176 research outputs found
Neutral carbon in the Egg Nebula (AFGL 2688)
A search for sub-mm C I emission from seven stars that are surrounded by dense molecular gas shells led to the detection, in the case of the "Egg Nebula' (AFGL 2688), of an 0.9 K line implying a C I/CO value greater than 5. The material surrounding this star must be extremely carbon-rich, and it is suggested that the apparently greater extent of the C I emission region may be due to the effects of the galactic UV field on the shell's chemistry, as suggested by Huggins and Glassgold (1982)
The molecular clump towards the eastern border of SNR G18.8+0.3
The eastern border of the SNR G18.8+0.3, close to an HII regions complex, is
a very interesting region to study the molecular gas that it is probably in
contact with the SNR shock front. We observed the aforementioned region using
the Atacama Submillimeter Telescope Experiment (ASTE) in the 12CO J=3-2, 13CO
J=3-2, HCO+ J=4-3, and CS J=7-6 lines with an angular resolution of 22". To
complement these observations, we analyzed IR, submillimeter and radio
continuum archival data. In this work, we clearly show that the radio continuum
"protrusion" that was early thought to belong to the SNR is an HII regions
complex deeply embedded in a molecular clump. The new molecular observations
reveal that this dense clump, belonging to an extended molecular cloud that
surrounds the SNR southeast border, is not physically in contact with SNR
G18.8+0.3, suggesting that the SNR shock front have not yet reached it or maybe
they are located at different distances. We found some young stellar objects
embedded in the molecular clump, suggesting that their formation should be
approximately coeval with the SN explosion.Comment: Accepted for publication in A&A (Sept. 7, 2012
Infrared Instrumentation and Astronomy
Contains reports on five research projects.Joint Services Electronics Program (Contract DAAB07-76-C-1400)National Aeronautics and Space Administration (Grant NGR 22-009-526)National Aeronautics and Space Administration (Grant NSG-7328)National Aeronautics and Space Administration (Contract NAS5-24096
Dynamic Power Convertor Development for Radioisotope Power Systems at NASA Glenn Research Center
The Thermal Energy Conversion Branch at NASA Glenn Research Center (GRC) is supporting the development of high-efficiency power convertors for use in Radioisotope Power Systems (RPS). Significant progress was made towards such a system that utilized Stirling conversion during the 2001 to 2015 timeframe. Flight development of the Advanced Stirling Radioisotope Generator (ASRG) was cancelled in 2013 by the Department of Energy (DOE) and NASA Headquarters primarily due to budget constraints, and the Advanced Stirling Convertor (ASC) technology contract was subsequently concluded in 2015. A new chapter of technology development has recently been initiated by the NASA RPS Program. This effort is considering all dynamic power convertor options, such as Stirling and Brayton cycles. Four convertor development contracts supporting this effort were awarded in 2017. The awarded contracts include two free-piston Stirling, one thermoacoustic Stirling, and one turbo-Brayton designs. The technology development contracts each consist of up to three phases: Design, Fabricate, and Test. As of May 2018, all contracts have completed the Design Phase, and each underwent a design review with an independent review board. Three of the contracts are planned to execute the Phase 2 option for fabrication. Convertors manifesting from these development efforts will then undergo independent validation and verification at NASA facilities, which will consist of convertor performance and RPS viability demonstrations. Example tests include launch vibration simulation, performance mapping over the environmental temperature range, and static acceleration exposure. In parallel with this renewed development effort, NASA GRC is still demonstrating free-piston Stirling convertor technology using assets from previous projects. The Stirling Research Laboratory (SRL) is still operating several convertors from previous development projects which have similarities and relevance to current contract designs. Four of which are flexure-bearing based, and another six are gas-bearing based. One of the flexure-bearing convertors has accumulated over 110,000 hours of operation, and holds the current record for maintenance-free heat-engine run-time. Another flexure-bearing convertor was recently manually shutdown after 105,620 hours of operation, then disassembled and inspected. This inspection produced a wealth of information about the effects of this amount of runtime on the technology's components. One of the engineering unit flexure-bearing convertors recently underwent launch simulation vibration test, a static acceleration exposure up to 20 g, and was then placed on extended operation. Amongst the gas-bearing convertors, the longest running unit has accumulated over 70,000 hours of operation. Four high-fidelity gas-bearing convertors from the ASRG project are still operating continuously, for which the longest runtime has reached 28,000 hours
