181 research outputs found
Majoranized Feynman rules
We point out that the compact Feynman rules for Majorana fermions proposed by
Denner et al. are in fact a convention for the complex phases of (anti)spinors,
valid for both Majorana and Dirac fermions. We establish the relation of this
phase convention with that common in the use of spinor techniques.Comment: 5 pages, comment and reference adde
X-Ray Spectroscopy of II Pegasi: Coronal Temperature Structure, Abundances, and Variability
We have obtained high resolution X-ray spectra of the coronally active
binary, II Pegasi (HD 224085), covering the wavelength range of 1.5-25
Angstroms. For the first half of our 44 ksec observation, the source was in a
quiescent state with constant X-ray flux, after which it flared, reaching twice
the quiescent flux in 12 ksec, then decreasing. We analyze the emission-line
spectrum and continuum during quiescent and flaring states. The differential
emission measure derived from lines fluxes shows a hot corona with a continuous
distribution in temperature. During the non-flare state, the distribution peaks
near log T = 7.2, and when flaring, near 7.6. High-temperature lines are
enhanced slightly during the flare, but most of the change occurs in the
continuum. Coronal abundance anomalies are apparent, with iron very deficient
relative to oxygen and significantly weaker than expected from photospheric
measurements, while neon is enhanced relative to oxygen. We find no evidence of
appreciable resonant scattering optical depth in line ratios of iron and
oxygen. The flare light curve is consistent with Solar two-ribbon flare models,
but with a very long reconnection time-constant of about 65 ks. We infer loop
lengths of about 0.05 stellar radii, to about 0.25 in the flare, if the flare
emission originated from a single, low-density loop.Comment: 25 pages, 5 figures, 3 tables, accepted by ApJ (scheduled for the
v559 n2 p1 Oct 1, 2001 issue
Structure and Stress: Trajectories of Depressive Symptoms across Adolescence and Young Adulthood
Previous research into the social distribution of early life depression has yielded inconsistent results regarding subgroup differences in depression levels and in the etiology of these differences. Using latent curve models and data from the National Longitudinal Study of Adolescent Health, this study investigates gender and racial/ethnic disparities in early life depressive symptoms and the explanatory roles of stress and socioeconomic status (SES). Results show that females and minorities experience higher levels of depressive symptoms across early life compared to males and Whites. Further, childhood SES and stressful life events (SLEs) explain much of the disparity for Blacks and Hispanics. Finally, Blacks, Hispanics, and females show greater sensitivity to the effects of low childhood SES and, in the case of females, SLEs. Overall, this study provides new insight into gender and racial/ethnic differences in the course of early life depression and in the role of the stress process during this important developmental stage
Rotation and X-ray emission from protostars
The ASCA satellite has recently detected variable hard X-ray emission from
two Class I protostars in the rho Oph cloud, YLW15 (IRS43) and WL6, with a
characteristic time scale ~20h. In YLW15, the X-ray emission is in the form of
quasi-periodic energetic flares, which we explain in terms of strong magnetic
shearing and reconnection between the central star and the accretion disk. In
WL6, X-ray flaring is rotationally modulated, and appears to be more like the
solar-type magnetic activity ubiquitous on T Tauri stars. We find that YLW15 is
a fast rotator (near break-up), while WL6 rotates with a significantly longer
period. We derive a mass M_\star ~ 2 M_\odot and \simlt 0.4 M_\odot for the
central stars of YLW15 and WL6 respectively. On the long term, the interactions
between the star and the disk results in magnetic braking and angular momentum
loss of the star. On time scales t_{br} ~ a few 10^5 yrs, i.e., of the same
order as the estimated duration of the Class~I protostar stage. Close to the
birthline there must be a mass-rotation relation, t_{br} \simpropto M_\star,
such that stars with M_\star \simgt 1-2 M_\odot are fast rotators, while their
lower-mass counterparts have had the time to spin down. The rapid rotation and
strong star-disk magnetic interactions of YLW15 also naturally explain the
observation of X-ray ``superflares''. In the case of YLW15, and perhaps also of
other protostars, a hot coronal wind (T~10^6 K) may be responsible for the VLA
thermal radio emission. This paper thus proposes the first clues to the
rotation status and evolution of protostars.Comment: 13 pages with 6 figures. To be published in ApJ (April 10, 2000 Part
1 issue
X-Ray Flaring on the dMe Star, Ross 154
We present results from two Chandra imaging observations of Ross 154, a
nearby flaring M dwarf star. During a 61-ks ACIS-S exposure, a very large flare
occurred (the equivalent of a solar X3400 event, with L_X = 1.8x10^30 ergs/s)
in which the count rate increased by a factor of over 100. The early phase of
the flare shows evidence for the Neupert effect, followed by a further rise and
then a two-component exponential decay. A large flare was also observed at the
end of a later 48-ks HRC-I observation. Emission from the non-flaring phases of
both observations was analyzed for evidence of low level flaring. From these
temporal studies we find that microflaring probably accounts for most of the
`quiescent' emission, and that, unlike for the Sun and the handful of other
stars that have been studied, the distribution of flare intensities does not
appear to follow a power-law with a single index. Analysis of the ACIS spectra,
which was complicated by exclusion of the heavily piled-up source core,
suggests that the quiescent Ne/O abundance ratio is enhanced by a factor of
