2,796 research outputs found
The Coronal X-ray Spectrum of the Multiple Weak-Lined T Tauri Star System HD 98800
We present high-resolution X-ray spectra of the multiple (hierarchical
quadruple) weak-lined T Tauri star system HD 98800, obtained with the High
Energy Transmission Gratings Spectrograph (HETGS) aboard the Chandra X-ray
Observatory (CXO). In the zeroth-order CXO/HETGS X-ray image, both principle
binary components of HD 98800 (A and B, separation 0.8'') are detected;
component A was observed to flare during the observation. The infrared excess
(dust disk) component, HD 98800B, is a factor ~4 fainter in X-rays than the
apparently ``diskless'' HD 98800A, in quiescence. The line ratios of He-like
species (e.g., Ne IX, O VII) in the HD 98800A spectrum indicate that the
X-ray-emitting plasma around HD 98800 is in a typical coronal density regime
(log n <~ 11). We conclude that the dominant X-ray-emitting component(s) of HD
98800 is (are) coronally active. The sharp spectral differences between HD
98800 and the classical T Tauri star TW Hya demonstrate the potential utility
of high-resolution X-ray spectroscopy in providing diagnostics of pre-main
sequence accretion processes.Comment: 11 pages, 5 figures; to appear in the Astrophysical Journal (Letters
CGRaBS: An All-Sky Survey of Gamma-Ray Blazar Candidates
We describe a uniform all-sky survey of bright blazars, selected primarily by
their flat radio spectra, that is designed to provide a large catalog of likely
gamma-ray AGN. The defined sample has 1625 targets with radio and X-ray
properties similar to those of the EGRET blazars, spread uniformly across the
|b| > 10 deg sky. We also report progress toward optical characterization of
the sample; of objects with known R < 23, 85% have been classified and 81% have
measured redshifts. One goal of this program is to focus attention on the most
interesting (e.g., high redshift, high luminosity, ...) sources for intensive
multiwavelength study during the observations by the Large Area Telescope (LAT)
on GLAST.Comment: 18 pages, 6 figures, 1 machine-readable table available at
http://astro.stanford.edu/CGRaBS/ ; accepted for publication in ApJ
Infrared Space Observatory Polarimetric Imaging of the Egg Nebula (RAFGL 2688)
We present polarimetric imaging of the protoplanetary nebula RAFGL 2688
obtained at 4.5 microns with the Infrared Space Observatory (ISO). We have
deconvolved the images to remove the signature of the point spread function of
the ISO telescope, to the extent possible. The deconvolved 4.5 micron image and
polarimetric map reveal a bright point source with faint, surrounding
reflection nebulosity. The reflection nebula is brightest to the
north-northeast, in agreement with previous ground- and space-based infrared
imaging. Comparison with previous near-infrared polarimetric imaging suggests
that the polarization of starlight induced by the dust grains in RAFGL 2688 is
more or less independent of wavelength between 2 microns and 4.5 microns. This,
in turn, indicates that scattering dominates over thermal emission at
wavelengths as long as ~5 microns, and that the dust grains have characteristic
radii < 1 micron.Comment: 27 pages, 9 figures; to appear in the Astronomical Journal, May 2002
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Characteristics of EGRET Blazars in the VLBA Imaging and Polarimetry Survey (VIPS)
We examine the radio properties of EGRET-detected blazars observed as part of
the VLBA Imaging and Polarimetry Survey (VIPS). VIPS has a flux limit roughly
an order of magnitude below the MOJAVE survey and most other samples that have
been used to study the properties of EGRET blazars. At lower flux levels, radio
flux density does not directly correlate with gamma-ray flux density. We do
find that the EGRET-detected blazars tend to have higher brightness
temperatures, greater core fractions, and possibly larger than average jet
opening angles. A weak correlation is also found with jet length and with
polarization. All of the well-established trends can be explained by
systematically larger Doppler factors in the gamma-ray loud blazars, consistent
with the measurements of higher apparent velocities found in monitoring
programs carried out at radio frequencies above 10 GHz.Comment: 20 pages, 7 figures, accepted to Ap
The Robinson Gravitational Wave Background Telescope (BICEP): a bolometric large angular scale CMB polarimeter
The Robinson Telescope (BICEP) is a ground-based millimeter-wave bolometric
array designed to study the polarization of the cosmic microwave background
radiation (CMB) and galactic foreground emission. Such measurements probe the
energy scale of the inflationary epoch, tighten constraints on cosmological
parameters, and verify our current understanding of CMB physics. Robinson
consists of a 250-mm aperture refractive telescope that provides an
instantaneous field-of-view of 17 degrees with angular resolution of 55 and 37
arcminutes at 100 GHz and 150 GHz, respectively. Forty-nine pair of
polarization-sensitive bolometers are cooled to 250 mK using a 4He/3He/3He
sorption fridge system, and coupled to incoming radiation via corrugated feed
horns. The all-refractive optics is cooled to 4 K to minimize polarization
systematics and instrument loading. The fully steerable 3-axis mount is capable
of continuous boresight rotation or azimuth scanning at speeds up to 5 deg/s.
