built a Fourier Optics framework, allowing us to evaluate
the small changes in the centroid position of the PSF as a function of
wavefront error. For a complete evaluation, we model both the low order surface
form errors, using Zernike polynomials, and the mid- and high-spatial
frequencies, using Power Spectral Density analysis. The described work will
then make it possible, performing full diffractive beam propagation, to assess
the expected astrometric performance of MICADO.Comment: 13 pages, 13 figures, to be submitted to the SPIE Astronomical
Telescopes & Instrumentation 2020 conferenc
Value of prominent flow voids without cord edema in the detection of spinal arteriovenous fistulae
Purpose: To determine the prevalence of spinal dural arteriovenous fistulae (SDAVF) in patients presenting with prominent vascular flow voids on imaging without other imaging findings suggestive of SDAVF. Methods: We retrospectively identified patients from January 1, 2005 to March 1, 2012 who underwent spinal angiography for suspected SDAVF with prominent vascular flow voids on prior imaging. We excluded patients with other major spinal pathology or other imaging findings of SDAVF including cord hyperintensity, enhancement, or expansion. We calculated the proportion of patients with positive findings for SDAVF on angiography and evaluated the prevalence of SDAVF for this finding alone and in correlation with clinical findings. Results: 18 patients underwent spinal angiography for prominent flow voids on imaging without other spinal pathology or imaging findings of SDAVF. Three had a SDAVF detected on angiography. The prevalence of SDAVF in this population was low, only 17% (95% CI 6-39%). All of the patients with positive angiography findings had myelopathy, increasing the prevalence to 100% if the additional clinical finding of myelopathy was present. Conclusions: Prominent flow voids without other imaging findings suggestive of SDAVF is poorly predictive of the presence of a SDAVF, unless myelopathy is present clinically. © 2014 Alhilali et al
Discovery of water vapour in the carbon star V Cygni from observations with Herschel/HIFI
We report the discovery of water vapour toward the carbon star V Cygni. We
have used Herschel's HIFI instrument, in dual beam switch mode, to observe the
1(11) - 0(00) para-water transition at 1113.3430 GHz in the upper sideband of
the Band 4b receiver. The observed spectral line profile is nearly parabolic,
but with a slight asymmetry associated with blueshifted absorption, and the
integrated antenna temperature is 1.69 \pm 0.17 K km/s. This detection of
thermal water vapour emission, carried out as part of a small survey of water
in carbon-rich stars, is only the second such detection toward a carbon-rich
AGB star, the first having been obtained by the Submillimeter Wave Astronomy
Satellite toward IRC+10216. For an assumed ortho-to-para ratio of 3 for water,
the observed line intensity implies a water outflow rate ~ (3 - 6) E-5 Earth
masses per year and a water abundance relative to H2 of ~ (2-5) E-6. This value
is a factor of at least 1E+4 larger than the expected photospheric abundance in
a carbon-rich environment, and - as in IRC+10216 - raises the intriguing
possibility that the observed water is produced by the vapourisation of
orbiting comets or dwarf planets. However, observations of the single line
observed to date do not permit us to place strong constraints upon the spatial
distribution or origin of the observed water, but future observations of
additional transitions will allow us to determine the inner radius of the
H2O-emitting zone, and the H2O ortho-to-para ratio, and thereby to place
important constraints upon the origin of the observed water emission.Comment: Accepted for publication in A&A (HIFI special issue
The origin of the [C II] emission in the S140 PDRs - new insights from HIFI
Using Herschel's HIFI instrument we have observed [C II] along a cut through
S140 and high-J transitions of CO and HCO+ at two positions on the cut,
corresponding to the externally irradiated ionization front and the embedded
massive star forming core IRS1. The HIFI data were combined with available
ground-based observations and modeled using the KOSMA-tau model for photon
dominated regions. Here we derive the physical conditions in S140 and in
particular the origin of [C II] emission around IRS1. We identify three
distinct regions of [C II] emission from the cut, one close to the embedded
source IRS1, one associated with the ionization front and one further into the
cloud. The line emission can be understood in terms of a clumpy model of
photon-dominated regions. At the position of IRS1, we identify at least two
distinct components contributing to the [C II] emission, one of them a small,
hot component, which can possibly be identified with the irradiated outflow
