134 research outputs found
On the Structural Differences between Disk and Dwarf Galaxies
Gas-rich dwarf and disk galaxies overlap in numerous physical quantities that
make their classification subjective. We report the discovery of a separation
between dwarfs and disks into two unique sequences in the mass (luminosity)
versus scale length plane. This provides an objective classification scheme for
late-type galaxies that only requires optical or near-IR surface photometry of
a galaxy. Since the baryonic Tully-Fisher relation for these samples produces a
continuous relation between baryonic mass and rotational velocity, we conclude
that the difference between dwarfs and disks must be because of their
distribution of stellar light such that dwarfs are more diffuse than disk
galaxies. This structural separation may be due to a primordial difference
between low and high mass galaxies or produced by hierarchical mergers where
disks are built up from dwarfs. Structural differences between dwarf and disk
galaxies may also be driven by the underlying kinematics where the strong
rotation in disks produces an axial symmetric object that undergoes highly
efficient star formation in contrast to the lower rotation, more disordered
motion of dwarfs that produces a diffuse, triaxial object with a history of
inefficient star formation.Comment: 16 pages, 2 figures, AJ in press, AASTeX5.
Galactic Spiral Shocks with Thermal Instability
Using one-dimensional hydrodynamic simulations including interstellar
heating, cooling, and thermal conduction, we investigate nonlinear evolution of
gas flow across galactic spiral arms. We model the gas as a
non-self-gravitating, unmagnetized fluid, and follow its interaction with a
stellar spiral potential in a local frame comoving with the stellar pattern.
Initially uniform gas rapidly separates into warm and cold phases as a result
of thermal instability (TI), and also forms a quasi-steady shock that prompts
phase transitions. After saturation, the flow follows a recurring cycle: warm
and cold phases in the interarm region are shocked and immediately cool to
become a denser cold medium in the arm; post-shock expansion reduces the mean
density to the unstable regime in the transition zone and TI subsequently
mediates evolution back into warm and cold interarm phases. For our standard
model with n_0 = 2 cm^-3, the gas resides in the dense arm, thermally-unstable
transition zone, and interarm region for 14%, 22%, 64% of the arm-to-arm
crossing time. These regions occupy 1%, 16%, and 83% of the arm-to-arm
distance, respectively. Gas at intermediate temperatures represents ~25-30% of
the total mass, similar to the fractions estimated from HI observations.
Despite transient features and multiphase structure, the time-averaged shock
profiles can be matched to that of a diffusive isothermal medium with
temperature 1,000 K and "particle" mean free path of l_0 = 100 pc. Finally, we
quantify numerical conductivity associated with translational motion of
phase-separated gas on the grid, and show that convergence of numerical results
requires the numerical conductivity to be comparable to or smaller than the
physical conductivity. (Abridged)Comment: 41 pages, 13 figures, 1 table. Accepted for publication in
Astrophysical Journal. PDF version with higher resolution figures is
available at http://astro.snu.ac.kr/~kimcg/AST/GSS_TI.pd
Dynamical models for sand ripples beneath surface waves
We introduce order parameter models for describing the dynamics of sand
ripple patterns under oscillatory flow. A crucial ingredient of these models is
the mass transport between adjacent ripples, which we obtain from detailed
numerical simulations for a range of ripple sizes. Using this mass transport
function, our models predict the existence of a stable band of wavenumbers
limited by secondary instabilities. Small ripples coarsen in our models and
this process leads to a sharply selected final wavenumber, in agreement with
experimental observations.Comment: 9 pages. Shortened version of previous submissio
Star Formation from Galaxies to Globules
The empirical laws of star formation suggest that galactic-scale gravity is
involved, but they do not identify the actual triggering mechanisms for
clusters in the final stages. Many other triggering processes satisfy the
empirical laws too, including turbulence compression and expanding shell
collapse. The self-similar nature of the gas and associated young stars
suggests that turbulence is more directly involved, but the small scale
morphology of gas around most embedded clusters does not look like a random
turbulent flow. Most clusters look triggered by other nearby stars. Such a
prominent local influence makes it difficult to understand the universality of
