273 research outputs found
Dense, Parsec-Scale Clumps near the Great Annihilator
We report on Combined Array for Research in Millimeter-Wave Astronomy (CARMA)
and James Clerk Maxwell Telescope (JCMT) observations toward the Einstein
source 1E 1740.7-2942, a LMXB commonly known as the "Great Annihilator." The
Great Annihilator is known to be near a small, bright molecular cloud on the
sky in a region largely devoid of emission in 12-CO surveys of the Galactic
Center. The region is of interest because it is interior to the dust lanes
which may be the shock zones where atomic gas from HI nuclear disk is converted
into molecular gas. We find that the region is populated with a number of dense
(n ~ 10^5 cm^-3) regions of excited gas with small filling factors, and
estimate that up to 1-3 x 10^5 solar masses of gas can be seen in our maps. The
detection suggests that a significant amount of mass is transported from the
shock zones to the GC star-forming regions in the form of small, dense bundles.Comment: 26 pages, 7 figures, accepted for publication by the Astrophysical
Journal, abstract abridge
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 Identification of DepB: An Enzyme Responsible for the Final Detoxification Step in the Deoxynivalenol Epimerization Pathway in Devosia mutans 17-2-E-8
Deoxynivalenol (DON) is one of the most common mycotoxins found in cereal grains and grains contaminated with DON can cause health issues for both humans and animals and result in severe economic losses. Currently there is no feasible method to remediate affected grains. The development of a biological method for detoxification is becoming increasingly more plausible with the discovery of microbes which can transform DON to a relatively non-toxic stereoisomer, 3-epi-DON. Although bacteria capable of detoxifying DON have been known for some time, it is only recently an enzyme responsible was identified. In Devosia mutans 17-2-E-8 (Devosia sp. 17-2-E-8) a two-step DON epimerization (Dep) pathway, designated as the Dep system, completes this reaction. DepA was recently identified as the enzyme responsible for the conversion of DON to 3-keto-DON, and in this report, DepB, a NADPH dependent dehydrogenase, is identified as the second and final step in the pathway. DepB readily catalyzes the reduction of 3-keto-DON to 3-epi-DON. DepB is shown to be moderately thermostable as it did not lose significant activity after a heat treatment at 55°C and it is amenable to lyophilization. DepB functions at a range of pH-values (5–9) and functions equally well in multiple common buffers. DepB is clearly a NADPH dependent enzyme as it utilizes it much more efficiently than NADH. The discovery of the final step in the Dep pathway may provide a means to finally mitigate the losses from DON contamination in cereal grains through an enzymatic detoxification system. The further development of this system will need to focus on the activity of the Dep enzymes under conditions mimicking industrially relevant conditions to test their functionality for use in areas such as corn milling, fuel ethanol fermentation or directly in animal feed
Influence heat-reflective coating on the decrease of heat losses of window constructions
Developed theoretical and methodological foundations of the optimal choice of space-planning and constructive decisions of low-rise buildings blocked type, aimed at improving efficiency of investment, energy and resource saving, creation of comfortable conditions for the population, ensure sustainable development of low-rise construction in the context of socio-economic priorities in the climatic zoning of the area of construction
Diagnostics of the Molecular Component of PDRs with Mechanical Heating
Context. Multitransition CO observations of galaxy centers have revealed that
significant fractions of the dense circumnuclear gas have high kinetic
temperatures, which are hard to explain by pure photon excitation, but may be
caused by dissipation of turbulent energy.
Aims. We aim to determine to what extent mechanical heating should be taken
into account while modelling PDRs. To this end, the effect of dissipated
turbulence on the thermal and chemical properties of PDRs is explored. Methods.
Clouds are modelled as 1D semi-infinite slabs whose thermal and chemical
equilibrium is solved for using the Leiden PDR-XDR code.
Results. In a steady-state treatment, mechanical heating seems to play an
important role in determining the kinetic temperature of the gas in molecular
clouds. Particularly in high-energy environments such as starburst galaxies and
galaxy centers, model gas temperatures are underestimated by at least a factor
of two if mechanical heating is ignored. The models also show that CO, HCN and
H2 O column densities increase as a function of mechanical heating. The HNC/HCN
integrated column density ratio shows a decrease by a factor of at least two in
high density regions with n \sim 105 cm-3, whereas that of HCN/HCO+ shows a
strong dependence on mechanical heating for this same density range, with
boosts of up to three orders of magnitude.
Conclusions. The effects of mechanical heating cannot be ignored in studies
of the molecular gas excitation whenever the ratio of the star formation rate
to the gas density is close to, or exceeds, 7 \times 10-6 M yr-1 cm4.5 . If
mechanical heating is not included, predicted column densities are
underestimated, sometimes even by a few orders of magnitude. As a lower bound
to its importance, we determined that it has non-negligible effects already
when mechanical heating is as little as 1% of the UV heating in a PDR.Comment: 26 pages, 14 figures in the text and 13 figures as supplementary
material. Accepted for publication in A&
Search for Interstellar Water in the Translucent Molecular Cloud toward HD 154368
We report an upper limit of 9 x 10^{12} cm-2 on the column density of water
in the translucent cloud along the line of sight toward HD 154368. This result
is based upon a search for the C-X band of water near 1240 \AA carried out
using the Goddard High Resolution Spectrograph of the Hubble Space Telescope.
