812 research outputs found
Ultraluminous X-ray Sources Powered by Radiatively Efficient Two-Phased Super-Eddington Accretion onto Stellar Mass Black holes
The radiation spectra of many of the brightest ultraluminous X-ray sources
(ULXs) are dominated by a hard power law component, likely powered by a hot,
optically thin corona that Comptonizes soft seed photons emitted from a cool,
optically thick black hole accretion disk. Before its dissipation and
subsequent conversion into coronal photon power, the randomized gravitational
binding energy responsible for powering ULX phenomena must separate from the
mass of its origin by a means other than, and quicker than, electron
scattering-mediated radiative diffusion. Therefore, the release of accretion
power in ULXs is not necessarily subject to Eddington-limited photon trapping,
as long as it occurs in a corona. Motivated by these basic considerations, we
present a model of ULXs powered by geometrically thin accretion onto stellar
mass black holes. We argue that the radiative efficiency of the flow remains
high if the corona is magnetized or optically thin and the majority of the
accretion power escapes in the form of radiation rather than an outflow. Within
the context of the current black hole X-ray binary paradigm, our ULX model may
be viewed as an extension of the very high state observed in Galactic sources.
(abridged)Comment: 11 page
Raman and nuclear magnetic resonance investigation of alkali metal vapor interaction with alkene-based anti-relaxation coating
The use of anti-relaxation coatings in alkali vapor cells yields substantial
performance improvements by reducing the probability of spin relaxation in wall
collisions by several orders of magnitude. Some of the most effective
anti-relaxation coating materials are alpha-olefins, which (as in the case of
more traditional paraffin coatings) must undergo a curing period after cell
manufacturing in order to achieve the desired behavior. Until now, however, it
has been unclear what physicochemical processes occur during cell curing, and
how they may affect relevant cell properties. We present the results of
nondestructive Raman-spectroscopy and magnetic-resonance investigations of the
influence of alkali metal vapor (Cs or K) on an alpha-olefin, 1-nonadecene
coating the inner surface of a glass cell. It was found that during the curing
process, the alkali metal catalyzes migration of the carbon-carbon double bond,
yielding a mixture of cis- and trans-2-nonadecene.Comment: 5 pages, 6 figure
An in-situ synchrotron XAS methodology for surface analysis under high temperature, pressure and shear
The complex tribochemical nature of lubricated tribological contacts is inaccessible in real time without altering their initial state. To overcome this issue, a new design of a pin-on-disc tribological apparatus was developed and combined with synchrotron X-ray absorption spectroscopy (XAS). Using the designed apparatus, it is possible to study in situ the transient decomposition reactions of various oil additives on different surfaces under a wide range of realistic operating conditions of contact pressure (1.0–3.0 GPa), temperature (25–120 °C), and sliding speed (30–3000 rpm or 0.15–15 m/s). To test the apparatus, several tribological tests were performed at different shearing times ranging from 2.5 to 60 min. These tests were carried out under helium atmosphere at a temperature of 80  °C, contact pressure of 2.2 GPa, and sliding speed of 50 rpm. The XAS experiments indicate that the zinc dialkyldithiophosphate antiwear additive decomposes in the oil to form a tribofilm on the iron surface at different reaction kinetics from the ones of the thermal film. The tribofilm composition evolves much faster than the one of the thermal film, which confirms that the formation of the tribofilm is a thermally activated process similar to the one of the thermal film but accelerated by shear. Furthermore, the results indicate that the sulfur of the formed film, whether a tribofilm or a thermal film, appears initially in the form of sulfate, with some sulfide, which under heat or shear is reduced into mainly sulfide
Inelastic electron tunneling via molecular vibrations in single-molecule transistors
In single-molecule transistors, we observe inelastic cotunneling features
that correspond energetically to vibrational excitations of the molecule, as
determined by Raman and infrared spectroscopy. This is a form of inelastic
electron tunneling spectroscopy of single molecules, with the transistor
geometry allowing in-situ tuning of the electronic states via a gate electrode.
The vibrational features shift and change shape as the electronic levels are
tuned near resonance, indicating significant modification of the vibrational
states. When the molecule contains an unpaired electron, we also observe
vibrational satellite features around the Kondo resonance.Comment: 5 pages, 4 figures. Supplementary information available upon reques
Direct Numerical Simulation of Radiation Pressure-Driven Turbulence and Winds in Star Clusters and Galactic Disks
[abridged] The pressure exerted by the radiation of young stars may be an
important feedback mechanism in forming star clusters and the disks of
starburst galaxies. However, there is great uncertainty in how efficiently
radiation couples to matter in these high optical depth environments. In
particular, it is unclear what levels of turbulence the radiation can produce,
and whether the infrared radiation trapped by the dust opacity can give rise to
heavily mass-loaded winds. In this paper we report a series of two-dimensional
flux-limited diffusion radiation-hydrodynamics calculations performed with the
code ORION in which we drive strong radiation fluxes through columns of dusty
matter confined by gravity. We consider both systems where the radiation flux
is sub-Eddington throughout the gas column, and where it is super-Eddington at
the midplane but sub-Eddington in the atmosphere. In the latter, we find that
the radiation-matter interaction gives rise to radiation-driven Rayleigh-Taylor
instability, which drives supersonic turbulence at a level sufficient to fully
explain the turbulence seen in Galactic protocluster gas clouds, and to make a
non-trivial contribution to the turbulence observed in starburst galaxy disks.
