419 research outputs found
Advances in understanding subglacial meltwater drainage from past ice sheets
Meltwater drainage beneath ice sheets is a fundamental consideration for understanding ice–bed
conditions and bed-modulated ice flow, with potential impacts on terminus behavior and iceshelf mass balance. While contemporary observations reveal the presence of basal water
movement in the subglacial environment and inferred styles of drainage, the geological record
of former ice sheets, including sediments and landforms on land and the seafloor, aids in understanding the spatiotemporal evolution of efficient and inefficient drainage systems and their
impact on ice-sheet behavior. We highlight the past decade of advances in geological studies
that focus on providing process-based information on subglacial hydrology of ice sheets, how
these studies inform theory, numerical models and contemporary observations, and address
the needs for future research
Modeling, Simulation and Control of Flat Panel Solar Collectors with Thermal Storage for Heating and Cooling Applications
AbstractThe focus of the present study is on the transient modeling of components in a solar system, controls, and heating and cooling loads. The system consists of solar flat plates and a thermal storage tank, which could provide a portion of heating requirements of a social enterprise building located in Winnipeg, Manitoba, Canada. This solar system and a natural gas hot water heater may replace the low efficiency boiler that supplies low quality steam for heat throughout the building using radiators. Results of the simulation performed in Simulink supports the proper selection of solar system components and controls that will be optimized for the climatic attributes of Winnipeg
First Detection of HCO+ Emission at High Redshift
We report the detection of HCO+(1-0) emission towards the Cloverleaf quasar
(z=2.56) through observations with the Very Large Array. This is the first
detection of ionized molecular gas emission at high redshift (z>2). HCO+
emission is a star formation indicator similar to HCN, tracing dense molecular
hydrogen gas (n(H_2) ~= 10^5 cm^{-3}) within star-forming molecular clouds. We
derive a lensing-corrected HCO+ line luminosity of L'(HCO+) = 3.5 x 10^9 K km/s
pc^2. Combining our new results with CO and HCN measurements from the
literature, we find a HCO+/CO luminosity ratio of 0.08 and a HCO+/HCN
luminosity ratio of 0.8. These ratios fall within the scatter of the same
relationships found for low-z star-forming galaxies. However, a HCO+/HCN
luminosity ratio close to unity would not be expected for the Cloverleaf if the
recently suggested relation between this ratio and the far-infrared luminosity
were to hold. We conclude that a ratio between HCO+ and HCN luminosity close to
1 is likely due to the fact that the emission from both lines is optically
thick and thermalized and emerges from dense regions of similar volumes. The
CO, HCN and HCO+ luminosities suggest that the Cloverleaf is a composite
AGN--starburst system, in agreement with the previous finding that about 20% of
the total infrared luminosity in this system results from dust heated by star
formation rather than heating by the AGN. We conclude that HCO+ is potentially
a good tracer for dense molecular gas at high redshift.Comment: 5 pages, 3 figures, ApJL, in press (accepted May 17, 2006
Detection of Emission from the CN Radical in the Cloverleaf Quasar at z=2.56
We report the detection of CN(N=3-2) emission towards the Cloverleaf quasar
(z=2.56) based on observations with the IRAM Plateau de Bure Interferometer.
This is the first clear detection of emission from this radical at high
redshift. CN emission is a tracer of dense molecular hydrogen gas (n(H2) > 10^4
cm^{-3}) within star-forming molecular clouds, in particular in regions where
the clouds are affected by UV radiation. The HCN/CN intensity ratio can be used
as a diagnostic for the relative importance of photodissociation regions (PDRs)
in a source, and as a sensitive probe of optical depth, the radiation field,
and photochemical processes. We derive a lensing-corrected CN(N=3-2) line
luminosity of L'(CN(3-2) = (4.5 +/- 0.5) x 10^9 K km/s pc^2. The ratio between
CN luminosity and far-infrared luminosity falls within the scatter of the same
relationship found for low-z (ultra-) luminous infrared galaxies. Combining our
new results with CO(J=3-2) and HCN(J=1-0) measurements from the literature and
assuming thermal excitation for all transitions, we find a CO/CN luminosity
ratio of 9.3 +/- 1.9 and a HCN/CN luminosity ratio of 0.95 +/- 0.15. However,
we find that the CN(N=3-2) line is likely only subthermally excited, implying
that those ratios may only provide upper limits for the intrinsic 1-0 line
luminosity ratios. We conclude that, in combination with other molecular gas
tracers like CO, HCN, and HCO+, CN is an important probe of the physical
conditions and chemical composition of dense molecular environments at high
redshift.Comment: 6 pages, 5 figures, 1 table, to appear in ApJ (accepted May 23, 2007
Cosmological Hydrodynamics with Multi-Species Chemistry and Nonequilibrium Ionization and Cooling
We have developed a method of solving for multi-species chemical reaction
flows in non--equilibrium and self--consistently with the hydrodynamic
equations in an expanding FLRW universe. The method is based on a backward
differencing scheme for the required stability when solving stiff sets of
equations and is designed to be efficient for three-dimensional calculations
without sacrificing accuracy. In all, 28 kinetic reactions are solved including
both collisional and radiative processes for the following nine separate
species: H, H+, He, He+, He++, H-, H2+, H2, and e-. The method identifies those
reactions (involving H- and H2+) ocurring on the shortest time scales,
decoupling them from the rest of the network and imposing equilibrium
concentrations to good accuracy over typical cosmological dynamical times.
