723 research outputs found
The role of electron-hole recombination in organic magnetoresistance
Magneto-electrical measurements were performed on diodes and bulk
heterojunction solar cells (BHSCs) to clarify the role of formation of
coulombically bound electron-hole (e-h) pairs on the magnetoresistance (MR)
response in organic thin film devices. BHSCs are suitable model systems because
they effectively quench excitons but the probability of forming e-h pairs in
them can be tuned over orders of magnitude by the choice of material and
solvent in the blend. We have systematically varied the e-h recombination
coefficients, which are directly proportional to the probability for the charge
carriers to meet in space, and found that a reduced probability of electrons
and holes meeting in space lead to disappearance of the MR. Our results clearly
show that MR is a direct consequence of e-h pair formation. We also found that
the MR line shape follows a power law-dependence of B0.5 at higher fields
Coronal mass ejections and angular momentum loss in young stars
Conference Proceeding© International Astronomical Union 2013In our own solar system, the necessity of understanding space weather is readily evident. Fortunately for Earth, our nearest stellar neighbor is relatively quiet, exhibiting activity levels several orders of magnitude lower than young, solar-type stars. In protoplanetary systems, stellar magnetic phenomena observed are analogous to the solar case, but dramatically enhanced on all physical scales: bigger, more energetic, more frequent. While coronal mass ejections (CMEs) could play a significant role in the evolution of protoplanets, they could also affect the evolution of the central star itself. To assess the consequences of prominence eruption/CMEs, we have invoked the solar-stellar connection to estimate, for young, solar-type stars, how frequently stellar CMEs may occur and their attendant mass and angular momentum loss rates. We will demonstrate the necessary conditions under which CMEs could slow stellar rotation. © 2013 International Astronomical Union
Linking Signatures of Accretion with Magnetic Field Measurements - Line Profiles are not Significantly Different in Magnetic and Non-Magnetic Herbig Ae/Be Stars
Herbig Ae/Be stars are young, pre-main-sequence stars that sample the
transition in structure and evolution between low- and high-mass stars,
providing a key test of accretion processes in higher-mass stars. Few Herbig
Ae/Be stars have detected magnetic fields, calling into question whether the
magnetospheric accretion paradigm developed for low-mass stars can be scaled to
higher masses. We present He I 10830 \AA\ line profiles for 64 Herbig Ae/Be
stars with a magnetic field measurement in order to test magnetospheric
accretion in the physical regime where its efficacy remains uncertain. Of the 5
stars with a magnetic field detection, 1 shows redshifted absorption,
indicative of infall, and 2 show blueshifted absorption, tracing mass outflow.
The fraction of redshifted and blueshifted absorption profiles in the
non-magnetic Herbig Ae/Be stars is remarkably similar, suggesting that the
stellar magnetic field does not affect gas kinematics traced by He I 10830 \AA.
Line profile morphology does not correlate with the luminosity, rotation rate,
mass accretion rate, or disk inclination. Only the detection of a magnetic
field and a nearly face-on disk inclination show a correlation (albeit for few
sources). This provides further evidence for weaker dipoles and more complex
field topologies as stars develop a radiative envelope. The small number of
magnetic Herbig Ae/Be stars has already called into question whether
magnetospheric accretion can be scaled to higher masses; accretion signatures
are not substantially different in magnetic Herbig Ae/Be stars, casting further
doubt that they accrete in the same manner as classical T Tauri stars.Comment: accepted to ApJ; 17 pages, 4 figures, 3 table
Health-related quality of life in relation to shark symptomatic and radiographic definitions of knee osteoarthritis : data from Osteoarthritis Initiative (OAI) 4-year follow- up study
Background: The purpose was to quantify the decrement in health utility (referred as disutility) associated with knee osteoarthritis (OA) and different symptomatic and radiographic uni- and bilateral definitions of knee OA in a repeated measures design of persons with knee OA or at increased risk of developing knee OA. Methods: Data were obtained from the Osteoarthritis Initiative database. SF-12 health-related quality of life was converted into SF-6D utilities, and were then handled as the health utility loss by subtracting 1.000 from the utility score, yielding a negative value (disutility). Symptomatic OA was defined by radiographic findings (Kellgren-Lawrence, K-L, grade >= 2) and frequent knee pain in the same knee. Radiographic OA was defined by five different definitions (K-L >= 2 unilaterally / bilaterally, or