1,126 research outputs found
Flash ionization of the partially ionized wind of the progenitor of SN 1987A
The H II region created by the progenitor of SN 1987A was further heated and
ionized by the supernova flash. Prior to the flash, the temperature of the gas
was 4000 - 5000 K, and helium was neutral, while the post-flash temperature was
only slightly less than 10^5 K, with the gas being ionized to helium-like
ionization stages of C, N and O. We have followed the slow post-flash cooling
and recombination of the gas, as well as its line emission, and find that the
strongest lines are N V 1240 and O VI 1034. Both these lines are good probes
for the density of the gas, and suitable instruments to detect the lines are
STIS on HST and FUSE, respectively. Other lines which may be detectable are N
IV] 1486 and [O III] 5007, though they are expected to be substantially weaker.
The relative strength of the oxygen lines is found to be a good tracer of the
color temperature of the supernova flash. From previous observations, we put
limits on the hydrogen density, n_H, of the H II region. The early N V 1240
flux measured by IUE gives an upper limit which is n_H ~ 180 \eta^{-0.40}
cm^{-3}, where \eta is the filling factor of the gas. The recently reported
emission in [O III] 5007 at 2500 days requires n_H = (160\pm12) \eta^{-0.19}
cm^{-3}, for a supernova burst similar to that in the 500full1 model of Ensman
& Burrows (1992). For the more energetic 500full2 burst the density is n_H =
(215\pm15) \eta^{-0.19} cm^{-3}. These values are much higher than in models of
the X-ray emission from the supernova (n_H ~ 75 cm^{-3}), and it seems
plausible that the observed [O III] emission is produced primarily elsewhere
than in the H II region. We also discuss the type of progenitor consistent with
the H II region. In particular, it seems unlikely that its spectral type was
much earlier than B2 Ia.Comment: LaTeX, 23 pages including 4 figures. To appear in ApJ (Main Journal
Identifying prognostic structural features in tissue sections of colon cancer patients using point pattern analysis
Diagnosis and prognosis of cancer is informed by the architecture inherent in cancer patient tissue sections. This architecture is typically identified by pathologists, yet advances in computational image analysis facilitate quantitative assessment of this structure. In this article we develop a spatial point process approach in order to describe patterns in cell distribution within tissue samples taken from colorectal cancer (CRC) patients. In particular, our approach is centered on the Palm intensity function. This leads to taking an approximate-likelihood technique in fitting point processes models. We consider two Neyman-Scott point processes and a void process, fitting these point process models to the CRC patient data. We find that the parameter estimates of these models may be used to quantify the spatial arrangement of cells. Importantly, we observe characteristic differences in the spatial arrangement of cells between patients who died from CRC and those alive at follow-up
SPIFI: a Direct-Detection Imaging Spectrometer for Submillimeter Wavelengths
The South Pole Imaging Fabry-Perot Interferometer (SPIFI) is the first instrument of its kind -a direct-detection imaging spectrometer for astronomy in the submillimeter band. SPIFI ’s focal plane is a square array of 25 silicon bolometers cooled to 60 mK; the spectrometer consists of two cryogenic scanning Fabry-Perot interferometers in series with a 60-mK bandpass filter. The instrument operates in the short submillimeter windows (350 and 450 μm) available from the ground, with spectral resolving power selectable between 500 and 10,000. At present, SPIFI’s sensitivity is within a factor of 1.5-3 of the photon background limit, comparable with the best heterodyne spectrometers. The instrument ’s large bandwidth and mapping capability provide substantial advantages for specific astrophysical projects, including deep extragalactic observations. We present the motivation for and design of SPIFI and its operational characteristics on the telescope
Early-Adulthood Cardiovascular Disease Risk Factor Profiles Among Individuals With and Without Diabetes in the Framingham Heart Study
OBJECTIVE Many studies of diabetes have examined risk factors at the time of diabetes diagnosis instead of considering the lifetime burden of adverse risk factor levels. We examined the 30-year cardiovascular disease (CVD) risk factor burden that participants have up to the time of diabetes diagnosis. RESEARCH DESIGN AND METHODS Among participants free of CVD, incident diabetes cases (fasting plasma glucose ≥126 mg/dL or treatment) occurring at examinations 2 through 8 (1979–2008) of the Framingham Heart Study Offspring cohort were age- and sex-matched 1:2 to controls. CVD risk factors (hypertension, high LDL cholesterol, low HDL cholesterol, high triglycerides, obesity) were measured at the time of diabetes diagnosis and at time points 10, 20, and 30 years prior. Conditional logistic regression was used