However, the instability also produces a channel structure in which the
radiation-matter interaction is reduced because the radiation field is not
fully trapped. For astrophysical parameters relevant to forming star clusters
and starburst galaxies, we find that this effect reduces the net momentum
deposition rate in the dusty gas by a factor of ~2-6 compared to simple
analytic estimates, and that in steady state the Eddington ratio reaches unity
and there are no strong winds. We provide an approximation formula, appropriate
for implementation in analytic models and non-radiative simulations, for the
force exerted by the infrared radiation field in this regime.Comment: 20 pages, 11 figures, emulateapj format, accepted to ApJ. This
version has extra discussion, but the results are unchanged. For movies of
simulation results, see http://www.ucolick.org/~krumholz/downloads.htm
Developing and evaluating JIApp: Acceptability and usability of a smartphone app system to improve self-management in young people with juvenile idiopathic arthritis
Background:
Flare-ups in juvenile idiopathic arthritis (JIA) are characterized by joint pain and swelling and often accompanied with fatigue, negative emotions, and reduced participation in activities. To minimize the impact of JIA on the physical and psychosocial development and well-being of young people (YP), it is essential to regularly monitor disease activity and side effects, as well as to support self-management such as adherence to treatment plans and engagement in general health-promoting behaviors. Smartphone technology has the potential to engage YP with their health care through convenient self-monitoring and easy access to information. In addition, having a more accurate summary of self-reported fluctuations in symptoms, behaviors, and psychosocial problems can help both YP and health care professionals (HCPs) better understand the patient’s condition, identify barriers to self-management, and assess treatment effectiveness and additional health care needs. No comprehensive smartphone app has yet been developed in collaboration with YP with JIA, their parents, and HCPs involved in their care.
Objectives:
The objective of this study was to design, develop, and evaluate the acceptability and usability of JIApp, a self-management smartphone app system for YP with JIA and HCPs.
Methods:
We used a qualitative, user-centered design approach involving YP, parents, and HCPs from the rheumatology team. The study was conducted in three phases: (1) phase I focused on developing consensus on the features, content, and design of the app; (2) phase II was used for further refining and evaluating the app prototype; and (3) phase III focused on usability testing of the app. The interview transcripts were analyzed using qualitative content analysis.
Results:
A total of 29 YP (aged 10-23, median age 17) with JIA, 7 parents, and 21 HCPs were interviewed. Major themes identified as the ones that helped inform app development in phase I were: (1) remote monitoring of symptoms, well-being, and activities; (2) treatment adherence; and (3) education and support. During phase II, three more themes emerged that informed further refinement of the app prototype. These included (4) adapting a reward system to motivate end users for using the app; (5) design of the app interface; and (6) clinical practice integration. The usability testing during phase III demonstrated high rates of overall satisfaction and further affirmed the content validity of the app.
Conclusions:
We present the development and evaluation of a smartphone app to encourage self-management and engagement with health care for YP with JIA. The app was found to have high levels of acceptability and usability among YP and HCPs and has the potential to improve health care and outcomes for this age group. Future feasibility testing in a prospective study will firmly establish the reliability, efficacy, and cost-effectiveness of such an app intervention for patients with arthritis
The Neutrino Bubble Instability: A Mechanism for Generating Pulsar Kicks
An explanation for the large random velocities of pulsars is presented. Like
many other models, we propose that the momentum imparted to the star is given
at birth. The ultimate source of energy is provided by the intense optically
thick neutrino flux that is responsible for radiating the proto-neutron star's
gravitational binding energy during the Kelvin-Helmholtz phase. The central
feature of the kick mechanism is a radiative-driven magnetoacoustic
instability, which we refer to as ``neutrino bubbles.'' Identical in nature to
the photon bubble instability, the neutrino bubble instability requires the
presence of an equilibrium radiative flux as well as a coherent steady
background magnetic field. Over regions of large magnetic flux densities, the
neutrino bubble instability is allowed to grow on dynamical timescales ~ 1ms,
potentially leading to large luminosity enhancements and density fluctuations.
