62 research outputs found
First interstellar detection of OH+
The Atacama Pathfinder Experiment (APEX) 12m telescope was used to observe
the N=1-0, J=0-1 ground state transitions of OH+ at 909.1588 GHz with the
CHAMP+ heterodyne array receiver. Two blended hyperfine structure transitions
were detected in absorption against the strong continuum source Sagittarius
B2(M) and in several pixels offset by 18". Both, absorption from Galactic
center gas as well as absorption from diffuse clouds in intervening spiral arms
in a velocity range from -116 to 38.5 km/s is observed. The total OH+ column
density of absorbing gas is 2.4 \times 10^15 cm-2. A column density local to
Sgr B2(M) of 2.6 \times 10^14 cm-2 is found. On the intervening line-of-sight
the column density per unit velocity interval are in the range from 1 to 40
\times 10^12 cm-2/(km/s). OH+ is found to be on average more abundant than
other hydrides such as SH+ and CH+. Abundance ratios of OH and atomic oxygen to
OH+ are found to be in the range of 10^1-2 and 10^3-4, respectively. The
detected absorption of a continuous velocity range on the line-of-sight shows
OH+ to be an abundant component of diffuse clouds.Comment: 5 pages, 4 figures. Accepted for publication in Astronomy and
Astrophysic
Deuterium chemistry in protoplanetary disks II The inner 30 AU
We present the results of models of the chemistry, including deuterium, in
the inner regions of protostellar disks. We find good agreement with recent gas
phase observations of several (non--deuterated) species. We also compare our
results with observations of comets and find that in the absence of other
processing e.g. in the accretion shock at the surface of the disk, or by mixing
in the disk, the calculated D/H ratios in ices are higher than measured and
reflect the D/H ratio set in the molecular cloud phase. Our models give quite
different abundances and molecular distributions to other inner disk models
because of the differences in physical conditions in the model disk. This
emphasizes how changes in the assumptions about the density and temperature
distribution can radically affect the results of chemical models.Comment: Accepted by Astrophysical Journa
Cosmic-ray and X-ray Heating of Interstellar Clouds and Protoplanetary Disks
Cosmic-ray and X-ray heating are derived from the electron energy loss
calculations of Dalgarno, Yan and Liu for hydrogen-helium gas mixtures. These
authors treated the heating from elastic scattering and collisional
de-excitation of rotationally excited hydrogen molecules. Here we consider the
heating that can arise from all ionization and excitation processes, with
particular emphasis on the reactions of cosmic-ray and X-ray generated ions
with the heavy neutral species, which we refer to as chemical heating. In
molecular regions, chemical heating dominates and can account for 50 per cent
of the energy expended in the creation of an ion pair. The heating per ion pair
ranges in the limit of negligible electron fraction from about 4.3 eV for
diffuse atomic gas, to about 13 eV for the moderately dense regions of
molecular clouds and to about 18 eV for the very dense regions of
protoplanetary disks. An important general conclusion of this study is that
cosmic-ray and X-ray heating depends on the physical properties of the medium,
i.e., on the molecular and electron fractions, the total density of hydrogen
nuclei, and to a lesser extent on the temperature. It is also noted that
chemical heating, the dominant process for cosmic-ray and X-ray heating, plays
a role in UV irradiated molecular gas.Comment: 39 pages, accepted for publication in the Astrophysical Journa
The generation of low-energy cosmic rays in molecular clouds
It is argued that if cosmic rays penetrate into molecular clouds, the total
energy they lose can exceed the energy from galactic supernovae shocks. It is
shown that most likely galactic cosmic rays interacting with the surface layers
of molecular clouds are efficiently reflected and do not penetrate into the
cloud interior. Low-energy cosmic rays ( GeV) that provide the primary
ionization of the molecular cloud gas can be generated inside such clouds by
multiple shocks arising due to supersonic turbulence.Comment: 11 pages, no figure
The chemistry of multiply deuterated molecules in protoplanetary disks. I. The outer disk
We present new models of the deuterium chemistry in protoplanetary disks,
including, for the first time, multiply deuterated species. We use these models
to explore whether observations in combination with models can give us clues as
to which desorption processes occur in disks. We find, in common with other
authors, that photodesorption can allow strongly bound molecules such as HDO to
exist in the gas phase in a layer above the midplane. Models including this
process give the best agreement with the observations. In the midplane, cosmic
ray heating can desorb weakly bound molecules such as CO and N. We find the
observations suggest that N is gaseous in this region, but that CO must be
retained on the grains to account for the observed DCO/HCO. This could
be achieved by CO having a higher binding energy than N (as may be the case
when these molecules are accreted onto water ice) or by a smaller cosmic ray
desorption rate for CO than assumed here, as suggested by recent theoretical
work.