Several tests of our code are presented, including radiative shock waves,
cosmological sheets, conservation constraints, and fully three-dimensional
simulations of CDM cosmological evolutions in which we compare our method to
results obtained when the packaged routine LSODAR is substituted for our
algorithms.Comment: Latex and postscript, 24 pages, with 6 figures. The paper is also
available at http://zeus.ncsa.uiuc.edu:8080/~abel/PGas/bib.html Submitted to
New Astronom
Modelling Primordial Gas in Numerical Cosmology
We have reviewed the chemistry and cooling behaviour of low-density (n<10^4
cm^-3) primordial gas and devised a cooling model wich involves 19 collisional
and 9 radiative processes and is applicable for temperatures in the range (1 K
< T < 10^8 K). We derived new fits of rate coefficients for the
photo-attachment of neutral hydrogen, the formation of molecular hydrogen via
H-, charge exchange between H2 and H+, electron detachment of H- by neutral
hydrogen, dissociative recombination of H2 with slow electrons,
photodissociation of H2+, and photodissociation of H2. Further it was found
that the molecular hydrogen produced through the gas-phase processes, H2+ + H
-> H2 + H+, and H- + H -> H2 + e-, is likely to be converted into its para
configuration on a faster time scale than the formation time scale. We have
tested the model extensively and shown it to agree well with former studies. We
further studied the chemical kinetics in great detail and devised a minimal
model which is substantially simpler than the full reaction network but
predicts correct abundances. This minimal model shows convincingly that 12
collisional processes are sufficient to model the H, He, H+, H-, He+, He++, and
H2 abundances in low density primordial gas for applications with no radiation
fields.Comment: 26 pages of text, 4 tables, and 6 eps figures. The paper is also
available at http://zeus.ncsa.uiuc.edu:8080/~abel/PGas/bib.html Submitted to
New Astronomy. Note that some of the hyperlinks given in the paper are still
under constructio
Simultaneous Observations of GRS 1758-258 in 1997 by VLA, IRAM, SEST, RXTE and OSSE: Spectroscopy and Timing
We report the results of our multi-wavelength observations of GRS 1758-258
made in August 1997. The energy bands include radio, millimeter, X-ray, and
gamma-ray. The observations enable us to obtain a complete spectrum of the
source over an energy range of 2 - 500 keV. The spectrum shows that GRS
1758-258 was in its hard state. It is well fitted by the Sunyaev-Titarchuk (ST)
Compton scattering model. The spectrum is also fit by a power law with an
exponential cutoff (PLE) plus a soft black-body component. The temperature of
the soft component is about 1.2 keV, and the energy flux is less than 1.5% of
the total X- and gamma-ray flux. The deduced hydrogen column density is in the
range of (0.93 - 2.0) 10^{22} cm^{-2}. No significant iron lines are detected.
The radio emission has a flat energy spectrum. The daily radio, X-ray and
gamma-ray light curves show that GRS 1758-258 was stable during the observation
period, but was highly variable on smaller time scales in X- and gamma-rays.
The power density spectra are typical for the low-state, but we find the photon
flux for the 5 to 10 keV band to be more variable than that in the other two
energy bands (2 - 5 keV and 10 - 40 keV). Harmonically spaced quasi-periodic
oscillations (QPOs) are observed in the power spectra. The phase lags between
the hard photons and the soft photons have a flat distribution over a wide
range of frequencies. A high coherence of about 1.0 (0.01 - 1 Hz) between the
hard photons and the soft photons is also obtained in our observations. We
compare these results with two variation models. Our millimeter observations
did not reveal any conclusive signatures of an interaction between the jet from
GRS 1758-258 and the molecular cloud that lies in the direction of GRS
1758-258.Comment: 32 pages, 13 figures, to appear in ApJ, 2000, V.533, no. 1, Apr. 10.
For better figure resolution, please directly download the paper from
http://spacsun.rice.edu/~lin/publication.htm
The Redshift Search Receiver 3 mm Wavelength Spectra of 10 Galaxies
The 3 mm wavelength spectra of 10 galaxies have been obtained at the Five
College Radio Astronomy Observatory using a new, very broadband receiver and
spectrometer, called the Redshift Search Receiver (RSR). The RSR has an
instantaneous bandwidth of 37 GHz covering frequencies from 74 to 111 GHz, and
has a spectral resolution of 31 MHz (~100 km/s). During tests of the RSR on the
FCRAO 14 m telescope the complete 3 mm spectra of the central regions of NGC
253, Maffei 2, NGC1068, IC 342, M82, NGC 3079, NGC 3690, NGC 4258, Arp 220 and
NGC 6240 were obtained. Within the wavelength band covered by the RSR, 20
spectral lines from 14 different atomic and molecular species were detected.
Based on simultaneous fits to the spectrum of each galaxy, a number of key
molecular line ratios are derived. A simple model which assumes the emission
arises from an ensemble of Milky Way-like Giant Nolecular Cloud cores can
adequately fit the observed line ratios using molecular abundances based on
Galactic molecular cloud cores. Variations seen in some line ratios, such as
13CO/HCN and HCO+/HCN, can be explained if the mean density of the molecular
gas varies from galaxy to galaxy. However, NGC 3690, NGC 4258 and NGC 6240 show
very large HCO+/HCN ratios and require significant abundance enhancement of
HCO+ over HCN, possible due to the proximity to active galactic nucleus
activity. Finally, the mass of dense molecular gas is estimated and we infer
that 25-85 % of the total molecular gas in the central regions of these
galaxies must have densities greater than 10^4 cm^-3.Comment: accepted for publication in A.
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