243 research outputs found
Analytical and experimental studies of thermionically emitting electrodes in contact with dense, seeded plasmas
Interactions are considered between a moving,
alkali-metal seeded, dense plasma and a
metallic electrode whose surface properties are
influenced by the absorption of seed particles. The
plasma behavior is governed by a set of differential
equations, which are coupled to the surface
through the boundary conditions. These conditions
are obtained by utilizing the particle desorption
rate expressions of Levine and Gyftopoulos. The
solution of the problem yields the state of the surface
as well as the spatial distribution of plasma
properties. In particular, electrode voltage drops
are predicted, which indicate whether the electrode
operates in a thermionic or arc mode. The
method has been applied to a potassiwn-seeded
argon plasma in contact with a tungsten electrode
in a stagnation flow geometry. The results show
that the plasma - surface interaction may lead to
large electrode currents at moderate voltage drops.
These currents can be up to an order of magnitude
greater than what the random electron current
would be at the surface under conditions of
perfect thermodynamic equilibrium at the surface
temperature. R.esults of a comparable experiment
show reasonably good agreement with the theory
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If someone cares about you, you are more apt to come around: improving HIV care engagement by strengthening the patient-provider relationship.
Purpose:The patient-provider relationship is a central factor that can promote or hinder long-term engagement in care among people living with chronic illnesses. In this paper, we explore characteristics of the patient-provider relationship that facilitated or hindered engagement in care among patients receiving care at HIV specialty clinics. Patients and methods:We conducted 6 focus group discussions with a total of 43 well-retained and less well-retained HIV+ patients in San Francisco, Seattle, and Birmingham, to elicit a wide range of perspectives on engagement in HIV care. Borrowing from the field of psychotherapy, we examined patient-provider relationship characteristics through the lens of the therapeutic alliance, and with regard to their therapeutic efficacy and impact on patient engagement. Results:The majority of participants emphasized how a strong patient-provider relationship defined by trust, intimacy, and collaboration promoted engagement, while a weak patient-provider relationship impeded engagement. Conclusion:We discuss practical strategies and therapeutic techniques that may be helpful to providers in building strong patient-provider relationship and contend that a strong patient-provider relationship is crucial for patients to feel cared for during clinical encounters, which can promote long-term and sustained engagement in HIV care
Physical Properties of Four SZE-Selected Galaxy Clusters in the Southern Cosmology Survey
We present the optical and X-ray properties of four clusters recently
discovered by the South Pole Telescope (SPT) using the Sunyaev-Zel'dovich
effect (SZE). The four clusters are located in one of the common survey areas
of the southern sky that is also being targeted by the Atacama Cosmology
Telescope (ACT) and imaged by the CTIO Blanco 4-m telescope. Based on publicly
available griz optical images and XMM-Newton and ROSAT X-ray observations we
analyse the physical properties of these clusters and obtain photometric
redshifts, luminosities, richness and mass estimates. Each cluster contains a
central elliptical whose luminosity is consistent with SDSS cluster studies.
Our mass estimates are well above the nominal detection limit of SPT and ACT;
the new SZE clusters are very likely massive systems with M>~5x10^14 M_sun.Comment: 5 pages, 2 figures. ApJL accepte
New Insights into the Problem of the Surface Gravity Distribution of Cool DA White Dwarfs
We review at length the longstanding problem in the spectroscopic analysis of
cool hydrogen-line (DA) white dwarfs (Teff < 13,000 K) where gravities are
significantly higher than those found in hotter DA stars. The first solution
that has been proposed for this problem is a mild and systematic helium
contamination from convective mixing that would mimic the high gravities. We
constrain this scenario by determining the helium abundances in six cool DA
white dwarfs using high-resolution spectra from the Keck I 10-m telescope. We
obtain no detections, with upper limits as low as He/H = 0.04 in some cases.
This allows us to put this scenario to rest for good. We also extend our model
grid to lower temperatures using improved Stark profiles with non-ideal gas
effects from Tremblay & Bergeron and find that the gravity distribution of cool
objects remains suspiciously high. Finally, we find that photometric masses
are, on average, in agreement with expected values, and that the high-log g
problem is so far unique to the spectroscopic approach.Comment: 44 pages, 14 figures, accepted for publication in the Astrophysical
Journa
Cool Customers in the Stellar Graveyard I: Limits to Extrasolar Planets Around the White Dwarf G29-38
We present high contrast images of the hydrogen white dwarf G 29-38 taken in
the near infrared with the Hubble Space Telescope and the Gemini North
Telescope as part of a high contrast imaging search for substellar objects in
orbit around nearby white dwarfs.
We review the current limits on planetary companions for G29-38, the only
nearby white dwarf with an infrared excess due to a dust disk. We add our
recent observations to these limits to produce extremely tight constraints on
the types of possible companions that could be present. No objects 6
M are detected in our data at projected separations 12 AU, and no
objects 16 M are detected for separations from 3 to 12 AU, assuming
a total system age of 1 Gyr. Limits for companions at separations 3 AU come
from a combination of 2MASS photometry and previous studies of G29-38's
pulsations. Our imaging with Gemini cannot confirm a tentative claim for the
presence of a low mass brown dwarf. These observations demonstrate that a
careful combination of several techniques can probe nearby white dwarfs for
large planets and low mass brown dwarfs.Comment: 20 pages, 4 figures, Accepted to Ap
Direct Distance Measurement to the Dusty White Dwarf GD 362
We present trigonometric parallax observations of GD 362 obtained over seven
epochs using the MDM 2.4m Hiltner Telescope. The existence of a dust disk
around this possibly massive white dwarf makes it an interesting target for
parallax observations. The measured parallax for GD 362 places it at a distance
of 50.6 pc, which implies that its radius and mass are ~ 0.0106 Rsun and 0.71
Msun, respectively. GD 362 is not as massive as initially thought (1.2Msun).
