1,015 research outputs found
Prototype solar heating and cooling systems, including potable hot water
Progress made in the development, delivery, and support of two prototype solar heating and cooling systems including potable hot water is reported. The system consists of the following subsystems: collector, auxiliary heating, potable hot water, storage, control, transport, and government-furnished site data acquisition. A comparison of the proposed Solaron Heat Pump and Solar Desiccant Heating and Cooling Systems, installation drawings, data on the Akron House at Akron, Ohio, and other program activities are included
Preliminary design of flight hardware for two-phase fluid research
This study defined the preliminary designs of flight software for the Space Shuttle Orbiter for three two-phase fluid research experiments: (1) liquid reorientation - to study the motion of liquid in tanks subjected to small accelerations; (2) pool boiling - to study low-gravity boiling from horizontal cylinders; and (3) flow boiling - to study low-gravity forced flow boiling heat transfer and flow phenomena in a heated horizontal tube. The study consisted of eight major tasks: reassessment of the existing experiment designs, assessment of the Spacelab facility approach, assessment of the individual carry-on approach, selection of the preferred approach, preliminary design of flight hardware, safety analysis, preparation of a development plan, estimates of detailed design, fabrication and ground testing costs. The most cost effective design approach for the experiments is individual carry-ons in the Orbiter middeck. The experiments were designed to fit into one or two middeck lockers. Development schedules for the detailed design, fabrication and ground testing ranged from 15 1/2 to 18 months. Minimum costs (in 1981 dollars) ranged from 998K for the pool boiling experiment
LOFAR observations of 4C+19.44. On the discovery of low frequency spectral curvature in relativistic jet knots
We present the first LOFAR observations of the radio jet in the quasar
4C+19.44 (a.k.a. PKS 1354+19) obtained with the long baselines. The achieved
resolution is very well matched to that of archival Jansky Very Large Array
(JVLA) observations at higher radio frequencies as well as the archival X-ray
images obtained with {\it Chandra}. We found that, for several knots along the
jet, the radio flux densities measured at hundreds of MHz lie well below the
values estimated by extrapolating the GHz spectra. This clearly indicates the
presence of spectral curvature. Radio spectral curvature has been already
observed in different source classes and/or extended radio structures and it
has been often interpreted as due to intrinsic processes, as a curved particle
energy distribution, rather than absorption mechanisms ({ Razin-Tsytovich}
effect, free-free or synchrotron self absorption to name a few). Here we
discuss our results according to the scenario where particles undergo
stochastic acceleration mechanisms also in quasar jet knots.Comment: 13 pages, 4 tables, 4 figures, pre-proof version, published on the
Astrophysical Journal (Harris, et al. 2019 ApJ, 873, 21
Carbon radio recombination lines from gigahertz to megahertz frequencies towards Orion A
Context. The combined use of carbon radio recombination lines (CRRLs) and the
158 m-[CII] line is a powerful tool for the study of the energetics and
physical conditions (e.g., temperature and density) of photodissociation
regions (PDRs). However, there are few observational studies that exploit this
synergy. Aims. Here we explore the relation between CRRLs and the 158
m-[CII] line in light of new observations and models. Methods. We present
new and existing observations of CRRLs in the frequency range 0.15--230 GHz
with ALMA, VLA, the GBT, Effelsberg 100m, and LOFAR towards Orion~A (M42). We
complement these observations with SOFIA observations of the 158 m-[CII]
line. We studied two PDRs: the foreground atomic gas, known as the Veil, and
the dense PDR between the HII region and the background molecular cloud.
Results. In the Veil we are able to determine the gas temperature and electron
density, which we use to measure the ionization parameter and the photoelectric
heating efficiency. In the dense PDR, we are able to identify a layered PDR
structure at the surface of the molecular cloud to the south of the Trapezium
cluster. There we find that the radio lines trace the colder portion of the
ionized carbon layer, the C/C/CO interface. By modeling the emission of
the ~m-[CII] line and CRRLs as arising from a PDR we derive a thermal
pressure K cm and a radiation field
close to the Trapezium. Conclusions. This work provides
additional observational support for the use of CRRLs and the 158 m-[CII]
line as complementary tools to study dense and diffuse PDRs, and highlights the
usefulness of CRRLs as probes of the C/C/CO interface.Comment: 18 pages, 16 figures, accepted for publication in A&
Spectroscopy with the Engineering Development Array: cold H at 63 MHz towards the Galactic Centre
The Engineering Development Array (EDA) is a single test station for Square
Kilometre Array (SKA) precursor technology. We have used the EDA to detect
low-frequency radio recombination lines (RRLs) from the Galactic Centre region.
Low-frequency RRLs are an area of interest for future low-frequency SKA work as
these lines provide important information on the physical properties of the
cold neutral medium. In this project we investigate the EDA, its bandpass and
the radio frequency interference environment for low-frequency spectroscopy. We
present line spectra from 30 to 325 MHz for the Galactic Centre region. The
decrease in sensitivity for the EDA at the low end of the receiver prevents
carbon and hydrogen RRLs to be detected below 40 and 60 MHz respectively. RFI
strongly affects frequencies in the range 276-292, 234-270, 131-138, 95-102 and
below 33 MHz. Cn RRLs were detected in absorption for quantum levels n
= 378 to 550 (39-121 MHz) and in emission for n = 272 to 306 (228-325 MHz).
Cn lines were detected in absorption for n = 387 to 696 (39-225 MHz).
Hn RRLs were detected in emission for n = 272 to 480 (59-325 MHz).
