2,850 research outputs found
The Production of HI in Photodissociation Regions and A Comparison with CO(1-0) Emission
The gas at the surfaces of molecular clouds in galaxies is heated and
dissociated by photons from young stars both near and far. HI resulting from
the dissociation of molecular hydrogen H2 emits hyperfine line emission at 21
cm, and warmed CO emits dipole rotational lines such as the 2.6 mm line of
CO(1-0). We use previously developed models for photodissociation regions
(PDRs) to compute the intensities of these HI and CO(1-0) lines as a function
of the total volume density n in the cloud and the far ultraviolet flux G0
incident upon it and present the results in units familiar to observers. The
intensities of these two lines behave differently with changing physical
conditions in the PDR, and, taken together, the two lines can provide a
ground-based radio astronomy diagnostic for determining n and G0 separately in
distant molecular clouds. This diagnostic is particularly useful in the range
Gzero <~ 100, 10 cm^{-3} <~ n <~ 10^5 cm^{-3}, which applies to a large
fraction of the volume of the interstellar medium in galaxies. If the molecular
cloud is located near discrete sources of far-UV (FUV) emission, the
PDR-generated HI and CO(1-0) emission on the cloud surface can be more easily
identified, appearing as layered ``blankets'' or ``blisters'' on the side of
the cloud nearest to the FUV source. As an illustration, we consider the
Galactic object G216 -2.5, i.e. ``Maddalena's Cloud'', which has been
previously identified as a large PDR in the Galaxy. We determine that this
cloud has n ~ 200 cm^{-3}, G0 ~ 0.8, consistent with other data.Comment: 13 Pages, 3 Figures. Accepted for publication in the Astrophysical
Journa
Photodissociation and the Morphology of HI in Galaxies
Young massive stars produce Far-UV photons which dissociate the molecular gas
on the surfaces of their parent molecular clouds. Of the many dissociation
products which result from this ``back-reaction'', atomic hydrogen \HI is one
of the easiest to observe through its radio 21-cm hyperfine line emission. In
this paper I first review the physics of this process and describe a simplified
model which has been developed to permit an approximate computation of the
column density of photodissociated \HI which appears on the surfaces of
molecular clouds. I then review several features of the \HI morphology of
galaxies on a variety of length scales and describe how photodissociation might
account for some of these observations. Finally, I discuss several consequences
which follow if this view of the origin of HI in galaxies continues to be
successful.Comment: 18 pages, 7 figures in 8 files, invited review paper for the
conference "Penetrating Bars Through Masks of Cosmic Dust: The Hubble Tuning
Fork Strikes a New Note", South Africa, June 2004. Proceedings to be
published by Kluwer, eds. D.L. Block, K.C. Freeman, I. Puerari, R. Groess, &
E.K. Bloc
Estimation of body fluid changes during peritoneal dialysis by segmental bioimpedance analysis
Estimation of body fluid changes during peritoneal dialysis by segmental bioimpedance analysis.BackgroundCommonly used bioimpedance analysis (BIA) is insensitive to changes in peritoneal fluid volume. The purpose of this study was to show, to our knowledge for the first time, that a new segmental approach accurately measures extracellular fluid changes during peritoneal dialysis (PD).MethodsFourteen stable PD patients were studied during a standard exchange with fluids of known conductivity. Bioimpedance was continuously measured in the arm, trunk, and leg and from wrist to ankle. Volume changes were calculated using both a newly developed sum of segmental BIA (SBIA) and current wrist-to-ankle BIA (WBIA) and were compared with actual volume changes measured gravimetrically.ResultsWhen 2.19 ± 0.48 L were removed from the peritoneal cavity during draining, 95.2 ± 13.8% of this volume was detected by SBIA compared with only 12.5 ± 24.3% detected by WBIA. When 2.11 ± 0.20 L of fresh dialysate was infused into the peritoneal cavity during filling, 91.1 ± 19.6% of this volume was detected by SBIA compared with only 8.8 ± 21.1% detected by WBIA.ConclusionThe good agreement between measured and calculated data using SBIA was due to: (a) improved placement of electrodes, (b) estimation of trunk extracellular volume based on a new algorithm, and (c) consideration of changes in dialysate conductivity. Correct estimation of fluid volume in the trunk is a prerequisite for applications in which direct analysis of fluid changes cannot be performed such as with peritoneal equilibration tests and continuous flow PD
Molecular clouds in the center of M81
We investigate the molecular gas content and the excitation and fragmentation
properties in the central region of the spiral galaxy Messier 81 in both the
^{12}CO(1-0) and ^{12}CO(2-1) transitions. We have recently observed the two
transitions of CO in the M~81 center with A, B, and HERA receivers of the IRAM
30-m telescope. We find no CO emission in the inner 300 pc and a weak
molecular gas clump structure at a distance of around 460 pc from the nucleus.