For gaseous molecules the calculated deuteration can be greatly changed by
chemical processing in the disk from the input molecular cloud values. On the
grains singly deuterated species tend to retain the D/H ratio set in the
molecular cloud, whereas multiply deuterated species are more affected by the
disk chemistry. Consequently the D/H ratios observed in comets may be partly
set in the parent cloud and partly in the disk, depending on the molecule.Comment: Accepted for publication in ApJ. 48 pages, 8 figure
Increased lipophilicity and subsequent cell partitioning decrease passive transcellular diffusion of novel, highly lipophilic antioxidants
ABSTRACT Oxidative stress is considered a cause or propagator of acute and chronic disorders of the central nervous system. Novel 2,4-diamino-pyrrolo [2,3-d]pyrimidines are potent inhibitors of iron-dependent lipid peroxidation, are cytoprotective in cell culture models of oxidative injury, and are neuroprotective in brain injury and ischemia models. The selection of lead candidates from this series required that they reach target cells deep within brain tissue in efficacious amounts after oral dosing. A homologous series of 26 highly lipophilic pyrrolopyrimidines was examined using cultured cell monolayers to understand the structure-permeability relationship and to use this information to predict brain penetration and residence time. Pyrrolopyrimidines were shown to be a more permeable structural class of membrane-interactive antioxidants where transepithelial permeability was inversely related to lipophilicity or to cell partitioning. Pyrrole substitutions influence cell partitioning where bulky hydrophobic groups increased partitioning and decreased permeability and smaller hydrophobic groups and more hydrophilic groups, especially those capable of weak hydrogen bonding, decreased partitioning, and increased permeability. Transmonolayer diffusion for these membrane-interactive antioxidants was limited mostly by desorption from the receiver-side membrane into the buffer. Thus, in this case, these in vitro cell monolayer models do not adequately mimic the in vivo situation by underestimating in vivo bioavailability of highly lipophilic compounds unless acceptors, such as serum proteins, are added to the receiving buffer. A series of novel 2,4-diamino-pyrrolo[2,3-d]pyrimidines were described as potent inhibitors of iron-dependent lipid peroxidation, and proved to be cytoprotective in cell culture models of oxidative injury and neuroprotective in brain injury and ischemia models Structural determinants of permeability and partitioning are discussed for a series of structurally similar homologs. In addition, detailed studies were conducted concurrently with two radiolabled compounds from the pyrrolopyrimidine series representing different physicochemical, permeability, and cell partitioning attributes to discern the roles of protein binding and cell partitioning on permeation and to complement ongoing pharmacological and pharmacokinetic studies. The data proved useful in predicting which compounds were most likely to leave the blood and penetrate underlying tissue. In a companion paper, brain uptake dynamics and cellular penetration of these compounds are confirmed in viv
The Influence of Deuteration and Turbulent Diffusion on the Observed D/H Ratio
The influence of turbulent mixing on the chemistry of the interstellar medium
has so far received little attention. Previous studies of this effect have
suggested that it might play an important role in mixing the various phases of
the interstellar medium. In this paper we examine the potential effects of
turbulent diffusion on the deuterium chemistry within molecular clouds. We find
that such mixing acts to reduce the efficiency of deuteration in these clouds
by increasing the ionization fraction and reducing freeze-out of heavy
molecules. This leads to lower abundances for many deuterated species. We also
examine the influence of turbulent mixing on the transition from atomic
hydrogen to H2 and from atomic deuterium to HD near the cloud edge. We find
that including turbulent diffusion in our models serves to push these
transitions deeper into the cloud and helps maintain a higher atomic fraction
throughout the cloud envelope. Based on these findings, we propose a new
process to account for the significant scatter in the observed atomic D/H ratio
for galactic sightlines extending beyond the Local Bubble. Although several
mechanisms have been put forward to explain this scatter, they are unable to
fully account for the range in D/H values. We suggest a scenario in which
turbulent mixing of atomic and molecular gas at the edges of molecular clouds
causes the observed atomic D/H ratio to vary by a factor of ~2.Comment: 14 pages, 14 figures, accepted for publication in Ap
Die prognostische Bedeutung der elektrophysiologischen Untersuchung bei Patienten mit Synkope unklarer Genese und normaler und reduzierter linksventrikulärer Funktion
Die prognostische Bedeutung der elektrophysiologischen Untersuchung bei Patienten mit Synkope unklarer Genese und normaler und reduzierter linksventrikulärer Funktion
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