3,185 research outputs found
Evolution of brown dwarf disks: A Spitzer survey in Upper Scorpius
We have carried out a Spitzer survey for brown dwarf (BD) disks in the ~5 Myr
old Upper Scorpius (UpSco) star forming region, using IRS spectroscopy from 8
to 12\mu m and MIPS photometry at 24\mu m. Our sample consists of 35 confirmed
very low mass members of UpSco. Thirteen objects in this sample show clear
excess flux at 24\mu m, explained by dust emission from a circum-sub-stellar
disk. Objects without excess emission either have no disks at all or disks with
inner opacity holes of at least ~5 AU radii. Our disk frequency of 37\pm 9% is
higher than what has been derived previously for K0-M5 stars in the same region
(on a 1.8 sigma confidence level), suggesting a mass-dependent disk lifetime in
UpSco. The clear distinction between objects with and without disks as well as
the lack of transition objects shows that disk dissipation inside 5 AU occurs
rapidly, probably on timescales of <~10^5 years. For the objects with disks,
most SEDs are uniformly flat with flux levels of a few mJy, well modeled as
emission from dusty disks affected by dust settling to the midplane, which also
provides indirect evidence for grain growth. The silicate feature around 10\mu
m is either absent or weak in our SEDs, arguing for a lack of hot, small dust
grains. Compared with younger objects in Taurus, BD disks in UpSco show less
flaring. Taken together, these results clearly demonstrate that we see disks in
an advanced evolutionary state: Dust settling and grain growth are ubiquituous
in circum-sub-stellar disks at ages of 5 Myr, arguing for planet forming
processes in BD disks. For almost all our targets, results from high-resolution
spectroscopy and high-spatial resolution imaging have been published before,
thus providing a large sample of BDs for which information about disks,
accretion, and binarity is available. (abridged)Comment: 39 pages, 7 figures, accepted for publication in Ap
Self-field effects upon the critical current density of flat superconducting strips
We develop a general theory to account self-consistently for self-field
effects upon the average transport critical current density Jc of a flat
type-II superconducting strip in the mixed state when the bulk pinning is
characterized by a field-dependent depinning critical current density Jp(B),
where B is the local magnetic flux density. We first consider the possibility
of both bulk and edge-pinning contributions but conclude that bulk pinning
dominates over geometrical edge-barrier effects in state-of-the-art YBCO films
and prototype second-generation coated conductors. We apply our theory using
the Kim model, JpK(B) = JpK(0)/(1+|B|/B0), as an example. We calculate Jc(Ba)
as a function of a perpendicular applied magnetic induction Ba and show how
Jc(Ba) is related to JpK(B). We find that Jc(Ba) is very nearly equal to
JpK(Ba) when Ba > Ba*, where Ba* is the value of Ba that makes the net flux
density zero at the strip's edge. However, Jc(Ba) is suppressed relative to
JpK(Ba) at low fields when Ba < Ba*, with the largest suppression occurring
when Ba*/B0 is of order unity or larger.Comment: 9 pages, 4 figures, minor revisions to add four reference
Recommended from our members
Comparing predictions made by a prediction model, clinical score, and physicians Pediatric asthma exacerbations in the emergency department
Background: Asthma exacerbations are one of the most common medical reasons for children to be brought to the hospital emergency department (ED). Various prediction models have been proposed to support diagnosis of exacerbations and evaluation of their severity. Objectives: First, to evaluate prediction models constructed from data using machine learning techniques and to select the best performing model. Second, to compare predictions from the selected model with predictions from the Pediatric Respiratory Assessment Measure (PRAM) score, and predictions made by ED physicians.
Design: A two-phase study conducted in the ED of an academic pediatric hospital. In phase 1 data collected prospectively using paper forms was used to construct and evaluate five prediction models, and the best performing model was selected. In phase 2, data collected prospectively using a mobile system was used to compare the predictions of the selected prediction model with those from PRAM and ED physicians.
Measurements: Area under the receiver operating characteristic curve and accuracy in phase 1; accuracy, sensitivity, specificity, positive and negative predictive values in phase 2.
Results: In phase 1 prediction models were derived from a data set of 240 patients and evaluated using 10-fold cross validation. A naive Bayes (NB) model demonstrated the best performance and it was selected for phase 2. Evaluation in phase 2 was conducted on data from 82 patients. Predictions made by the NB model were less accurate than the PRAM score and physicians (accuracy of 70.7%, 73.2% and 78.0% respectively), however, according to McNemar’s test it is not possible to conclude that the differences between predictions are statistically significant.