to compare risk factor levels at each time point between diabetes cases and controls. RESULTS We identified 525 participants with new-onset diabetes who were matched to 1,049 controls (mean age, 60 years; 40% women). Compared with those without diabetes, individuals who eventually developed diabetes had higher levels of hypertension (odds ratio [OR], 2.2; P = 0.003), high LDL (OR, 1.5; P = 0.04), low HDL (OR, 2.1; P = 0.0001), high triglycerides (OR, 1.7; P = 0.04), and obesity (OR, 3.3; P < 0.0001) at time points 30 years before diabetes diagnosis. After further adjustment for BMI, the ORs for hypertension (OR, 1.9; P = 0.02) and low HDL (OR, 1.7; P = 0.01) remained statistically significant. CONCLUSIONS CVD risk factors are increased up to 30 years before diagnosis of diabetes. These findings highlight the importance of a life course approach to CVD risk factor identification among individuals at risk for diabetes
Spontaneous Stratification in Granular Mixtures
Granular materials size segregate when exposed to external periodic
perturbations such as vibrations. Moreover, mixtures of grains of different
sizes spontaneously segregate in the absence of external perturbations: when a
mixture is simply poured onto a pile, the large grains are more likely to be
found near the base, while the small grains are more likely to be near the top.
Here, we report a spontaneous phenomenon arising when we pour a mixture between
two vertical plates: the mixture spontaneously stratifies into alternating
layers of small and large grains whenever the large grains are rougher than the
small grains. In contrast, we find only spontaneous segregation when the large
grains are more rounded than the small grains. The stratification is related to
the occurrence of avalanches; during each avalanche the grains comprising the
avalanche spontaneously stratify into a pair of layers through a "kink"
mechanism, with the small grains forming a sublayer underneath the layer of
large grains.Comment: 4 pages, 6 figures, http://polymer.bu.edu/~hmakse/Home.htm
A direct method for measuring discounting and QALYs more easily and reliably
Time discounting and quality of life are two important factors in evaluations of medical interventions. The measurement of these two factors is complicated because they interact. Existing methods either simply assume one factor given, based on heuristic assumptions, or invoke complicating extraneous factors, such as risk, that generate extra biases. The authors introduce a method for measuring discounting (and then quality of life) that involves no extraneous factors and that avoids distorting interactions. Their method is considerably simpler and more realistic for subjec
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Next Generation Waveform Based Three-Dimensional Models and Metrics to Improve Nuclear Explosion Monitoring in the Middle East
The Effect of Air on Granular Size Separation in a Vibrated Granular Bed
Using high-speed video and magnetic resonance imaging (MRI) we study the
motion of a large sphere in a vertically vibrated bed of smaller grains. As
previously reported we find a non-monotonic density dependence of the rise and
sink time of the large sphere. We find that this density dependence is solely
due to air drag. We investigate in detail how the motion of the intruder sphere
is influenced by size of the background particles, initial vertical position in
the bed, ambient pressure and convection. We explain our results in the
framework of a simple model and find quantitative agreement in key aspects with
numerical simulations to the model equations.Comment: 14 pages, 16 figures, submitted to PRE, corrected typos, slight
change
Combinatorial realizations of crystals via torus actions on quiver varieties
Consider Kashiwara's crystal associated to a highest weight representation of
a symmetric Kac-Moody algebra. There is a geometric realization of this object
using Nakajima's quiver varieties, but in many particular cases it can also be
realized by elementary combinatorial methods. Here we propose a framework for
extracting combinatorial realizations from the geometric picture: We construct
certain torus actions on the quiver varieties and use Morse theory to index the
irreducible components by connected components of the subvariety of torus fixed
points. We then discuss the case of affine sl(n). There the fixed point
components are just points, and are naturally indexed by multi-partitions.
There is some choice in our construction, leading to a family of combinatorial
models for each highest weight crystal. Applying this construction to the
crystal of the fundamental representation recovers a family of combinatorial
realizations recently constructed by Fayers. This gives a more conceptual proof
of Fayers' result as well as a generalization to higher level. We also discuss
a relationship with Nakajima's monomial crystal.Comment: 23 pages, v2: added Section 8 on monomial crystals and some
references; v3: many small correction
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