1,275 research outputs found
Density Variations over Subparsec Scales in Diffuse Molecular Gas
We present high-resolution observations of interstellar CN, CH, CH^{+},
\ion{Ca}{1}, and \ion{Ca}{2} absorption lines toward the multiple star systems
HD206267 and HD217035. Substantial variations in CN absorption are observed
among three sight lines of HD206267, which are separated by distances of order
10,000 AU; smaller differences are seen for CH, CH^{+}, and \ion{Ca}{1}. Gas
densities for individual velocity components are inferred from a chemical
model, independent of assumptions about cloud shape. While the component
densities can differ by factors of 5.0 between adjacent sightlines, the
densities are always less than 5000 cm^{-3}. Calculations show that the derived
density contrasts are not sensitive to the temperature or reaction rates used
in the chemical model. A large difference in the CH^{+} profiles (a factor of 2
in column density) is seen in the lower density gas toward HD217035.Comment: 9 pages, 2 figures. Accepted for publication in ApJ
OH+ in Diffuse Molecular Clouds
Near ultraviolet observations of OH+ and OH in diffuse molecular clouds
reveal a preference for different environments. The dominant absorption feature
in OH+ arises from a main component seen in CH+ (that with the highest CH+/CH
column density ratio), while OH follows CN absorption. This distinction
provides new constraints on OH chemistry in these clouds. Since CH+ detections
favor low-density gas with small fractions of molecular hydrogen, this must be
true for OH+ as well, confirming OH+ and H2O+ observations with the Herschel
Space Telescope. Our observed correspondence indicates that the cosmic ray
ionization rate derived from these measurements pertains to mainly atomic gas.
The association of OH absorption with gas rich in CN is attributed to the need
for high enough density and molecular fraction before detectable amounts are
seen. Thus, while OH+ leads to OH production, chemical arguments suggest that
their abundances are controlled by different sets of conditions and that they
coexist with different sets of observed species. Of particular note is that
non-thermal chemistry appears to play a limited role in the synthesis of OH in
diffuse molecular clouds.Comment: 15 pages, 4 figures, to appear in ApJ Letter
Sport for Social Change: Bridging the Theory–Practice Divide
Theory development around sport for social change agendas has received greater attention from scholars over the past 10 years. Yet, it remains underdeveloped when compared with theoretical advancements and innovations in other aspects of the sport industry. In this special issue, we bring to light some of the most recent conceptual and empirical work exploring the theory–practice connection in the field of sport for social change
Key signal contributions in photothermal deflection spectroscopy
We report on key signal contributions in photothermal deflection spectroscopy
(PDS) of semiconductors at photon energies below the bandgap energy and show
how to extract the actual absorption properties from the measurement data. To
this end, we establish a rigorous computation scheme for the deflection signal
including semi-analytic raytracing to analyze the underlying physical effects.
The computation takes into account linear and nonlinear absorption processes
affecting the refractive index and thus leading to a deflection of the probe
beam. We find that beside the linear mirage effect, nonlinear absorption
mechanisms make a substantial contribution to the signal for strongly focussed
pump beams and sample materials with high two-photon absorption coefficients.
For example, the measured quadratic absorption contribution exceeds 5% at a
pump beam intensity of about in Si and at
in GaAs. In addition, our method also
includes thermal expansion effects as well as spatial gradients of the
attenuation properties. We demonstrate that these effects result in an
additional deflection contribution which substantially depends on the distance
of the photodetector from the readout point. This distance dependent
contribution enhances the surface related PDS signal up to two orders of
magnitude and may be misinterpreted as surface absorption if not corrected in
the analysis of the measurement data. We verify these findings by PDS
measurements on crystalline silicon at a wavelength of 1550 nm and provide
guidelines how to extract the actual attenuation coefficient from the PDS
signal.Comment: 10 pages, 16 figures, submitted to Journal of Applied Physiv
Monitoring the Variable Interstellar Absorption toward HD 219188 with HST/STIS
We discuss the results of continued spectroscopic monitoring of the variable
intermediate-velocity (IV) absorption at v = -38 km/s toward HD 219188. After
reaching maxima in mid-2000, the column densities of both Na I and Ca II in
that IV component declined by factors >= 2 by the end of 2006. Comparisons
between HST/STIS echelle spectra obtained in 2001, 2003, and 2004 and HST/GHRS
echelle spectra obtained in 1994--1995 indicate the following: (1) The
absorption from the dominant species S II, O I, Si II, and Fe II is roughly
constant in all four sets of spectra -- suggesting that the total N(H) and the
(mild) depletions have not changed significantly over a period of nearly ten
years. (2) The column densities of the trace species C I (both ground and
excited fine-structure states) and of the excited state C II* all increased by
factors of 2--5 between 1995 and 2001 -- implying increases in the hydrogen
density n_H (from about 20 cm^{-3} to about 45 cm^{-3}) and in the electron
density n_e (by a factor >= 3) over that 6-year period. (3) The column
densities of C I and C II* -- and the corresponding inferred n_H and n_e --
then decreased slightly between 2001 and 2004. (4) The changes in C I and C II*
are very similar to those seen for Na I and Ca II. The relatively low total
N(H) and the modest n_H suggest that the -38 km/s cloud toward HD 219188 is not
a very dense knot or filament. Partial ionization of hydrogen appears to be
responsible for the enhanced abundances of Na I, C I, Ca II, and C II*. In this
case, the variations in those species appear to reflect differences in density
and ionization [and not N(H)] over scales of tens of AU.Comment: 33 pages, 6 figures, aastex, accepted to Ap
Lyman Alpha Forest towards B2 1225+317
We present observations of the Lyman alpha forest towards B2 1225+317 taken
at a resolution of 18 km/s. A clean sample of Lyman alpha forest lines is
extracted after a careful profile fitting analysis and removal of absorption
lines of heavy elements. The sample is analyzed for statistical properties.
