3,189 research outputs found
Probing the properties of Be star discs with spectroastrometry and NLTE radiative transfer modelling: beta CMi
While the presence of discs around classical Be stars is well established,
their origin is still uncertain. To understand what processes result in the
creation of these discs and how angular momentum is transported within them,
their physical properties must be constrained. This requires comparing high
spatial and spectral resolution data with detailed radiative transfer
modelling. We present a high spectral resolution, R~80,000, sub milli-arcsecond
precision, spectroastrometric study of the circumstellar disc around the Be
star beta CMi. The data are confronted with three-dimensional, NLTE radiative
transfer calculations to directly constrain the properties of the disc.
Furthermore, we compare the data to disc models featuring two velocity laws;
Keperian, the prediction of the viscous disc model, and angular momentum
conserving rotation. It is shown that the observations of beta CMi can only be
reproduced using Keplerian rotation. The agreement between the model and the
observed SED, polarisation and spectroastrometric signature of beta CMi
confirms that the discs around Be stars are well modelled as viscous decretion
discs.Comment: Accepted for publication in MNRA
TURBULENT HEATING OF THE DISTANT SOLAR WIND BY INTERSTELLAR PICKUP PROTONS IN A DECELERATING FLOW
Previous models of solar wind heating by interstellar pickup proton-driven turbulence have assumed that the wind speed is a constant in heliocentric radial position. However, the same pickup process, which is taken to provide the turbulent energy, must also decelerate the wind. In this paper, we extend our phenomenological turbulence model to include variable wind speed, and then incorporate the deceleration due to interstellar pickup protons into the model. We compare the model results with plasma and field data from Voyager 2, taking this opportunity to present an extended and improved data set of proton core temperature, magnetic field fluctuation intensity, and correlation length along the Voyager trajectory. A particular motivation for including the solar wind deceleration in this model is the expectation that a slower wind would reduce the resulting proton core temperature in the region beyond ~60 AU, where the previous model predictions were higher than the observed values. However, we find instead that the deceleration of the steady-state wind increases the energy input to the turbulence, causing even higher temperatures in that region. The increased heating is shown to result from the larger values of the ratio of Alfven speed to solar wind speed that develop in the decelerating wind.Jet Propulsion Laboratory (U.S.) (NASA contract 959203)United States. National Aeronautics and Space Administration (NASA grant NNX08A147G)United States. National Aeronautics and Space Administration (NASA Guest Investigator grant NNX07AH75G)United States. National Aeronautics and Space Administration (NASA Guest Investigator grant NNX08AJ19G
Statistical Inference for Valued-Edge Networks: Generalized Exponential Random Graph Models
Across the sciences, the statistical analysis of networks is central to the
production of knowledge on relational phenomena. Because of their ability to
model the structural generation of networks, exponential random graph models
are a ubiquitous means of analysis. However, they are limited by an inability
to model networks with valued edges. We solve this problem by introducing a
class of generalized exponential random graph models capable of modeling
networks whose edges are valued, thus greatly expanding the scope of networks
applied researchers can subject to statistical analysis
Quasi-Periodic Releases of Streamer Blobs and Velocity Variability of the Slow Solar Wind near the Sun
We search for persistent and quasi-periodic release events of streamer blobs
during 2007 with the Large Angle Spectrometric Coronagraph on the \textit{Solar
and Heliospheric Observatory} and assess the velocity of the slow solar wind
along the plasma sheet above the corresponding streamer by measuring the
dynamic parameters of blobs. We find 10 quasi-periodic release events of
streamer blobs lasting for three to four days. In each day of these events, we
observe three-five blobs. The results are in line with previous studies using
data observed near the last solar minimum. Using the measured blob velocity as
a proxy for that of the mean flow, we suggest that the velocity of the
background slow solar wind near the Sun can vary significantly within a few
hours. This provides an observational manifestation of the large velocity
variability of the slow solar wind near the Sun.Comment: 14 pages, 5 figures, accepted by Soalr Physic
Cyclic Variability of the Circumstellar Disc of the Be Star Tau. II. Testing the 2D Global Disc Oscillation Model
Aims. In this paper we model, in a self-consistent way, polarimetric,
photometric, spectrophotometric and interferometric observations of the
classical Be star Tauri. Our primary goal is to conduct a critical
quantitative test of the global oscillation scenario. Methods. We have carried
out detailed three-dimensional, NLTE radiative transfer calculations using the
radiative transfer code HDUST. For the input for the code we have used the most
up-to-date research on Be stars to include a physically realistic description
for the central star and the circumstellar disc. We adopt a rotationally
deformed, gravity darkened central star, surrounded by a disc whose unperturbed
state is given by a steady-state viscous decretion disc model. We further
assume that disc is in vertical hydrostatic equilibrium. Results. By adopting a
viscous decretion disc model for Tauri and a rigorous solution of the
radiative transfer, we have obtained a very good fit of the time-average
properties of the disc. This provides strong theoretical evidence that the
viscous decretion disc model is the mechanism responsible for disc formation.