Conclusion: Both the PRAM score and the NB model were less accurate than physicians. The NB model can handle incomplete patient data and as such may complement the PRAM score. However, it requires further research to improve its accuracy
Influence of atmospheric turbulence on states of light carrying orbital angular momentum
We have experimentally studied the degradation of mode purity for light beams carrying orbital angular momentum (OAM) propagating through simulated atmospheric turbulence. The turbulence is modeled as a randomly varying phase aberration, which obeys statistics postulated by Kolmogorov turbulence theory. We introduce this simulated turbulence through the use of a phase-only spatial light modulator. Once the turbulence is introduced, the degradation in mode quality results in crosstalk between OAM modes. We study this crosstalk in OAM for 11 modes, showing that turbulence uniformly degrades the purity of all the modes within this range, irrespective of mode number
Feasibility of direct digital sampling for diffuse optical frequency domain spectroscopy in tissue
Frequency domain optical spectroscopy in the diffusive regime is currently being investigated for biomedical applications including tumor detection, therapy monitoring, exercise metabolism, and others. Analog homodyne or heterodyne detection of sinusoidally modulated signals have been the predominant method for measuring phase and amplitude of photon density waves that have traversed through tissue. Here we demonstrate the feasibility of utilizing direct digital sampling of modulated signals using a 3.6 Gigasample/second 12 bit Analog to Digital Converter. Digitally synthesized modulated signals between 50MHz and 400MHz were measured on tissue simulating phantoms at six near-infrared wavelengths. An amplitude and phase precision of 1% and 0.6 degrees were achieved during drift tests. Amplitude, phase, scattering and absorption values were compared with a well-characterized network analyzer based diffuse optical device. Measured optical properties measured with both systems were within 3.6% for absorption and 2.8% for scattering over a range of biologically relevant values. Direct digital sampling represents a viable method for frequency domain diffuse optical spectroscopy and has the potential to reduce system complexity, size, and cost
Cancer incidence among children and young adults who have undergone x-ray guided cardiac catheterization procedures
Children and young adults with heart disease appear to be at increased risk of developing cancer, although the reasons for this are unclear. A cohort of 11,270 individuals, who underwent cardiac catheterizations while aged B 22 years in the UK, was established from hospital records. Radiation doses from cardiac catheterizations and CT scans were estimated. The cohort was matched with the NHS Central Register and NHS Transplant Registry to determine cancer incidence and transplantation status. Standardized incidence ratios (SIR) with associated confidence intervals (CI) were calculated. The excess relative risk (ERR) of lymphohaematopoietic neoplasia was also calculated using Poisson regression. The SIR was raised for all malignancies (2.32, 95% CI 1.65, 3.17), lymphoma (8.34, 95% CI 5.22, 12.61) and leukaemia (2.11, 95% CI 0.82, 4.42). After censoring transplant recipients, post-transplant, the SIR was reduced to 0.90 (95% CI 0.49, 1.49) for all malignancies. All lymphomas developed post-transplant. The SIR for all malignancies developing 5 years from the first cardiac catheterization (2 years for leukaemia/lymphoma) remained raised (3.01, 95% CI 2.09, 4.19) but was again reduced after censoring transplant recipients (0.98, 95% CI 0.48, 1.77). The ERR per mGy bone marrow dose for lympho-haematopoietic neoplasia was reduced from 0.541 (95% CI 0.104, 1.807) to 0.018 (95% CI - 0.002, 0.096) where transplantation status was accounted for as a time-dependent background risk factor. In conclusion, transplantation appears to be a large contributor to elevated cancer rates in this patient group. This is likely to be mainly due to associated immunosuppression, however, radiation exposure may also be a contributing factor
Observation of single collisionally cooled trapped ions in a buffer gas
Individual Ba ions are trapped in a gas-filled linear ion trap and observed
with a high signal-to-noise ratio by resonance fluorescence. Single-ion storage
times of ~5 min (~1 min) are achieved using He (Ar) as a buffer gas at
pressures in the range 8e-5 - 4e-3 torr. Trap dynamics in buffer gases are
experimentally studied in the simple case of single ions. In particular, the
cooling effects of light gases such as He and Ar and the destabilizing
properties of heavier gases such as Xe are studied. A simple model is offered
to explain the observed phenomenology.Comment: 5 pages, 4 figures, accepted for publication in Phys. Rev. A. Minor
text and figure change
- …