3,268 research outputs found
Gas and Dust Dynamics in Starlight-heated Protoplanetary Disks
Theoretical models of the ionization state in protoplanetary disks suggest
the existence of large areas with low ionization and weak coupling between the
gas and magnetic fields. In this regime hydrodynamical instabilities may become
important. In this work we investigate the gas and dust structure and dynamics
for a typical T Tauri system under the influence of the vertical shear
instability (VSI). We use global 3D radiation hydrodynamics simulations
covering all of azimuth with embedded particles of 0.1 and 1mm
size, evolved for 400 orbits. Stellar irradiation heating is included with
opacities for 0.1- to 10-m-sized dust. Saturated VSI turbulence produces a
stress-to-pressure ratio of . The value of is
lowest within 30~au of the star, where thermal relaxation is slower relative to
the orbital period and approaches the rate below which VSI is cut off. The rise
in from 20 to 30~au causes a dip in the surface density near 35~au,
leading to Rossby wave instability and the generation of a stationary,
long-lived vortex spanning about 4~au in radius and 40~au in azimuth. Our
results confirm previous findings that mm size grains are strongly vertically
mixed by the VSI. The scale height aspect ratio for 1mm grains is determined to
be 0.037, much higher than the value obtained from millimeter-wave
observations of the HL~Tau system. The measured aspect ratio is better fit by
non-ideal MHD models. In our VSI turbulence model, the mm grains drift radially
inwards and many are trapped and concentrated inside the vortex. The turbulence
induces a velocity dispersion of ~m/s for the mm grains, indicating
that grain-grain collisions could lead to fragmentation.Comment: ApJ accepte
Global Hydromagnetic Simulations of Protoplanetary Disks with Stellar Irradiation and Simplified Thermochemistry
Outflows driven by large-scale magnetic fields likely play an important role
in the evolution and dispersal of protoplanetary disks, and in setting the
conditions for planet formation. We extend our 2-D axisymmetric non-ideal MHD
model of these outflows by incorporating radiative transfer and simplified
thermochemistry, with the twin aims of exploring how heating influences wind
launching, and illustrating how such models can be tested through observations
of diagnostic spectral lines. Our model disks launch magnetocentrifugal
outflows primarily through magnetic tension forces, so the mass-loss rate
increases only moderately when thermochemical effects are switched on. For
typical field strengths, thermochemical and irradiation heating are more
important than magnetic dissipation. We furthermore find that the entrained
vertical magnetic flux diffuses out of the disk on secular timescales as a
result of non-ideal MHD. Through post-processing line radiative transfer, we
demonstrate that spectral line intensities and moment-1 maps of atomic oxygen,
the HCN molecule, and other species show potentially observable differences
between a model with a magnetically driven outflow and one with a weaker,
photoevaporative outflow. In particular, the line shapes and velocity
asymmetries in the moment-1 maps could enable the identification of outflows
emanating from the disk surface.Comment: 35 pages, 20 figures, accepted for publication in Ap
Bounds on SCFTs from Conformal Perturbation Theory
The operator product expansion (OPE) in 4d (super)conformal field theory is
of broad interest, for both formal and phenomenological applications. In this
paper, we use conformal perturbation theory to study the OPE of nearly-free
fields coupled to SCFTs. Under fairly general assumptions, we show that the OPE
of a chiral operator of dimension with its complex
conjugate always contains an operator of dimension less than . Our
bounds apply to Banks-Zaks fixed points and their generalizations, as we
illustrate using several examples.Comment: 36 pages; v2: typos fixed, minor change
Language and social/emotional problems identified at a universal developmental assessment at 30 months
Non peer reviewedPublisher PD
Multiple reassortment events in the evolutionary history of H1N1 influenza A virus since 1918
The H1N1 subtype of influenza A virus has caused substantial morbidity and mortality in humans, first documented in the global pandemic of 1918 and continuing to the present day. Despite this disease burden, the evolutionary history of the A/H1N1 virus is not well understood, particularly whether there is a virological basis for several notable epidemics of unusual severity in the 1940s and 1950s. Using a data set of 71 representative complete genome sequences sampled between 1918 and 2006, we show that segmental reassortment has played an important role in the genomic evolution of A/H1N1 since 1918. Specifically, we demonstrate that an A/H1N1 isolate from the 1947 epidemic acquired novel PB2 and HA genes through intra-subtype reassortment, which may explain the abrupt antigenic evolution of this virus. Similarly, the 1951 influenza epidemic may also have been associated with reassortant A/H1N1 viruses. Intra-subtype reassortment therefore appears to be a more important process in the evolution and epidemiology of H1N1 influenza A virus than previously realized
Translation and Community in the work of Elizabeth Cary
Explores the role of female community within Elizabeth Cary\u27s translations and her play, The Tragedy of Mariam
Solar-type dynamo behaviour in fully convective stars without a tachocline
In solar-type stars (with radiative cores and convective envelopes), the
magnetic field powers star spots, flares and other solar phenomena, as well as
chromospheric and coronal emission at ultraviolet to X-ray wavelengths. The
dynamo responsible for generating the field depends on the shearing of internal
magnetic fields by differential rotation. The shearing has long been thought to
take place in a boundary layer known as the tachocline between the radiative
core and the convective envelope. Fully convective stars do not have a
tachocline and their dynamo mechanism is expected to be very different,
although its exact form and physical dependencies are not known. Here we report
observations of four fully convective stars whose X-ray emission correlates
with their rotation periods in the same way as in Sun-like stars. As the X-ray
activity - rotation relationship is a well-established proxy for the behaviour
of the magnetic dynamo, these results imply that fully convective stars also
operate a solar-type dynamo. The lack of a tachocline in fully convective stars
therefore suggests that this is not a critical ingredient in the solar dynamo
and supports models in which the dynamo originates throughout the convection
zone.Comment: 6 pages, 1 figure. Accepted for publication in Nature (28 July 2016).
