476 research outputs found
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
Boron Abundances in Diffuse Interstellar Clouds
We present a comprehensive survey of B abundances in diffuse interstellar
clouds from HST/STIS observations along 56 Galactic sight lines. Our sample is
the result of a complete search of archival STIS data for the B II resonance
line at 1362 angstroms, with each detection confirmed by the presence of
absorption from other dominant ions at the same velocity. The data probe a
range of astrophysical environments including both high-density regions of
massive star formation as well as low-density paths through the Galactic halo,
allowing us to clearly define the trend of B depletion onto interstellar grains
as a function of gas density. Many extended sight lines exhibit complex
absorption profiles that trace both local gas and gas associated with either
the Sagittarius-Carina or Perseus spiral arm. Our analysis indicates a higher
B/O ratio in the inner Sagittarius-Carina spiral arm than in the vicinity of
the Sun, which may suggest that B production in the current epoch is dominated
by a secondary process. The average gas-phase B abundance in the warm diffuse
ISM is consistent with the abundances determined for a variety of Galactic disk
stars, but is depleted by 60 percent relative to the solar system value. Our
survey also reveals sight lines with enhanced B abundances that potentially
trace recent production of B-11 either by cosmic-ray or neutrino-induced
spallation. Such sight lines will be key to discerning the relative importance
of the two production routes for B-11 synthesis.Comment: To be published in the proceedings of the IAU Symposium 268, Light
Elements in the Universe, C. Charbonnel, M. Tosi, F. Primas & C. Chiappini,
ed
Monte Carlo Calculation of the Spin-Stiffness of the Two-Dimensional Heisenberg Model
Using a collective-mode Monte Carlo method (the Wolff-Swendsen-Wang
algorithm), we compute the spin-stiffness of the two-dimensional classical
Heisenberg model. We show that it is the relevant physical quantity to
investigate the behaviour of the model in the very low temperature range
inaccessible to previous studies based on correlation length and susceptibility
calculations.Comment: 6 pages, latex, 3 postscript figures appended, DIM preprint 93-3
XY checkerboard antiferromagnet in external field
Ordering by thermal fluctuations is studied for the classical XY
antiferromagnet on a checkerboard lattice in zero and finite magnetic fields by
means of analytical and Monte Carlo methods. The model exhibits a variety of
novel broken symmetries including states with nematic ordering in zero field
and with triatic order parameter at high fields.Comment: 6 page
The Nature of Interstellar Gas toward the Pleiades Revealed in Absorption Lines
We present high-resolution, high signal to noise absorption-line observations
of CN, Ca II, Ca I, CH^+, and CH along twenty lines of sight toward members of
the Pleiades. The acquired data enable the most detailed study to date of the
interaction between cluster stars and the surrounding interstellar gas. Total
equivalent widths are consistent with previous investigations except where
weaker features are detected owing to our greater sensitivity. Mean b-values
for the molecular species indicate that toward most of the Pleiades CH is
associated with the production of CH^+ rather than CN. An analysis of radial
velocities reveals a kinematic distinction between ionized atomic gas and
molecular and neutral gas. Molecular components are found with velocities in
the local standard of rest of either ~ +7 km s^-1 or ~ +9.5 km s^-1, with the
higher-velocity components associated with the strongest absorption. Atomic gas
traced by Ca II shows a strong central component at v_LSR ~ +7 km s^-1
exhibiting velocity gradients indicative of cloud-cluster interactions. Gas
density estimates derived from measured CH/CH^+ column density ratios show good
agreement with those inferred from H_2 rotational populations, yielding typical
values of n ~ 50 cm^-3. Our models do not include the important time-dependent
effects on CH^+ formation which may ultimately be needed to extract physical
conditions in these clouds.Comment: 18 pages, 10 figures, emulateapj style, Accepted for publication in
Ap
Interstellar CN and CH+ in Diffuse Molecular Clouds: 12C/13C Ratios and CN Excitation
We present very high signal-to-noise ratio absorption-line observations of CN
and CH+ along 13 lines of sight through diffuse molecular clouds. The data are
examined to extract precise isotopologic ratios of 12CN/13CN and 12CH+/13CH+ in
order to assess predictions of diffuse cloud chemistry. Our results on
12CH+/13CH+ confirm that this ratio does not deviate from the ambient 12C/13C
ratio in local interstellar clouds, as expected if the formation of CH+
involves nonthermal processes. We find that 12CN/13CN, however, can be
significantly fractionated away from the ambient value. The dispersion in our
sample of 12CN/13CN ratios is similar to that found in recent surveys of
12CO/13CO. For sight lines where both ratios have been determined, the
12CN/13CN ratios are generally fractionated in the opposite sense compared to
12CO/13CO. Chemical fractionation in CO results from competition between
selective photodissociation and isotopic charge exchange. An inverse
relationship between 12CN/13CN and 12CO/13CO follows from the coexistence of CN
and CO in diffuse cloud cores. However, an isotopic charge exchange reaction
with CN may mitigate the enhancements in 12CN/13CN for lines of sight with low
12CO/13CO ratios. For two sight lines with high values of 12CO/13CO, our
results indicate that about 50 percent of the carbon is locked up in CO, which
is consistent with the notion that these sight lines probe molecular cloud
envelopes where the transition from C+ to CO is expected to occur. An analysis
of CN rotational excitation yields a weighted mean value for T_01(12CN) of
2.754 +/- 0.002 K, which implies an excess over the temperature of the cosmic
microwave background of only 29 +/- 3 mK. This modest excess eliminates the
need for a local excitation mechanism beyond electron and neutral collisions.