Ultrastructural and functional fate of recycled vesicles in hippocampal synapses
Efficient recycling of synaptic vesicles is thought to be critical for sustained information transfer at central terminals. However, the specific contribution that retrieved vesicles make to future transmission events remains unclear. Here we exploit fluorescence and time-stamped electron microscopy to track the functional and positional fate of vesicles endocytosed after readily releasable pool (RRP) stimulation in rat hippocampal synapses. We show that most vesicles are recovered near the active zone but subsequently take up random positions in the cluster, without preferential bias for future use. These vesicles non-selectively queue, advancing towards the release site with further stimulation in an actin-dependent manner. Nonetheless, the small subset of vesicles retrieved recently in the stimulus train persist nearer the active zone and exhibit more privileged use in the next RRP. Our findings reveal heterogeneity in vesicle fate based on nanoscale position and timing rules, providing new insights into the origins of future pool constitution
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
On the internal structure of starless cores. II. A molecular survey of L1498 and L1517B
[Abridged] We present a molecular survey of the starless cores L1498 and
L1517B. These cores have been selected for their relative isolation and
close-to-round shape, and they have been observed in a number of lines of 13
molecular species (4 already presented in the first part of this series): CO,
CS, N2H+, NH3, CH3OH, SO, C3H2, HC3N, C2S, HCN, H2CO, HCO+, and DCO+. Using a
physical model of core structure and a Monte Carlo radiative transfer code, we
determine for each core a self-consistent set abundances that fits
simultaneously the observed radial profile of integrated intensity and the
emergent spectrum towards the core center (for abundant species, optically thin
isopologues are used). From this work, we find that L1498 and L1517B have
similar abundance patterns, with most species suffering a significant drop
toward the core center. This occurs for CO, CS, CH3OH, SO, C3H2, HC3N, C2S,
HCN, H2CO, HCO+, and DCO+, which we fit with profiles having a sharp central
hole. The size of this hole varies with molecule: DCO+, HCN, and HC3N have the
smallest holes while SO, C2S and CO have the largest holes. Only N2H+ and NH3
are present in the gas phase at the core centers. From the different behavior
of molecules, we select SO, C2S, and CH3OH as the most sensitive tracers of
molecular depletion. Comparing our abundance determinations with the
predictions from current chemical models we find order of magnitude
discrepancies. Finally, we show how the ``contribution function'' can be used
to study the formation of line profiles from the different regions of a core.Comment: 22 pages, 12 figures, A&A accepte
Optical and submillimetre observations of Bok globules -- tracing the magnetic field from low to high density
We present optical and submillimetre polarimetry data of the Bok globule CB3
and optical polarimetry data of the Bok globule CB246. We use each set of
polarimetry data to infer the B-field orientation in each of the clouds. The
optical data can only be used in the low density, low extinction edge regions
of clouds. The submillimetre data can only be used in the high column-density,
central regions of the clouds.
It has previously been found that near-infrared polarisation mapping of
background stars does not accurately trace the magnetic field in dense cloud
regions. This may be due to a lack of aligned grains in dense regions. We test
this by comparing the field orientations measured by our two independent
polarimetry methods. We find that the field orientation deduced from the
optical data matches up well with the orientation estimated from the
submillimetre data. We therefore claim that both methods are accurately tracing
the same magnetic field in CB3. Hence, in this case, there must be significant
numbers of aligned dust grains in the high density region, and they do indeed
trace the magnetic field in the submillimetre.
We find an offset of 4014 degrees between the magnetic field orientation
and the short axis of the globule. This is consistent with the mean value of
313 degrees found in our previous work on prestellar cores, even though
CB3 is a protostellar core. Taken together, the six prestellar cores that we
have now studied in this way show a mean offset between magnetic field
orientation and core short axis of 3 degrees, in apparent
contradiction with some models of magnetically dominated star formation.Comment: 8 pages, 3 figures, accepted for publication in MNRA
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