Robinson has begun its first season of observation at the South Pole. Given the
measured performance of the instrument along with the excellent observing
environment, Robinson will measure the E-mode polarization with high
sensitivity, and probe for the B-modes to unprecedented depths. In this paper
we discuss aspects of the instrument design and their scientific motivations,
scanning and operational strategies, and the results of initial testing and
observations.Comment: 18 pages, 11 figures. To appear in Millimeter and Submillimeter
Detectors and Instrumentation for Astronomy III, Proceedings of SPIE, 6275,
200
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Clinical Prediction Model Suitable for Assessing Hospital Quality for Patients Undergoing Carotid Endarterectomy
Background: Assessing hospital quality in the performance of carotid endarterectomy (CEA) requires appropriate risk adjustment across hospitals with varying case mixes. The aim of this study was to develop and validate a prediction model to assess the risk of inâhospital stroke or death after CEA that could aid in the assessment of hospital quality. Methods and Results: Patients from National Cardiovascular Data Registry (NCDR)'s Carotid Artery Revascularization and Endarterectomy (CARE) Registry undergoing CEA without acute evolving stroke from 2005 to 2013 were included. Inâhospital stroke or death was modeled using hierarchical logistic regression with 20 candidate variables and accounting for hospitalâlevel clustering. Internal validation was achieved with bootstrapping; model discrimination and calibration were assessed. A total of 213 (1.7%) primary end point events occurred during 12 889 procedures. Independent predictors of stroke or death included age, prior peripheral artery disease, diabetes mellitus, prior coronary artery disease, having a symptomatic carotid lesion, having a contralateral carotid occlusion, or having New York Heart Association Class III or IV heart failure. The model was well calibrated and demonstrated moderate discriminative ability (câstatistic 0.65). The NCDR CEA score was then developed to support simple, prospective risk quantification in the clinical setting. Conclusions: The NCDR CEA score, comprising 7 clinical variables, predicts inâhospital stroke or death after CEA. This model can be used to estimate hospital riskâadjusted outcomes for CEA and to assist with the assessment of hospital quality
Investigating grain growth in disks around southern T Tauri stars at millimetre wavelengths
CONTEXT - Low-mass stars form with disks in which the coagulation of grains
may eventually lead to the formation of planets. It is not known when and where
grain growth occurs, as models that explain the observations are often
degenerate. A way to break this degeneracy is to resolve the sources under
study. AIMS - To find evidence for the existence of grains of millimetre sizes
in disks around in T Tauri stars, implying grain growth. METHODS - The
Australia Telescope Compact Array (ATCA) was used to observe 15 southern T
Tauri stars, five in the constellation Lupus and ten in Chamaeleon, at 3.3
millimetre. The five Lupus sources were also observed with the Submillimeter
Array (SMA) at 1.4 millimetre. Our new data are complemented with data from the
literature to determine the slopes of the spectral energy distributions in the
millimetre regime. RESULTS - Ten sources were detected at better than 3sigma
with the ATCA, with sigma ~1-2 mJy, and all sources that were observed with the
SMA were detected at better than 15sigma, with sigma ~4 mJy. Six of the sources
in our sample are resolved to physical radii of ~100 AU. Assuming that the
emission from such large disks is predominantly optically thin, the millimetre
slope can be related directly to the opacity index. For the other sources, the
opacity indices are lower limits. Four out of six resolved sources have opacity
indices <~1, indicating grain growth to millimetre sizes and larger. The masses
of the disks range from < 0.01 to 0.08 MSun, which is comparable to the minimum
mass solar nebula. A tentative correlation is found between the millimetre
slope and the strength and shape of the 10-micron silicate feature, indicating
that grain growth occurs on similar (short) timescales in both the inner and
outer disk.Comment: 13 pages, 7 figures, 5 tables, accepted for publication in A&
Impact of Scottish smoke-free legislation on smoking quit attempts and prevalence
<p><b>Objectives:</b> In Scotland, legislation was implemented in March 2006 prohibiting smoking in all wholly or partially enclosed public spaces. We investigated the impact on attempts to quit smoking and smoking prevalence.</p>
<p><b>Methods:</b> We performed time series models using Box-Jenkins autoregressive integrated moving averages (ARIMA) on monthly data on the gross ingredient cost of all nicotine replacement therapy (NRT) prescribed in Scotland in 2003â2009, and quarterly data on self-reported smoking prevalence between January 1999 and September 2010 from the Scottish Household Survey.</p>