the Kennicutt and Schmidt laws on galactic scales. A unified view of
multi-scale star formation avoids most of these problems. Ambient self-gravity
produces spiral arms and drives much of the turbulence that leads to
self-similar structures, while localized energy input from existing clusters
and field supernovae triggers new clusters in pre-existing clouds. The
hierarchical structure in the gas made by turbulence ensures that the
triggering time scales with size, giving the Schmidt law over a wide range of
scales and the size-duration correlation for young star fields. The efficiency
of star formation is determined by the fraction of the gas above a critical
density of around 10^5 m(H2)/cc. Star formation is saturated to its largest
possible value given the fractal nature of the interstellar medium.Comment: accepted for ApJ, 42 pages, Dannie Heineman prize lecture, January
200
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Observation of diffraction contrast in scanning helium microscopy
Abstract: Scanning helium microscopy is an emerging form of microscopy using thermal energy neutral helium atoms as the probe particle. The very low energy combined with lack of charge gives the technique great potential for studying delicate systems, and the possibility of several new forms of contrast. To date, neutral helium images have been dominated by topographic contrast, relating to the height and angle of the surface. Here we present data showing contrast resulting from specular reflection and diffraction of helium atoms from an atomic lattice of lithium fluoride. The signature for diffraction is evident by varying the scattering angle and observing sharp features in the scattered distribution. The data indicates the viability of the approach for imaging with diffraction contrast and suggests application to a wide variety of other locally crystalline materials
Development of a new diabetes medication self-efficacy scale and its association with both reported problems in using diabetes medications and self-reported adherence
Background: Although there are several different general diabetes self-efficacy scales, there is a need to develop a self-efficacy scale that providers can use to assess patientĂąâŹâąs self-efficacy regarding medication use. The purpose of this study was to: 1) develop a new diabetes medication self-efficacy scale and 2) examine how diabetes medication self-efficacy is associated with patient-reported problems in using diabetes medications and self-reported adherence. Patients and methods: Adult English-speaking patients with type 2 diabetes were recruited from a family medicine clinic and a pharmacy in Eastern North Carolina, USA. The patients were eligible if they reported being nonadherent to their diabetes medicines on a visual analog scale. Multivariable regression was used to examine the relationship between self-efficacy and the number of reported diabetes medication problems and adherence.
Results: The diabetes medication self-efficacy scale had strong reliability (CronbachĂąâŹâąs alpha =0.86). Among a sample (N=51) of mostly African-American female patients, diabetes medication problems were common (6.1ñ3.1) and a greater number of diabetes medications were associated with lower medication adherence (odds ratio: 0.35; 95% confidence interval: 0.13, 0.89). Higher medication self-efficacy was significantly related to medication adherence (odds ratio: 1.17; 95% confidence interval: 1.05, 1.30) and inversely related to the number of self-reported medication problems (ĂÂČ=-0.13; P=0.006).
Conclusion: Higher diabetes medication self-efficacy was associated with fewer patient- reported medication problems and better medication adherence. Assessing medication-specific self-efficacy may help to identify medication-related problems that providers can help the patients address, potentially improving adherence and patient outcomes.
Keywords: diabetes, adherence, self-efficacy, literac
Observations of velocities, sand concentrations, and fluxes under velocity-asymmetric oscillatory flows
Peer reviewedPublisher PD
Consumers' experiences and values in conventional and alternative medicine paradigms: a problem detection study (PDS)
Background: This study explored consumer perceptions of complementary and alternative medicine (CAM) and relationships with CAM and conventional medicine practitioners. A problem detection study (PDS) was used. The qualitative component to develop the questionnaire used a CAM consumer focus group to explore conventional and CAM paradigms in healthcare. 32 key issues, seven main themes, informed the questionnaire (the quantitative PDS component - 36 statements explored using five-point Likert scales.
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