Author's version, including Method
Dietary Patterns and Practices and Leucocyte Telomere Length: Findings from the UK Biobank
BACKGROUND: Shorter telomere length (TL) is associated with risk of several age-related diseases and decreased life span, but the extent to which dietary patterns and practices associate with TL is uncertain. OBJECTIVE: This study aimed to investigate the association of dietary patterns and practices and leucocyte TL (LTL). DESIGN: This was a cross-sectional study. PARTICIPANTS AND SETTING: Data collected voluntarily from up to 422,797 UK Biobank participants, during 2006-2010. MAIN OUTCOME MEASURES: LTL was measured as a ratio of the telomere repeat number to a single-copy gene and was loge-transformed and standardized (z-LTL). STATISTICAL ANALYSES PERFORMED: Adherence a priori to a Mediterranean-style diet was assessed through the MedDietScore. Principal component analysis was used to a posteriori extract the "Meat" and "Prudent" dietary patterns. Additional dietary practices considered were the self-reported adherence to "Vegetarian" diet, "Eating 5-a-day of fruit and vegetables" and "Abstaining from eggs/dairy/wheat/sugar." Associations between quintiles of dietary patterns or adherence to dietary practices with z-LTL were investigated through multivariable linear regression models (adjusted for demographic, lifestyle, and clinical characteristics). RESULTS: Adherence to the "Mediterranean" and the "Prudent" patterns, was positively associated with LTL, with an effect magnitude in z-LTL of 0.020 SD and 0.014 SD, respectively, for the highest vs the lowest quintile of adherence to the pattern (both P values < 0.05). Conversely, a reversed association between quintile of the "Meat" pattern and LTL was observed, with z-LTL being on average shorter by 0.025 SD (P = 6.12×10-05) for participants in the highest quintile of the pattern compared with the lowest quintile. For adherents to "5-a-day" z-LTL was on average longer by 0.027 SD (P = 5.36×10-09), and for "abstainers," LTL was shorter by 0.016 SD (P = 2.51×10-04). The association of LTL with a vegetarian diet was nonsignificant after adjustment for demographic, lifestyle, and clinical characteristics. CONCLUSIONS: Several dietary patterns and practices associated with beneficial health effects are significantly associated with longer LTL. However, the magnitude of the association was small, and any clinical relevance is uncertain
HUNGARIAN EXPERIENCE IN STRUCTURAL DESIGN CODING (HISTORICAL ANTECEDENTS OF EUROCODE-2)
This paper gives review of the historical antecedents of Eurocode-2 in Hungary and East Europe. The method of permissible stresses, using uniform safety factor was first changed in 1950 in Hungary by the semi-probabilistic method using partial safety factors. This new method was accepted with some resistance on the part of the leading structural engineers. Nevertheless most of the East-European countries accepted the new method with some political overtones', to be follow the Soviet example. The authors assert in the papaer that due to the economic necessities. Hungary and the other East European countries gained experience with the regulations affording less safety than the EC2, and this offers an interesting set of experience to the West European
countries which have intoduced or are introducing the semi-probabilistic procedure. The most significant point all the experience is the recognition that only one part of the parameters in the structural analysis determining safety can be handled statistically. During design the statistically not significant data such as the error of the structural model must also be taken into consideration. Based on the experience, the authors propose an alternative design method
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