~2.5 compared to the commonly adopted solar abundance ratio, and that the Ne/O
ratio and overall coronal metallicity during the flare appear to be enhanced
relative to quiescent abundances. Based on the temperatures and emission
measures derived from the spectral fits, we estimate the length scales and
plasma densities in the flaring volume and also track the evolution of the
flare in color-intensity space. Lastly, we searched for a stellar-wind
charge-exchange X-ray halo around the star but without success; because of the
relationship between mass-loss rate and the halo surface brightness, not even
an upper limit on the stellar mass-loss rate can be determined.Comment: 20 pages, 12 figures (4 color), accepted by ApJ, expected publication
April 1, 200
Genome-wide pharmacogenomic analysis of response to treatment with antipsychotics
Schizophrenia is an often devastating neuropsychiatric illness. Understanding the genetic variation affecting response to antipsychotics is important to develop novel diagnostic tests to match individual schizophrenic patients to the most effective and safe medication. Here we use a genomewide approach to detect genetic variation underlying individual differences in response to treatment with the antipsychotics olanzapine, quetiapine, risperidone, ziprasidone and perphenazine. Our sample consisted of 738 subjects with DSM-IV schizophrenia who took part in the Clinical Antipsychotic Trials of Intervention Effectiveness (CATIE). Subjects were genotyped using the Affymetrix 500K genotyping platform plus a custom 164K chip to improve genomewide coverage. Treatment outcome was measured using the Positive and Negative Syndrome Scale (PANSS). Our criterion for genomewide significance was a pre-specified threshold that ensures, on average, only 10% of the significant findings are false discoveries. The top statistical result reached significance at our pre-specified threshold and involved a SNP in an intergenic region on chromosome 4p15. In addition, SNPs in ANKS1B and CNTNAP5 that mediated the effects of olanzapine and risperidone on Negative symptoms were very close to our threshold for declaring significance. The most significant SNP in CNTNAP5 is nonsynonymous, giving rise to an amino acid substitution. In addition to highlighting our top results, we provide all p-values for download as a resource for investigators with the requisite samples to carry out replication. This study demonstrates the potential of GWAS to discover novel genes that mediate effects of antipsychotics, which eventually could help to tailor drug treatment to schizophrenic patients
Recent Advances in Understanding Particle Acceleration Processes in Solar Flares
We review basic theoretical concepts in particle acceleration, with
particular emphasis on processes likely to occur in regions of magnetic
reconnection. Several new developments are discussed, including detailed
studies of reconnection in three-dimensional magnetic field configurations
(e.g., current sheets, collapsing traps, separatrix regions) and stochastic
acceleration in a turbulent environment. Fluid, test-particle, and
particle-in-cell approaches are used and results compared. While these studies
show considerable promise in accounting for the various observational
manifestations of solar flares, they are limited by a number of factors, mostly
relating to available computational power. Not the least of these issues is the
need to explicitly incorporate the electrodynamic feedback of the accelerated
particles themselves on the environment in which they are accelerated. A brief
prognosis for future advancement is offered.Comment: This is a chapter in a monograph on the physics of solar flares,
inspired by RHESSI observations. The individual articles are to appear in
Space Science Reviews (2011
Stellar Coronal and Wind Models: Impact on Exoplanets
Surface magnetism is believed to be the main driver of coronal heating and
stellar wind acceleration. Coronae are believed to be formed by plasma confined
in closed magnetic coronal loops of the stars, with winds mainly originating in
open magnetic field line regions. In this Chapter, we review some basic
properties of stellar coronae and winds and present some existing models. In
the last part of this Chapter, we discuss the effects of coronal winds on
exoplanets.Comment: Chapter published in the "Handbook of Exoplanets", Editors in Chief:
Juan Antonio Belmonte and Hans Deeg, Section Editor: Nuccio Lanza. Springer
Reference Work
Power calculations using exact data simulation: A useful tool for genetic study designs.
Statistical power calculations constitute an essential first step in the planning of scientific studies. If sufficient summary statistics are available, power calculations are in principle straightforward and computationally light. In designs, which comprise distinct groups (e.g., MZ & DZ twins), sufficient statistics can be calculated within each group, and analyzed in a multi-group model. However, when the number of possible groups is prohibitively large (say, in the hundreds), power calculations on the basis of the summary statistics become impractical. In that case, researchers may resort to Monte Carlo based power studies, which involve the simulation of hundreds or thousands of replicate samples for each specified set of population parameters. Here we present exact data simulation as a third method of power calculation. Exact data simulation involves a transformation of raw data so that the data fit the hypothesized model exactly. As in power calculation with summary statistics, exact data simulation is computationally light, while the number of groups in the analysis has little bearing on the practicality of the method. The method is applied to three genetic designs for illustrative purposes
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