Local luminosity enhancements, which preferentially occur over regions of
strong magnetic field, lead to a net global asymmetry in the neutrino emission
and the young neutron star is propelled in the direction opposite to these
regions. For favorable values of magnetic field structure, size, and strength
as well as neutrino bubble saturation amplitude, momentum kicks in excess of
1000 km/s can be achieved. Since the neutrino-powered kick is delivered over
the duration of the Kelvin-Helmholtz time ~ a few seconds, one expects
spin-kick alignment from this neutrino bubble powered model.Comment: submitted to Ap
Prednisone treatment of elderly-onset rheumatoid arthritis: Disease activity and bone mass in comparison with chloroquine treatment
Objective. Prednisone is frequently used in the treatment of elderly-onset rheumatoid arthritis (RA), but the balance between efficacy and toxicity, including the effect on bone mass, has not been investigated in long-term studies. This prospective, randomized study was undertaken to compare disease activity and bone mass during long-term treatment with prednisone versus chloroquine in this patient population.
Methods. Patients with active RA diagnosed at age ≥ 60 were randomized to receive prednisone (15 mg/day for 1 month, with the dosage tapered as low as possible thereafter) (n = 28) or chloroquine (n = 28). Patients who did not show a response received other second-line drugs as an adjunct to prednisone or as a replacement for chloroquine. Bone mass was measured by dual-energy x-ray absorptiometry. The study duration was 2 years.
Results. During the 2 years, treatment with other second-line drugs was needed for 12 patients in the prednisone group (43%) and 8 in the chloroquine group (29%). Functional capacity and disease activity improved significantly in both groups and did not differ significantly between the groups, except for a greater improvement in the prednisone group at 1 month. Radiographic scores for joint destruction progressed similarly in both groups. There was a nonsignificant excess bone loss of 1.8% in the spine and 1.5% in the hip in the prednisone group, compared with the chloroquine group.
Conclusion. Neither treatment was entirely satisfactory since a significant number of patients needed an additional second-line drug over the 2-year period
The Persistence of Cool Galactic Winds in High Stellar Mass Galaxies Between z~1.4 and ~1
We present an analysis of the MgII 2796, 2803 and FeII 2586, 2600 absorption
line profiles in coadded spectra of 468 galaxies at 0.7 < z < 1.5. The galaxy
sample, drawn from the Team Keck Treasury Redshift Survey of the GOODS-N field,
has a range in stellar mass (M_*) comparable to that of the sample at z~1.4
analyzed in a similar manner by Weiner et al. (2009; W09), but extends to lower
redshifts and has specific star formation rates which are lower by ~0.6 dex. We
identify outflows of cool gas from the Doppler shift of the MgII absorption
lines and find that the equivalent width (EW) of absorption due to outflowing
gas increases on average with M_* and star formation rate (SFR). We attribute
the large EWs measured in spectra of the more massive, higher-SFR galaxies to
optically thick absorbing clouds having large velocity widths. The outflows
have hydrogen column densities N(H) > 10^19.3 cm^-2, and extend to velocities
of ~500 km/s. While galaxies with SFR > 10 Msun/yr host strong outflows in both
this and the W09 sample, we do not detect outflows in lower-SFR (i.e., log
M_*/Msun < 10.5) galaxies at lower redshifts. Using a simple galaxy evolution
model which assumes exponentially declining SFRs, we infer that strong outflows
persist in galaxies with log M_*/Msun > 10.5 as they age between z=1.4 and z~1,
presumably because of their high absolute SFRs. Finally, using high resolution
HST/ACS imaging in tandem with our spectral analysis, we find evidence for a
weak trend (at 1 sigma significance) of increasing outflow absorption strength
with increasing galaxy SFR surface density.Comment: Submitted to ApJ. 25 pages, 19 figures, Figure 2 reduced in
resolution. Uses emulateapj forma
Local Radiative Hydrodynamic and Magnetohydrodynamic Instabilities in Optically Thick Media
We examine the local conditions for radiative damping and driving of short
wavelength, propagating hydrodynamic and magnetohydrodynamic (MHD) waves in
static, optically thick, stratified equilibria. We show that so-called strange
modes in stellar oscillation theory and magnetic photon bubbles are intimately
related and are both fundamentally driven by the background radiation flux
acting on compressible waves. We identify the necessary criteria for unstable
driving of these waves, and show that this driving can exist in both gas and
radiation pressure dominated media, as well as pure Thomson scattering media in
the MHD case. The equilibrium flux acting on opacity fluctuations can drive
both hydrodynamic acoustic waves and magnetosonic waves unstable. In addition,
magnetosonic waves can be driven unstable by a combination of the equilibrium
flux acting on density fluctuations and changes in the background radiation
pressure along fluid displacements. We briefly describe the conditions under
which these instabilities might be manifested in both main sequence stellar
envelopes and accretion disks.Comment: 55 pages, revised version accepted for publication by ApJ. New
appendix added justifying WKB analysi
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