Our observational limit on the water abundance together with detailed chemical
models of translucent clouds and previous measurements of OH along the line of
sight constrain the branching ratio in the dissociative recombination of H_3O+
to form water. We find at the level that no more than 30% of
dissociative recombinations of H_3O+ can lead to H_2O. The observed spectrum
also yielded high-resolution observations of the Mg II doublet at 1239.9 \AA
and 1240.4 \AA, allowing the velocity structure of the dominant ionization
state of magnesium to be studied along the line of sight. The Mg II spectrum is
consistent with GHRS observations at lower spectral resolution that were
obtained previously but allow an additional velocity component to be
identified.Comment: Accepted by ApJ, uses aasp
HD/H2 Molecular Clouds in the Early Universe: The Problem of Primordial Deuterium
We have detected new HD absorption systems at high redshifts, z_abs=2.626 and
z_abs=1.777, identified in the spectra of the quasars J0812+3208 and Q1331+170,
respectively. Each of these systems consists of two subsystems. The HD column
densities have been determined: log(N(HD),A)=15.70+/-0.07 for z_A=2.626443(2)
and log(N(HD),B)=12.98+/-0.22 for z_B=2.626276(2) in the spectrum of J0812+3208
and log(N(HD),C)=14.83+/-0.15 for z_C=1.77637(2) and log(N(HD),D)=14.61+/-0.20
for z_D=1.77670(3) in the spectrum of Q1331+170. The measured HD/H2 ratio for
three of these subsystems has been found to be considerably higher than its
values typical of clouds in our Galaxy. We discuss the problem of determining
the primordial deuterium abundance, which is most sensitive to the baryon
density of the Universe \Omega_{b}. Using a well-known model for the chemistry
of a molecular cloud, we have estimated the isotopic ratio
D/H=HD/2H_2=(2.97+/-0.55)x10^{-5} and the corresponding baryon density
\Omega_{b}h^2=0.0205^{+0.0025}_{-0.0020}. This value is in good agreement with
\Omega_{b}h^2=0.0226^{+0.0006}_{-0.0006} obtained by analyzing the cosmic
microwave background radiation anisotropy. However, in high-redshift clouds,
under conditions of low metallicity and low dust content, hydrogen may be
incompletely molecularized even in the case of self-shielding. In this
situation, the HD/2H_2 ratio may not correspond to the actual D/H isotopic
ratio. We have estimated the cloud molecularization dynamics and the influence
of cosmological evolutionary effects on it
Warm molecular hydrogen in the Spitzer SINGS galaxy sample
(simplified) Results on the properties of warm H2 in 57 normal galaxies are
derived from H2 rotational transitions, obtained as part of SINGS. This study
extends previous extragalactic surveys of H2, the most abundant constituent of
the molecular ISM, to more common systems (L_FIR = e7 to 6e10 L_sun) of all
morphological and nuclear types. The S(1) transition is securely detected in
the nuclear regions of 86% of SINGS galaxies with stellar masses above 10^9.5
M_sun. The derived column densities of warm H2 (T > ~100 K), even though
averaged over kiloparsec-scale areas, are commensurate with those of resolved
PDRs; the median of the sample is 3e20 cm-2. They amount to between 1% and >30%
of the total H2. The power emitted in the sum of the S(0) to S(2) transitions
is on average 30% of the [SiII] line power, and ~4e-4 of the total infrared
power (TIR) within the same area for star-forming galaxies, which is consistent
with excitation in PDRs. The fact that H2 emission scales tightly with PAH
emission, even though the average radiation field intensity varies by a factor
ten, can also be understood if both tracers originate predominantly in PDRs,
either dense or diffuse. A large fraction of the 25 LINER/Sy targets, however,
strongly depart from the rest of the sample, in having warmer H2 in the excited
states, and an excess of H2 emission with respect to PAHs, TIR and [SiII]. We
propose a threshold in H2 to PAH power ratios, allowing the identification of
low-luminosity AGNs by an excess H2 excitation. A dominant contribution from
shock heating is favored in these objects. Finally, we detect, in nearly half
the star-forming targets, non-equilibrium ortho to para ratios, consistent with
FUV pumping combined with incomplete ortho-para thermalization by collisions,
or possibly non-equilibrium PDR fronts advancing into cold gas.Comment: ApJS, in pres
Environmental Emission of Pharmaceuticals from Wastewater Treatment Plants in the USA
The residual drugs, drug bioconjugates, and their metabolites, mostly from human and veterinary usage, are routinely flushed down the drain, and enter wastewater treatment plants (WWTP). Increasing population, excessive use of allopathic medicine, continual introduction of novel drugs, and existing inefficient wastewater treatment processes result in the discharge of large volumes of pharmaceuticals and their metabolites from the WWTPs into the environment. The effluent from the WWTPs globally contaminate ~25% of rivers and the lakes. Pharmaceuticals in the environment, as contaminants of emerging concerns, behave as pseudo-persistent despite their relatively short environmental half-lives in the environment. Therefore, residual levels of pharmaceuticals in the environment not only pose a threat to the wildlife but also affect human health through contaminated food and drinking water. This chapter highlights WWTPs as point-sources of their environmental emissions and various effects on the aquatic and terrestrial ecosystem
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