the highest / mean / combination of K-L grades of both knees). Repeated measures generalized estimating equation (GEE) models were used to investigate disutility in relation to these different definitions. Results: Utility decreased with worsening of symptomatic or radiographic status of knee OA. The participants with bilateral and unilateral symptomatic knee OA had 0.03 (p <0.001) and 0.02 (p <0.001) points lower utility scores, respectively, compared with the reference group. The radiographic K-L grade 4 defined as the mean or the highest grade of both knees was related to a decrease of 0.04 (p <0.001) and 0.03 (p <0.001) points in utility scores, respectively, compared to the reference group. Conclusions: Knee OA is associated with diminished health-related quality of life. Health utility can be quantified in relation to both symptomatic and radiographic uni- and bilateral definitions of knee OA, and these definitions are associated with differing disutilities. The performance of symptomatic definition was better, indicating that pain experience is an important factor in knee OA related quality of life.Peer reviewe
A Search for Star-Disk Interaction Among the Strongest X-ray Flaring Stars in the Orion Nebula Cluster
The Chandra Orion Ultradeep Project observed hundreds of young, low-mass
stars undergoing highly energetic X-ray flare events. The 32 most powerful
cases have been modeled with the result that the magnetic structures
responsible for these flares can be many stellar radii in extent. In this
paper, we model the observed spectral energy distributions of these 32 stars in
order to determine, in detail for each star, whether there is circumstellar
disk material situated in sufficient proximity to the stellar surface for
interaction with the large magnetic loops inferred from the observed X-ray
flares. Our spectral energy distributions span the wavelength range 0.3-8 um
(plus 24 um for some stars), allowing us to constrain the presence of dusty
circumstellar material out to >10 AU from the stellar surface in most cases.
For 24 of the 32 stars in our sample the available data are sufficient to
constrain the location of the inner edge of the dusty disks. Six of these (25%)
have spectral energy distributions consistent with inner disks within reach of
the observed magnetic loops. Another four stars may have gas disks interior to
the dust disk and extending within reach of the magnetic loops, but we cannot
confirm this with the available data. The remaining 14 stars (58%) appear to
have no significant disk material within reach of the large flaring loops.
Thus, up to ~40% of the sample stars exhibit energetic X-ray flares that
possibly arise from a magnetic star-disk interaction, and the remainder are
evidently associated with extremely large, free-standing magnetic loops
anchored only to the stellar surface.Comment: Accepted to the ApJ; 26 pages, 6 tables, 6 figure
Mass Loss in Pre-main-sequence Stars via Coronal Mass Ejections and Implications for Angular Momentum Loss
We develop an empirical model to estimate mass-loss rates via coronal mass ejections (CMEs) for solar-type pre-main-sequence (PMS) stars. Our method estimates the CME mass-loss rate from the observed energies of PMS X-ray flares, using our empirically determined relationship between solar X-ray flare energy and CME mass: log ( M CME [g]) = 0.63 × log ( E flare [erg]) – 2.57. Using masses determined for the largest flaring magnetic structures observed on PMS stars, we suggest that this solar-calibrated relationship may hold over 10 orders of magnitude in flare energy and 7 orders of magnitude in CME mass. The total CME mass-loss rate we calculate for typical solar-type PMS stars is in the range 10 –12 -10 –9 M ☉ yr –1 . We then use these CME mass-loss rate estimates to infer the attendant angular momentum loss leading up to the main sequence. Assuming that the CME outflow rate for a typical ~1 M ☉ T Tauri star is <10 –10 M ☉ yr –1 , the resulting spin-down torque is too small during the first ~1 Myr to counteract the stellar spin-up due to contraction and accretion. However, if the CME mass-loss rate is ##IMG## [http://ej.iop.org/icons/Entities/gsim.gif] {gsim 10 –10 M ☉ yr –1 , as permitted by our calculations, then the CME spin-down torque may influence the stellar spin evolution after an age of a few Myr.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/98581/1/0004-637X_760_1_9.pd
Review of the Properties of the W Boson at LEP, and the Precision Determination of its Mass
We review the precision measurement of the mass and couplings of the W Boson
at LEP. The total and differential W+W- cross section is used to extract the
WWZ and WWgamma couplings. We discuss the techniques used by the four LEP
experiments to determine the W mass in different decay channels, and present
the details of methods used to evaluate the sources of systematic uncertainty.Comment: 97 page
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