With the global oscillation model we have successfully fitted spatially
resolved VLTI/AMBER observations and the temporal V/R variations of the
H and Br lines. This result convincingly demonstrates that the
oscillation pattern in the disc is a one-armed spiral. Possible model
shortcomings, as well as suggestions for future improvements, are also
discussed.Comment: 14 pages, 9 figures, accepted to A&
Origins of the Ambient Solar Wind: Implications for Space Weather
The Sun's outer atmosphere is heated to temperatures of millions of degrees,
and solar plasma flows out into interplanetary space at supersonic speeds. This
paper reviews our current understanding of these interrelated problems: coronal
heating and the acceleration of the ambient solar wind. We also discuss where
the community stands in its ability to forecast how variations in the solar
wind (i.e., fast and slow wind streams) impact the Earth. Although the last few
decades have seen significant progress in observations and modeling, we still
do not have a complete understanding of the relevant physical processes, nor do
we have a quantitatively precise census of which coronal structures contribute
to specific types of solar wind. Fast streams are known to be connected to the
central regions of large coronal holes. Slow streams, however, appear to come
from a wide range of sources, including streamers, pseudostreamers, coronal
loops, active regions, and coronal hole boundaries. Complicating our
understanding even more is the fact that processes such as turbulence,
stream-stream interactions, and Coulomb collisions can make it difficult to
unambiguously map a parcel measured at 1 AU back down to its coronal source. We
also review recent progress -- in theoretical modeling, observational data
analysis, and forecasting techniques that sit at the interface between data and
theory -- that gives us hope that the above problems are indeed solvable.Comment: Accepted for publication in Space Science Reviews. Special issue
connected with a 2016 ISSI workshop on "The Scientific Foundations of Space
Weather." 44 pages, 9 figure
Application of Genetic Programming to High Energy Physics Event Selection
We review genetic programming principles, their application to FOCUS data
samples, and use the method to study the doubly Cabibbo suppressed decay D+ ->
K+ pi+ pi- relative to its Cabibbo favored counterpart, D+ -> K- pi+ pi+. We
find that this technique is able to improve upon more traditional analysis
methods. To our knowledge, this is the first application of the genetic
programming technique to High Energy Physics data.Comment: 39 page
Search for the standard model Higgs boson in the H to ZZ to 2l 2nu channel in pp collisions at sqrt(s) = 7 TeV
A search for the standard model Higgs boson in the H to ZZ to 2l 2nu decay
channel, where l = e or mu, in pp collisions at a center-of-mass energy of 7
TeV is presented. The data were collected at the LHC, with the CMS detector,
and correspond to an integrated luminosity of 4.6 inverse femtobarns. No
significant excess is observed above the background expectation, and upper
limits are set on the Higgs boson production cross section. The presence of the
standard model Higgs boson with a mass in the 270-440 GeV range is excluded at
95% confidence level.Comment: Submitted to JHE
Measurement of the cross-section and charge asymmetry of bosons produced in proton-proton collisions at TeV with the ATLAS detector
This paper presents measurements of the and cross-sections and the associated charge asymmetry as a
function of the absolute pseudorapidity of the decay muon. The data were
collected in proton--proton collisions at a centre-of-mass energy of 8 TeV with
the ATLAS experiment at the LHC and correspond to a total integrated luminosity
of 20.2~\mbox{fb^{-1}}. The precision of the cross-section measurements
varies between 0.8% to 1.5% as a function of the pseudorapidity, excluding the
1.9% uncertainty on the integrated luminosity. The charge asymmetry is measured
with an uncertainty between 0.002 and 0.003. The results are compared with
predictions based on next-to-next-to-leading-order calculations with various
parton distribution functions and have the sensitivity to discriminate between
them.Comment: 38 pages in total, author list starting page 22, 5 figures, 4 tables,
submitted to EPJC. All figures including auxiliary figures are available at
https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/PAPERS/STDM-2017-13
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