58 research outputs found
A 490 GHz planar circuit balanced Nb-AlO-Nb quasiparticle mixer for radio astronomy: Application to quantitative local oscillator noise determination
This article presents a heterodyne experiment which uses a 380-520 GHz planar
circuit balanced Nb--Nb
superconductor-insulator-superconductor (SIS) quasiparticle mixer with 4-8 GHz
instantaneous intermediate frequency (IF) bandwidth to quantitatively determine
local oscillator (LO) noise. A balanced mixer is a unique tool to separate
noise at the mixer's LO port from other noise sources. This is not possible in
single-ended mixers. The antisymmetric IV characteristic of a SIS mixer further
helps to simplify the measurements. The double-sideband receiver sensitivity of
the balanced mixer is 2-4 times the quantum noise limit over the
measured frequencies with a maximum LO noise rejection of 15 dB. This work
presents independent measurements with three different LO sources that produce
the reference frequency but also an amount of near-carrier noise power which is
quantified in the experiment as a function of the LO and IF frequency in terms
of an equivalent noise temperature . In a second experiment we use only
one of two SIS mixers of the balanced mixer chip, in order to verify the
influence of near-carrier LO noise power on a single-ended heterodyne mixer
measurement. We find an IF frequency dependence of near-carrier LO noise power.
The frequency-resolved IF noise temperature slope is flat or slightly negative
for the single-ended mixer. This is in contrast to the IF slope of the balanced
mixer itself which is positive due to the expected IF roll-off of the mixer.
This indicates a higher noise level closer to the LO's carrier frequency. Our
findings imply that near-carrier LO noise has the largest impact on the
sensitivity of a receiver system which uses mixers with a low IF band, for
example superconducting hot-electron bolometer (HEB) mixers.Comment: 13 pages, 8 figures, 2 tables, see manuscript for complete abstrac
Observation of Andreev Reflection Enhanced Shot Noise
We have experimentally investigated the quasiparticle shot noise in
NbN/MgO/NbN superconductor - insulator - superconductor tunnel junctions. The
observed shot noise is significantly larger than theoretically expected. We
attribute this to the occurrence of multiple Andreev reflection processes in
pinholes present in the MgO barrier. This mechanism causes the current to flow
in large charge quanta (Andreev clusters), with a voltage dependent average
value of m = 1+ 2 Delta/eV times the electron charge. Because of this charge
enhancement effect, the shot noise is increased by the factor m.Comment: 4 pages, 5 figures include
Atomic Carbon in M82: Physical conditions derived from simultaneous observations of the [CI] fine structure submillimeter wave transitions
We report the first extragalactic detection of the neutral carbon [CI]
3P2-3P1 fine structure line at 809 GHz. The line was observed towards M82
simultaneously with the 3P1-3P0 line at 492 GHz, providing a precise
measurement of the J=2-1/J=1-0 integrated line ratio of 0.96 (on a [K km s^-1]
-scale). This ratio constrains the [CI] emitting gas to have a temperature of
at least 50 K and a density of at least 10^4 cm^-3. Already at this minimum
temperature and density, the beam averaged CI-column density is large, 2.1
10^18 cm^-2, confirming the high CI/CO abundance ratio of approximately 0.5
estimated earlier from the 492 GHz line alone. We argue that the [CI] emission
from M82 most likely arises in clouds of linear size around a few pc with a
density of about 10^4 cm^-3 or slightly higher and temperatures of 50 K up to
about 100 K.Comment: 4 pages, 2 figures, ApJL in press, postscript also available at
ftp://apollo.ph1.uni-koeln.de/pub/stutzki/m82_pap.ps.gz
e-mail-contact:[email protected]
QRiH: Towards a fitting system for humanities research evaluation .
Merit, Expertise and Measuremen
First observations with CONDOR, a 1.5 THz heterodyne receiver
The THz atmospheric windows centered at roughly 1.3 and 1.5~THz, contain
numerous spectral lines of astronomical importance, including three high-J CO
lines, the N+ line at 205 microns, and the ground transition of para-H2D+. The
CO lines are tracers of hot (several 100K), dense gas; N+ is a cooling line of
diffuse, ionized gas; the H2D+ line is a non-depleting tracer of cold (~20K),
dense gas. As the THz lines benefit the study of diverse phenomena (from
high-mass star-forming regions to the WIM to cold prestellar cores), we have
built the CO N+ Deuterium Observations Receiver (CONDOR) to further explore the
THz windows by ground-based observations. CONDOR was designed to be used at the
Atacama Pathfinder EXperiment (APEX) and Stratospheric Observatory For Infrared
Astronomy (SOFIA). CONDOR was installed at the APEX telescope and test
observations were made to characterize the instrument. The combination of
CONDOR on APEX successfully detected THz radiation from astronomical sources.
CONDOR operated with typical Trec=1600K and spectral Allan variance times of
30s. CONDOR's first light observations of CO 13-12 emission from the hot core
Orion FIR4 (= OMC1 South) revealed a narrow line with T(MB) = 210K and
delta(V)=5.4km/s. A search for N+ emission from the ionization front of the
Orion Bar resulted in a non-detection. The successful deployment of CONDOR at
APEX demonstrates the potential for making observations at THz frequencies from
ground-based facilities.Comment: 4 pages + list of objects, 3 figures, to be published in A&A special
APEX issu
Photon Dominated Regions in NGC 3603
Aims: We aim at deriving the excitation conditions of the interstellar gas as
well as the local FUV intensities in the molecular cloud surrounding NGC 3603
to get a coherent picture of how the gas is energized by the central stars.