Hn lines were detected for n = 387 to 453 (141-225 MHz). The stacked
Hn detection at 63 MHz is the lowest frequency detection made for
hydrogen RRLs and shows that a cold (partially) ionized medium exists along the
line of sight to the Galactic Centre region. The size and velocity of this cold
H gas indicates that it is likely associated with the nearby
Riegel-Crutcher cloud.Comment: 18 pages, 6 figures and 5 table
Hydrogen Two-Photon Continuum Emission from the Horseshoe Filament in NGC 1275
Far ultraviolet emission has been detected from a knot of Halpha emission in
the Horseshoe filament, far out in the NGC 1275 nebula. The flux detected
relative to the brightness of the Halpha line in the same spatial region is
very close to that expected from Hydrogen two-photon continuum emission in the
particle heating model of Ferland et al. (2009) if reddening internal to the
filaments is taken into account. We find no need to invoke other sources of far
ultraviolet emission such as hot stars or emission lines from CIV in
intermediate temperature gas to explain these data.Comment: 9 pages, 8 figures. Accepted for publication in MNRA
Searching for the largest bound atoms in space
(abridged) Radio recombination lines (RRLs) at frequencies < 250 MHz
trace the cold, diffuse phase of the ISM. Next generation low frequency
interferometers, such as LOFAR, MWA and the future SKA, with unprecedented
sensitivity, resolution, and large fractional bandwidths, are enabling the
exploration of the extragalactic RRL universe. We observed the radio quasar 3C
190 (z~1.2) with the LOFAR HBA. In reducing this data for spectroscopic
analysis, we have placed special emphasis on bandpass calibration. We devised
cross-correlation techniques to significantly identify the presence of RRLs in
a low frequency spectrum. We demonstrate the utility of this method by applying
it to existing low-frequency spectra of Cassiopeia A and M 82, and to the new
observations of 3C 190. RRLs have been detected in the foreground of 3C 190 at
z = 1.12355 (assuming a carbon origin), owing to the first detection of RRLs
outside of the local universe (first reported in Emig et al. 2019). Towards the
Galactic supernova remnant Cas A, we uncover three new detections: (1)
C-transitions (n = 5) for the first time at low radio
frequencies, (2) H-transitions at 64 MHz with a FWHM of 3.1 km/s, the
most narrow and one of the lowest frequency detections of hydrogen to date, and
(3) C at v = 0 km/s in the frequency range 55-78 MHz for the
first time. Additionally we recover C, C, C, and
C from the -47 km/s and -38 km/s components. In the nearby starburst
galaxy, M 82, we do not find a significant feature. Our current searches for
RRLs in LOFAR observations are limited to narrow (< 100 km/s) features, owing
to the relatively small number of channels available for continuum estimation.
Future strategies making use of larger contiguous frequency coverage would aid
calibration to deeper sensitivities and broader features.Comment: 21 pages, 21 figures, accepted in A&
Cosmic evolution of FRI and FRII sources out to z=2.5
Radio-loud active galactic nuclei (RLAGN) play an important role in the
evolution of galaxies through the effects on their environment. The two major
morphological classes are core-bright (FRI) and edge-bright (FRII) sources.
With the LOw-Frequency ARray (LOFAR) we compare the FRI and FRII evolution down
to lower flux densities and with larger samples than before with the aim to
examine the cosmic space density evolution for FRIs and FRIIs by analyzing
their space density evolution between L_150~10^24.5 W/Hz and L_150~10^28.5 W/Hz
and up to z=2.5. We construct radio luminosity functions (RLFs) from FRI and
FRII catalogues based on recent data from LOFAR at 150MHz to study the space
densities as a function of radio luminosity and redshift. To partly correct for
selection biases and completeness, we simulate how sources appear at a range of
redshifts. We report a space density enhancement from low to high redshift for
FRI and FRII sources brighter than L_150~10^27 W/Hz. This is possibly related
to the higher gas availability in the earlier denser universe. The constant
FRI/FRII space density ratio evolution as a function of radio luminosity and
redshift in our results suggests that the jet-disruption of FRIs might be
primarily caused by events occurring on scales within the host galaxy, rather
than being driven by changes in the overall large-scale environment. Remaining
selection biases in our results also highlight the need to resolve more sources
at angular scales below 40 arcsec and therefore strengthens the motivation for
the further development and automation of the calibration and imaging pipeline
of LOFAR data to produce images at sub-arcsecond resolution
Lofar low-band antenna observations of the 3C 295 and boötes fields : Source counts and ultra-steep spectrum sources
© 2018 The American Astronomical Society. All rights reserved.We present Low Frequency Array (LOFAR) Low Band observations of the Boötes and 3C 295 fields. Our images made at 34, 46, and 62 MHz reach noise levels of 12, 8, and 5 mJy beam-1, making them the deepest images ever obtained in this frequency range. In total, we detect between 300 and 400 sources in each of these images, covering an area of 17-52 deg2. From the observations, we derive Euclidean-normalized differential source counts. The 62 MHz source counts agree with previous GMRT 153 MHz and Very Large Array 74 MHz differential source counts, scaling with a spectral index of -0.7. We find that a spectral index scaling of -0.5 is required to match up the LOFAR 34 MHz source counts. This result is also in agreement with source counts from the 38 MHz 8C survey, indicating that the average spectral index of radio sources flattens toward lower frequencies. We also find evidence for spectral flattening using the individual flux measurements of sources between 34 and 1400 MHz and by calculating the spectral index averaged over the source population. To select ultra-steep spectrum (α < -1.1) radio sources that could be associated with massive high-redshift radio galaxies, we compute spectral indices between 62 MHz, 153 MHz, and 1.4 GHz for sources in the Boötes field. We cross-correlate these radio sources with optical and infrared catalogs and fit the spectral energy distribution to obtain photometric redshifts. We find that most of these ultra-steep spectrum sources are located in the 0.7 ≲ z ≲ 2.5 range.Peer reviewe
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