<p><b>Results:</b> NRT prescription costs were significantly higher than expected over the three months prior to implementation of the legislation. Prescription costs peaked at ÂŁ1.3 million in March 2006; ÂŁ292,005.9 (95% CI ÂŁ260,402.3, ÂŁ323,609, p<0.001) higher than the monthly norm. Following implementation of the legislation, costs fell exponentially by around 26% per month (95% CI 17%, 35%, p<0.001). Twelve months following implementation, the costs were not significantly different to monthly norms. Smoking prevalence fell by 8.0% overall, from 31.3% in January 1999 to 23.7% in JulyâSeptember 2010. In the quarter prior to implementation of the legislation, smoking prevalence fell by 1.7% (95% CI 2.4%, 1.0%, p<0.001) more than expected from the underlying trend.</p>
<p><b>Conclusions:</b> Quit attempts increased in the three months leading up to Scotland's smoke-free legislation, resulting in a fall in smoking prevalence. However, neither has been sustained suggesting the need for additional tobacco control measures and ongoing support.</p>
Feasibility and acceptability of a rural, pragmatic, telemedicineâ delivered healthy lifestyle programme
Background: The public health crisis of obesity leads to increasing morbidity that are even more profound in certain populations such as rural adults. Live, twoâway videoâconferencing is a modality that can potentially surmount geographic barriers and staffing shortages. Methods: Patients from the DartmouthâHitchcock Weight and Wellness Center were recruited into a pragmatic, singleâarm, nonrandomized study of a remotely delivered 16âweek evidenceâbased healthy lifestyle programme. Patients were provided hardware and appropriate software allowing for remote participation in all sessions, outside of the clinic setting. Our primary outcomes were feasibility and acceptability of the telemedicine intervention, as well as potential effectiveness on anthropometric and functional measures. Results: Of 62 participants approached, we enrolled 37, of which 27 completed at least 75% of the 16âweek programme sessions (27% attrition). Mean age was 46.9 ± 11.6 years (88.9% female), with a mean body mass index of 41.3 ± 7.1 kg/m2 and mean waist circumference of 120.7 ± 16.8 cm. Mean patient participant satisfaction regarding the telemedicine approach was favourable (4.48 ± 0.58 on 1â5 Likert scaleâlow to high) and 67.6/75 on standardized questionnaire. Mean weight loss at 16 weeks was 2.22 ± 3.18 kg representing a 2.1% change (P \u3c .001), with a loss in waist circumference of 3.4% (P = .001). Fat mass and visceral fat were significantly lower at 16 weeks (2.9% and 12.5%; both P \u3c .05), with marginal improvement in appendicular skeletal muscle mass (1.7%). In the 30âsecond sitâtoâstand test, a mean improvement of 2.46 stands (P = .005) was observed. Conclusion: A telemedicineâdelivered, intensive weight loss intervention is feasible, acceptable, and potentially effective in rural adults seeking weight loss
Mapping the Shores of the Brown Dwarf Desert III: Young Moving Groups
We present the results of an aperture masking interferometry survey for
substellar companions around 67 members of the young (~8-200Myr) nearby
(~5-86pc) AB Doradus, Beta Pictoris, Hercules-Lyra, TW Hya, and
Tucana-Horologium stellar associations. Observations were made at near infrared
wavelengths between 1.2-3.8 microns using the adaptive optics facilities of the
Keck II, VLT UT4, and Palomar Hale Telescopes. Typical contrast ratios of
~100-200 were achieved at angular separations between ~40-320mas, with our
survey being 100% complete for companions with masses below 0.25\msolar across
this range. We report the discovery of a \msolar companion to
HIP14807, as well as the detections and orbits of previously known stellar
companions to HD16760, HD113449, and HD160934. We show that the companion to
HD16760 is in a face-on orbit, resulting in an upward revision of its mass from
\mjupiter to \msolar. No substellar
companions were detected around any of our sample members, despite our ability
to detect companions with masses below 80\mjupiter for 50 of our targets: of
these, our sensitivity extended down to 40\mjupiter around 30 targets, with a
subset of 22 subject to the still more stringent limit of 20\mjupiter. A
statistical analysis of our non-detection of substellar companions allows us to
place constraints on their frequency around ~0.2-1.5\msolar stars. In
particular, considering companion mass distributions that have been proposed in
the literature, we obtain an upper limit estimate of ~9-11% for the frequency
of 20-80\mjupiter companions between 3-30AU at 95% confidence, assuming that
their semimajor axes are distributed according to in this range.Comment: Accepted by Ap
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