Methods: The NANTEN2-4m submillimeter antenna is used to map the [CI] 1-0, 2-1
and CO 4-3, 7-6 lines in a 2' x 2' region around the young OB cluster NGC 3603
YC. These data are combined with C18O 2-1 data, HIRES-processed IRAS 60 and 100
micron maps of the FIR continuum, and Spitzer/IRAC maps. Results: The NANTEN2
observations show the presence of two molecular clumps located south-east and
south-west of the cluster and confirm the overall structure already found by
previous CS and C18O observations. We find a slight position offset of the peak
intensity of CO and [CI], and the atomic carbon appears to be further extended
compared to the molecular material. We used the HIRES far-infrared dust data to
derive a map of the FUV field heating the dust. We constrain the FUV field to
values of \chi = 3 - 6 \times 10^3 in units of the Draine field across the
clouds. Approximately 0.2 to 0.3 % of the total FUV energy is re-emitted in the
[CII] 158 {\mu}m cooling line observed by ISO. Applying LTE and escape
probability calculations, we derive temperatures (TMM1 = 43 K, TMM2 = 47 K),
column densities (N(MM1) = 0.9 \times 10^22 cm^-2, N(MM2) = 2.5 \times 10^22
cm^-2) and densities (n(MM1) = 3 \times 10^3 cm^-3, n(MM2) = 10^3 -10^4 cm^-3)
for the two observed molecular clumps MM1 and MM2. Conclusions: The cluster is
strongly interacting with the ambient molecular cloud, governing its structure
and physical conditions. A stability analysis shows the existence of
gravitationally collapsing gas clumps which should lead to star formation.
Embedded IR sources have already been observed in the outskirts of the
molecular cloud and seem to support our conclusions.Comment: 13 pages, 10 figures, accepted for publication by A&
Herschel observations of EXtra-Ordinary Sources: The Terahertz spectrum of Orion KL seen at high spectral resolution
We present the first high spectral resolution observations of Orion KL in the
frequency ranges 1573.4 - 1702.8 GHz (band 6b) and 1788.4 - 1906.8 GHz (band
7b) obtained using the HIFI instrument on board the Herschel Space Observatory.
We characterize the main emission lines found in the spectrum, which primarily
arise from a range of components associated with Orion KL including the hot
core, but also see widespread emission from components associated with
molecular outflows traced by H2O, SO2, and OH. We find that the density of
observed emission lines is significantly diminished in these bands compared to
lower frequency Herschel/HIFI bands.Comment: Accepted for publication in the Herschel HIFI special issue of
Astronomy and Astrophysics Letters, 5 pages, 3 figure
Herschel observations of extra-ordinary sources: Detection of Hydrogen Fluoride in absorption towards Orion~KL
We report a detection of the fundamental rotational transition of hydrogen
fluoride in absorption towards Orion KL using Herschel/HIFI. After the removal
of contaminating features associated with common molecules ("weeds"), the HF
spectrum shows a P-Cygni profile, with weak redshifted emission and strong
blue-shifted absorption, associated with the low-velocity molecular outflow. We
derive an estimate of 2.9 x 10^13 cm^-2 for the HF column density responsible
for the broad absorption component. Using our best estimate of the H2 column
density within the low-velocity molecular outflow, we obtain a lower limit of
~1.6 x 10^-10 for the HF abundance relative to hydrogen nuclei, corresponding
to 0.6% of the solar abundance of fluorine. This value is close to that
inferred from previous ISO observations of HF J=2--1 absorption towards Sgr B2,
but is in sharp contrast to the lower limit of 6 x 10^-9 derived by Neufeld et
al. (2010) for cold, foreground clouds on the line of sight towards G10.6-0.4.Comment: 5 pages, 3 figures, paper to be published in the Herschel special
issue of A&A letter
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&
Herschel observations of extra-ordinary sources: Detecting spiral arm clouds by CH absorption lines
We have observed CH absorption lines ()
against the continuum source Sgr~B2(M) using the \textit{Herschel}/HIFI
instrument. With the high spectral resolution and wide velocity coverage
provided by HIFI, 31 CH absorption features with different radial velocities
and line widths are detected and identified. The narrower line width and lower
column density clouds show `spiral arm' cloud characteristics, while the
absorption component with the broadest line width and highest column density
corresponds to the gas from the Sgr~B2 envelope. The observations show that
each `spiral arm' harbors multiple velocity components, indicating that the
clouds are not uniform and that they have internal structure. This
line-of-sight through almost the entire Galaxy offers unique possibilities to
study the basic chemistry of simple molecules in diffuse clouds, as a variety
of different cloud classes are sampled simultaneously. We find that the linear
relationship between CH and H column densities found at lower by UV
observations does not continue into the range of higher visual extinction.
There, the curve flattens, which probably means that CH is depleted in the
denser cores of these clouds.Comment: Accepted for publication in A&A, HIFI Special Issu
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