Our results are entirely consistent with the distance and mass estimates (52.2
pc and 0.73 Msun) by Zuckerman et al., who demonstrated that GD 362 has a
helium dominated atmosphere. Dropping GD 362 from the list of massive white
dwarfs, there are no white dwarfs with M > 0.9 Msun that are known to host
circumstellar dust disks.Comment: ApJ Letters, in pres
Southern Cosmology Survey II: Massive Optically-Selected Clusters from 70 square degrees of the SZE Common Survey Area
We present a catalog of 105 rich and massive (M>3\times10^{14}M_{\sun})
optically-selected clusters of galaxies extracted from 70 square-degrees of
public archival griz imaging from the Blanco 4-m telescope acquired over 45
nights between 2005 and 2007. We use the clusters' optically-derived properties
to estimate photometric redshifts, optical luminosities, richness, and masses.
We complement the optical measurements with archival XMM-Newton and ROSAT X-ray
data which provide additional luminosity and mass constraints on a modest
fraction of the cluster sample. Two of our clusters show clear evidence for
central lensing arcs; one of these has a spectacular large-diameter,
nearly-complete Einstein Ring surrounding the brightest cluster galaxy. A
strong motivation for this study is to identify the massive clusters that are
expected to display prominent signals from the Sunyaev-Zeldovich Effect (SZE)
and therefore be detected in the wide-area mm-band surveys being conducted by
both the Atacama Cosmology Telescope and the South Pole Telescope. The optical
sample presented here will be useful for verifying new SZE cluster candidates
from these surveys, for testing the cluster selection function, and for
stacking analyzes of the SZE data.Comment: 13 pages, 7 Figures. Accepted for publication to ApJSS. Full
resolution plots and additional material available at
http://peumo.rutgers.edu/~felipe/e-prints
AN ECCENTRIC BINARY MILLISECOND PULSAR with A HELIUM WHITE DWARF COMPANION in the GALACTIC FIELD
Low-mass white dwarfs (LMWDs) are believed to be exclusive products of binary evolution, as the universe is not old enough to produce them from single stars. Because of the strong tidal forces operating during the binary interaction phase, the remnant systems observed today are expected to have negligible eccentricities. Here, we report on the first unambiguous identification of an LMWD in an eccentric (e = 0.13) orbit around the millisecond pulsar PSR J2234+0511, which directly contradicts this picture. We use our spectra and radio-timing solution (derived elsewhere) to infer the WD temperature (Teff = 8600 ±190 K), and peculiar systemic velocity relative to the local standard of rest ( km s-1). We also place model-independent constraints on the WD radius ( ) and surface gravity ( dex). The WD and kinematic properties are consistent with the expectations for low-mass X-ray binary evolution and disfavor a dynamic three-body formation channel. In the case of the high eccentricity being the result of a spontaneous phase transition, we infer a mass of ∼1.60 M o for the pulsar progenitor, which is too low for the quark-nova mechanism proposed by Jiang et al., and too high for the scenario of Freire & Tauris, in which a WD collapses into a neutron star via a rotationally delayed accretion-induced collapse. We find that eccentricity pumping via interaction with a circumbinary disk is consistent with our inferred parameters. Finally, we report tentative evidence for pulsations that, if confirmed, would transform the star into an unprecedented laboratory for WD physics
Assessing readiness to implement long-acting injectable HIV antiretroviral therapy: Provider and staff perspectives
Background: Long-acting injectable antiretroviral therapy (LAI-ART) represents the next innovation in HIV therapy. Pre-implementation research is needed to develop effective strategies to ensure equitable access to LAI-ART to individuals living with HIV. Methods: We conducted focus group discussions (FGDs) with providers and staff affiliated with HIV clinics in San Francisco, Chicago, and Atlanta to understand barriers to and facilitators of LAI-ART implementation. Participants also completed a short survey about implementation intentions. FGDs were held via video conference, recorded, transcribed, and thematically analyzed using domains associated with the Consolidated Framework for Implementation Research (CFIR). Results: Between September 2020 and April 2021, we led 10 FDGs with 49 participants, of whom ~60% were prescribing providers. Organizational readiness for implementing change was high, with 85% agreeing to being committed to figuring out how to implement LAI-ART. While responses were influenced by the unique inner and outer resources available in each setting, several common themes, including implementation mechanisms, dominated: (1) optimism and enthusiasm about LAI-ART was contingent on ensuring equitable access to LAI-ART; (2) LAI-ART shifts the primary responsibility of ART adherence from the patient to the clinic; and (3) existing clinic systems require strengthening to meet the needs of patients with adherence challenges. Current systems in all sites could support the use of LAI-ART in a limited number of stable patients. Scale-up and equitable use would be challenging or impossible without additional personnel. Participants outlined programmatic elements necessary to realize equitable access including centralized tracking of patients, capacity for in-depth, hands-on outreach, and mobile delivery of LAI-ART. Sites further specified unknown logistical impacts on implementation related to billing/payer source as well as shipping and drug storage. Conclusions: Among these HIV care sites, clinic readiness to offer LAI-ART to a subset of patients is high. The main challenges to implementation include concerns about unequal access and a recognition that strengthening the clinic system is critical.</p
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