walls. This is consistent with the fact that the [C II] peak at IRS1 coincides
with shocked H2 emission at the edges of the outflow cavity. We note that
previously available observations of IRS1 can be well reproduced by a
single-component KOSMA-tau model. Thus it is HIFI's unprecedented spatial and
spectral resolution, as well as its sensitivity which has allowed us to uncover
an additional hot gas component in the S140 region.Comment: accepted for publication in Astronomy and Astrophysics (HIFI special
issue
Polarisation Observations of VY Canis Majoris Water Vapour 5{32}-4{41} 620.701 GHz Maser Emission with HIFI
CONTEXT: Water vapour maser emission from evolved oxygen-rich stars remains
poorly understood. Additional observations, including polarisation studies and
simultaneous observation of different maser transitions may ultimately lead to
greater insight. AIMS: We have aimed to elucidate the nature and structure of
the VY CMa water vapour masers in part by observationally testing a theoretical
prediction of the relative strengths of the 620.701 GHz and the 22.235 GHz
maser components of ortho water vapour. METHODS: In its high-resolution mode
(HRS) the Herschel Heterodyne Instrument for the Infrared (HIFI) offers a
frequency resolution of 0.125 MHz, corresponding to a line-of-sight velocity of
0.06 km/s, which we employed to obtain the strength and linear polarisation of
maser spikes in the spectrum of VY CMa at 620.701 GHz. Simultaneous ground
based observations of the 22.235 GHz maser with the Max-Planck-Institut f\"ur
Radioastronomie 100-meter telescope at Effelsberg, provided a ratio of 620.701
GHz to 22.235 GHz emission. RESULTS:We report the first astronomical detection
to date of water vapour maser emission at 620.701 GHz. In VY CMa both the
620.701 and the 22.235 GHz polarisation are weak. At 620.701 GHz the maser
peaks are superposed on what appears to be a broad emission component, jointly
ejected asymmetrically from the star. We observed the 620.701 GHz emission at
two epochs 21 days apart, both to measure the potential direction of linearly
polarised maser components and to obtain a measure of the longevity of these
components. Although we do not detect significant polarisation levels in the
core of the line, they rise up to approximately 6% in its wings
HERSCHEL-HIFI spectroscopy of the intermediate mass protostar NGC7129 FIRS 2
HERSCHEL-HIFI observations of water from the intermediate mass protostar
NGC7129 FIRS 2 provide a powerful diagnostic of the physical conditions in this
star formation environment. Six spectral settings, covering four H216O and two
H218O lines, were observed and all but one H218O line were detected. The four
H2 16 O lines discussed here share a similar morphology: a narrower, \approx 6
km/s, component centered slightly redward of the systemic velocity of NGC7129
FIRS 2 and a much broader, \approx 25 km/s component centered blueward and
likely associated with powerful outflows. The narrower components are
consistent with emission from water arising in the envelope around the
intermediate mass protostar, and the abundance of H2O is constrained to \approx
10-7 for the outer envelope. Additionally, the presence of a narrow
self-absorption component for the lowest energy lines is likely due to
self-absorption from colder water in the outer envelope. The broader component,
where the H2O/CO relative abundance is found to be \approx 0.2, appears to be
tracing the same energetic region that produces strong CO emission at high J.Comment: 6 pages, 4 figures, accepted by A&
Hydrides in Young Stellar Objects: Radiation tracers in a protostar-disk-outflow system
Context: Hydrides of the most abundant heavier elements are fundamental
molecules in cosmic chemistry. Some of them trace gas irradiated by UV or
X-rays. Aims: We explore the abundances of major hydrides in W3 IRS5, a
prototypical region of high-mass star formation. Methods: W3 IRS5 was observed
by HIFI on the Herschel Space Observatory with deep integration (about 2500 s)
in 8 spectral regions. Results: The target lines including CH, NH, H3O+, and
the new molecules SH+, H2O+, and OH+ are detected. The H2O+ and OH+ J=1-0 lines
are found mostly in absorption, but also appear to exhibit weak emission
(P-Cyg-like). Emission requires high density, thus originates most likely near
the protostar. This is corroborated by the absence of line shifts relative to
the young stellar object (YSO). In addition, H2O+ and OH+ also contain strong
absorption components at a velocity shifted relative to W3 IRS5, which are
attributed to foreground clouds. Conclusions: The molecular column densities
derived from observations correlate well with the predictions of a model that
assumes the main emission region is in outflow walls, heated and irradiated by
protostellar UV radiation.Comment: Astronomy and Astrophysics Letters, in pres
- …