Eighty percent of the column densities are < 10^{14} cm^{-2}. A single power
law is inconsistent with the column density distribution and a steepening/break
in the distribution is indicated. The average velocity dispersion parameter is
29.4 km/s. We find 3 sigma evidence for a correlation between column density
and the velocity dispersion parameter. The correlation, however, is mainly due
to narrow lines and weakens to 1.2 sigma if lines with velocity dispersion
parameter smaller than 20 km/s are excluded. An excess of line pairs with
velocity separation 100 km/s over the expected number is found.Comment: latex(mn.sty), 6 figures (available on request from
[email protected] or [email protected]), to appear in MNRA
Simulation of tethered oligomers in nanochannels using multi-particle collision dynamics
The effect of a high Reynold's number, pressure-driven flow of a compressible
gas on the conformation of an oligomer tethered to the wall of a square-channel
is studied under both ideal solvent and poor solvent conditions using a hybrid
multiparticle collision dynamics and molecular dynamics algorithm. Unlike
previous studies, the flow field contains an elongational component in addition
to a shear component as well as fluid slip near the walls and results in a
Schmidt number for the polymer beads that is less than unity. In both solvent
regimes the oligomer is found to extend in the direction of flow. Under the
ideal solvent conditions, torsional twisting of the chain and aperiodic
cyclical dynamics are observed for the end of the oligomer. Under poor solvent
conditions, a metastable helix forms in the end of the chain despite the lack
of any attractive potential between beads in the oligomeric chain. The
formation of the helix is postulated to be the result of a solvent induced
chain collapse that has been confined to a single dimension by a strong flow
field.Comment: 15 pages, 13 figures, to appear in J. Chem. Phys., 201
High-Resolution Observations of Interstellar Ca I Absorption -- Implications for Depletions and Electron Densities in Diffuse Clouds
We present high-resolution (FWHM ~ 0.3-1.5 km/s) spectra of interstellar Ca I
absorption toward 30 Galactic stars. Comparisons of the column densities of Ca
I, Ca II, K I, and other species -- for individual components identified in the
line profiles and also when integrated over entire lines of sight -- yield
information on relative electron densities and depletions. There is no obvious
relationship between the ratio N(Ca I)/N(Ca II) [equal to n_e/(Gamma/alpha_r)
for photoionization equilibrium] and the fraction of hydrogen in molecular form
f(H2) (often taken to be indicative of the local density n_H). For a smaller
sample of sightlines for which the thermal pressure (n_H T) and local density
can be estimated via analysis of the C I fine-structure excitation, the average
electron density inferred from C, Na, and K (assuming photoionization
equilibrium) seems to be independent of n_H and n_H T. While the n_e obtained
from the ratio N(Ca I)/N(Ca II) is often significantly higher than the values
derived from other elements, the patterns of relative n_e derived from
different elements show both similarities and differences for different lines
of sight -- suggesting that additional processes besides photoionization and
radiative recombination commonly and significantly affect the ionization
balance of heavy elements in diffuse IS clouds. Such additional processes may
also contribute to the (apparently) larger than expected fractional ionizations
(n_e/n_H) found for some lines of sight with independent determinations of n_H.
In general, inclusion of ``grain-assisted'' recombination does reduce the
inferred n_e, but it does not reconcile the n_e estimated from different
elements. The depletion of calcium may have a much weaker dependence on density
than was suggested by earlier comparisons with CH and CN.Comment: aastex, 70 pages, accepted to ApJ
Red Cells Genetically Deficient in Carbonic Anhydrase II Have Elevated Levels of a Carbonic Anhydrase Indistinguishable from Muscle CA III a
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/71425/1/j.1749-6632.1984.tb12351.x.pd
CO emission and variable CH and CH+ absorption towards HD34078: evidence for a nascent bow shock ?
The runaway star HD34078, initially selected to investigate small scale
structure in a foreground diffuse cloud has been shown to be surrounded by
highly excited H2. We first search for an association between the foreground
cloud and HD34078. Second, we extend previous investigations of temporal
absorption line variations (CH, CH+, H2) in order to better characterize them.
We have mapped the CO(2-1) emission at 12 arcsec resolution around HD34078's
position, using the 30 m IRAM antenna. The follow-up of CH and CH+ absorption
lines has been extended over 5 more years. In parallel, CH absorption towards
the reddened star Zeta Per have been monitored to check the homogeneity of our
measurements. Three more FUSE spectra have been obtained to search for N(H2)
variations. CO observations show a pronounced maximum near HD34078's position,
clearly indicating that the star and diffuse cloud are associated. The optical
spectra confirm the reality of strong, rapid and correlated CH and CH+
fluctuations. On the other hand, N(H2, J=0) has varied by less than 5 % over 4
years. We also discard N(CH) variations towards Zeta Per at scales less than 20
AU. Observational constraints from this work and from 24 micron dust emission
appear to be consistent with H2 excitation but inconsistent with steady-state
bow shock models and rather suggest that the shell of compressed gas
surrounding HD34078, is seen at an early stage of the interaction. The CH and
CH+ time variations as well as their large abundances are likely due to
chemical structure in the shocked gas layer located at the stellar wind/ambient
cloud interface. Finally, the lack of variations for both N(H2, J=0) towards
HD34078 and N(CH) towards Zeta Per suggests that quiescent molecular gas is not
subject to pronounced small-scale structure.Comment: 19 pages, 15 figures, accepted for publication in A&
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