Observations of the first two CO transitions allowed us to compute the line
ratio, and the average I_{21}/I_{10} ratio is 0.68 for the M~81 center. This
low value, atypical both of the galactic nuclei of spiral galaxies and of
interacting systems, is probably associated to diffuse gas with molecular
hydrogen density that is not high enough to excite the CO molecules. After
analyzing the clumping properties of the molecular gas in detail, we identify
very massive giant molecular associations (GMAs) in CO(2-1) emission with
masses of 10 M and diameters of 250 pc. The deduced
N(H_{2})/I_{CO} ratio for the individually resolved GMAs, assumed to be
virialized, is a factor of 15 higher than the \textit{standard} Galactic
value, showing - as suspected - that the X ratio departs significantly from the
mean for galaxies with an unusual physics of the molecular gas.Comment: 12 pages, 7 figures, 6 tables. Accepted for pubblication in A&
Age Differences in Central (Semantic) and Peripheral Processing: The importance of Considering Both Response Times and Errors
In this project we examined the effect of adult age on visual word recognition by using combined reaction time (RT) and accuracy methods based on the Hick–Hyman law. This was necessary because separate Brinley analyses of RT and errors resulted in contradicting results. We report the results of a lexical decision task experiment (with 96 younger adults and 97 older adults). We transformed the error data into entropy and then predicted RT by using entropy values separately for exposure duration (thought to influence peripheral processes) and word frequency (thought to influence central processes). For exposure duration, the entropy–RT functions indicate that older adults show higher intercepts and slopes than do younger adults, suggesting an encoding decrement for older adults. However, for word frequency, older adults show higher intercepts but not steeper slopes than younger adults. Older adults thus show a peripheral processing decrement but not a central processing decrement for lexical decision
Age Differences in Central (Semantic) and Peripheral Processing: The importance of Considering Both Response Times and Errors
In this project we examined the effect of adult age on visual word recognition by using combined reaction time (RT) and accuracy methods based on the Hick–Hyman law. This was necessary because separate Brinley analyses of RT and errors resulted in contradicting results. We report the results of a lexical decision task experiment (with 96 younger adults and 97 older adults). We transformed the error data into entropy and then predicted RT by using entropy values separately for exposure duration (thought to influence peripheral processes) and word frequency (thought to influence central processes). For exposure duration, the entropy–RT functions indicate that older adults show higher intercepts and slopes than do younger adults, suggesting an encoding decrement for older adults. However, for word frequency, older adults show higher intercepts but not steeper slopes than younger adults. Older adults thus show a peripheral processing decrement but not a central processing decrement for lexical decision
Corals of the genus Porites are a locally abundant component of the epibiont community on mangrove prop roots at Calabash Caye, Turneffe Atoll, Belize
Mangroves are generally regarded as inhospitable for corals, but recent reports suggest they provide ecological refuge for some species. We surveyed diverse mangrove habitats on Turneffe Atoll, Belize, documenting 127 colonies of Porites divaricata (Thin Finger Coral) along 1858 m of mangrove prop roots at Calabash Caye and a much more diverse coral assemblage at Crooked Creek. At Calabash, corals were highly clumped, and varied widely in size and morphology, including large well-arborized colonies, encrusting forms with few branches, and new recruits with no branches, suggesting an age-structuredpopulation exhibiting extensive morphological plasticity. The data described here contributeto an emerging picture of mangroves as potentially critical habitat for many Caribbeancoral species.Accepted manuscrip
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Amphiprion barberi, a new species of anemonefish (Pomacentridae) from Fiji, Tonga, and Samoa
Amphiprion barberi, a new species of anemonefish fish, is described from 46 specimens, 16.3-85.8 mm SL, collected at depths of 2-10 m from coral reefs of Fiji, Tonga, and Samoa. It is closely allied to A. melanopus, which is widely distributed in the western Pacific. The two species exhibit significant colour-pattern differences, including a mainly reddish orange body in A. barberi and dark brown or blackish body in A. melanopus. Adults of the new species also possess fewer spinules (11-19 versus 19-26) in the upper-opercular series than A. melanopus. Genetic data presented here confirms the separation of these species
Spiral shocks and the formation of molecular clouds in a two phase medium
We extend recent numerical results (Dobbs et. al. 2006) on molecular cloud
formation in spiral galaxies by including a multi-phase medium. The addition of
a hot phase of gas enhances the structure in the cold gas, and significantly
increases the fraction of molecular hydrogen that is formed when the cold gas
passes through a spiral shock. The difference in structure is reflected in the
mass power spectrum of the molecular clouds, which is steeper for the
multi-phase calculations. The increase in molecular gas occurs as the addition
of a hot phase leads to higher densities in the cold gas. In particular, cold
gas is confined in clumps between the spiral arms and retains a higher
molecular fraction. Unlike the single phase results, molecular clouds are
present in the inter-arm regions for the multi-phase medium. However the
density of the inter-arm molecular hydrogen is generally below that which can
be reliably determined from CO measurements. We therefore predict that for a
multi-phase medium, there will be low density clouds containing cold atomic and
molecular hydrogen, which are potentially entering the spiral arms.Comment: 11 pages, 14 figures, accepted for publication in MNRA
An adaptive hierarchical domain decomposition method for parallel contact dynamics simulations of granular materials
A fully parallel version of the contact dynamics (CD) method is presented in
this paper. For large enough systems, 100% efficiency has been demonstrated for
up to 256 processors using a hierarchical domain decomposition with dynamic
load balancing. The iterative scheme to calculate the contact forces is left
domain-wise sequential, with data exchange after each iteration step, which
ensures its stability. The number of additional iterations required for
convergence by the partially parallel updates at the domain boundaries becomes
negligible with increasing number of particles, which allows for an effective
parallelization. Compared to the sequential implementation, we found no
influence of the parallelization on simulation results.Comment: 19 pages, 15 figures, published in Journal of Computational Physics
(2011
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