The rotational excitation temperatures in 13CN show no excess over the
temperature of the CMB.Comment: 27 pages, 21 figures, emulateapj style, accepted for publication in
Ap
Power-law spin correlations in pyrochlore antiferromagnets
The ground state ensemble of the highly frustrated pyrochlore-lattice
antiferromagnet can be mapped to a coarse-grained ``polarization'' field
satisfying a zero-divergence condition From this it follows that the
correlations of this field, as well as the actual spin correlations, decay with
separation like a dipole-dipole interaction (). Furthermore, a lattice
version of the derivation gives an approximate formula for spin correlations,
with several features that agree well with simulations and neutron-diffraction
measurements of diffuse scattering, in particular the pinch-point
(pseudo-dipolar) singularities at reciprocal lattice vectors. This system is
compared to others in which constraints also imply diffraction singularities,
and other possible applications of the coarse-grained polarization are
discussed.Comment: 13 pp, revtex, two figure
Superconducting Phase with Fractional Vortices in the Frustrated Kagome Wire Network at f=1/2
In classical XY kagome antiferromagnets, there can be a novel low temperature
phase where has quasi-long-range order but is
disordered, as well as more conventional antiferromagnetic phases where
is ordered in various possible patterns ( is the angle of orientation
of the spin). To investigate when these phases exist in a physical system, we
study superconducting kagome wire networks in a transverse magnetic field when
the magnetic flux through an elementary triangle is a half of a flux quantum.
Within Ginzburg-Landau theory, we calculate the helicity moduli of each phase
to estimate the Kosterlitz-Thouless (KT) transition temperatures. Then at the
KT temperatures, we estimate the barriers to move vortices and effects that
lift the large degeneracy in the possible patterns. The effects we have
considered are inductive couplings, non-zero wire width, and the
order-by-disorder effect due to thermal fluctuations. The first two effects
prefer patterns while the last one selects a
pattern of supercurrents. Using the parameters of recent experiments, we
conclude that at the KT temperature, the non-zero wire width effect dominates,
which stabilizes a conventional superconducting phase with a current
pattern. However, by adjusting the experimental parameters, for example by
bending the wires a little, it appears that the novel superconducting
phase can instead be stabilized. The barriers to vortex motion are low enough
that the system can equilibrate into this phase.Comment: 30 pages including figure
Guiding Post-Hospital Recovery By \u27What Matters:\u27 Implementation of Patient Priorities Identification in a Va Community Living Center
BACKGROUND: Patient priorities care (PPC) is an effective age-friendly health systems (AFHS) approach to aligning care with goals derived from \u27what matters\u27. The purpose of this quality improvement program was to evaluate the fidelity and feasibility of the health priorities identification (HPI) process in VA Community Living Centers (CLC).
METHODS: PPC experts worked with local CLC staff to guide the integration of HPI into the CLC and utilized a Plan-Do-Study-Act (PDSA) model for this quality improvement project. PPC experts reviewed health priorities identification (HPI) encounters and interdisciplinary team (IDT) meetings for fidelity to the HPI process of PPC. Qualitative interviews with local CLC staff determined the appropriateness of the health priorities identification process in the CLC.
RESULTS: Over 8 months, nine facilitators completed twenty HPI encounters. Development of a Patient Health Priorities note template, staff education and PPC facilitator training improved fidelity and documentation of HPI encounters in the electronic health record. Facilitator interviews suggested that PPC is appropriate in this setting, not burdensome to staff and fostered a person-centered approach to AFHS.
CONCLUSIONS: The HPI process is an acceptable and feasible approach to ask the \u27what matters\u27 component of AFHS in a CLC setting
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