304 research outputs found
Ionization Equilibrium Timescales in Collisional Plasmas
Astrophysical shocks or bursts from a photoionizing source can disturb the
typical collisional plasma found in galactic interstellar media or the
intergalactic medium. The spectrum emitted by this plasma contains diagnostics
that have been used to determine the time since the disturbing event, although
this determination becomes uncertain as the elements in the plasma return to
ionization equilibrium. A general solution for the equilibrium timescale for
each element arises from the elegant eigenvector method of solution to the
problem of a non-equilibrium plasma described by Masai (1984) and Hughes &
Helfand (1985). In general the ionization evolution of an element Z in a
constant electron temperature plasma is given by a coupled set of Z+1 first
order differential equations. However, they can be recast as Z uncoupled first
order differential equations using an eigenvector basis for the system. The
solution is then Z separate exponential functions, with the time constants
given by the eigenvalues of the rate matrix. The smallest of these eigenvalues
gives the scale of slowest return to equilibrium independent of the initial
conditions, while conversely the largest eigenvalue is the scale of the fastest
change in the ion population. These results hold for an ionizing plasma, a
recombining plasma, or even a plasma with random initial conditions, and will
allow users of these diagnostics to determine directly if their best-fit result
significantly limits the timescale since a disturbance or is so close to
equilibrium as to include an arbitrarily-long time.Comment: 4 pages, 2 figures. Accepted for publication by the Astrophysical
Journa
Size dependence of solar X-ray flare properties
Non-thermal and thermal parameters of 85 solar flares of GOES class B1 to M6
(background subtracted classes A1 to M6) have been compared to each other. The
hard X-ray flux has been measured by RHESSI and a spectral fitting provided
flux and spectral index of the non-thermal emission, as well as temperature and
emission measure of the thermal emission. The soft X-ray flux was taken from
GOES measurements. We find a linear correlation in a double logarithmic plot
between the non-thermal flux and the spectral index. The higher the
acceleration rate of a flare, the harder the non-thermal electron distribution.
The relation is similar to the one found by a comparison of the same parameters
from several sub-peaks of a single flare. Thus small flares behave like small
subpeaks of large flares. Thermal flare properties such as temperature,
emission measure and the soft X-ray flux also correlate with peak non-thermal
flux. A large non-thermal peak flux entails an enhancement in both thermal
parameters. The relation between spectral index and the non-thermal flux is an
intrinsic feature of the particle acceleration process, depending on flare
size. This property affects the reported frequency distribution of flare
energies.Comment: Astronomy and Astrophysics, in pres
Stellar wind bubbles around WR and [WR] stars
We study the dynamics of stellar wind bubbles around hydrogen-deficient stars
using numerical simulations with time- and ion dependent cooling. We consider
two types of hydrogen-deficient stars, massive WR stars, producing Ring
Nebulae, and low mass [WR] stars, producing Planetary Nebulae. We show that for
the Planetary Nebulae, the different cooling properties of the
hydrogen-deficient wind lead to a later transition from momentum- to
energy-driven flow, which could explain the observed turbulence of these
nebulae. We find that Ring Nebulae should all be energy-driven, and show how
comparing the bubble's momentum and kinetic energy to the input wind momentum
and kinetic energy, can give misleading information about the dynamics of the
bubble.Comment: 9 pages, 3 figures, to be published in A&
In vivo validation of an experimental adaptive quantitative coronary angiography algorithm to circumvent overestimation of small luminal diameters
The reliability of quantitative coronary angiography (QCA) measurements is of fundamental importance for the study and practice of interventional cardiology. In vivo validation results have consistently reported a tendency for QCA systems to overestimate small luminal diameters. Such a systematic error may result in the underestimation of luminal gain during intracoronary procedures and in the underestimation of progression of coronary artery disease during longitudinal studies. We report the in vivo validation results of an experimental adaptive edge‐detection algorithm that was developed to reduce overestimation of small luminal diameters by incorporating a dynamic function of variable kernel size of the derivative operator and variable weighting of the first and second derivatives of the brightness profile. The results of the experimental algorithm were compared to those of the conventional parent edge detection algorithm with fixed parameters. Dynamic adjustment of the edge‐detection algorithm parameters was found to improve measurements of small (lt;0.8‐mm) luminal diameters as evidenced by an intercept of +.07 mm for the algorithm with variable weighting compared to +0.21 mm for the parent algorithm with fixed weighting. A slope of <1 was found for both the parent and experimental algorithms with subsequent underestimation of large luminal diameters. S
Reproducibility of scientific workflows execution using cloud-aware provenance (ReCAP)
© 2018, Springer-Verlag GmbH Austria, part of Springer Nature. Provenance of scientific workflows has been considered a mean to provide workflow reproducibility. However, the provenance approaches adopted so far are not applicable in the context of Cloud because the provenance trace lacks the Cloud information. This paper presents a novel approach that collects the Cloud-aware provenance and represents it as a graph. The workflow execution reproducibility on the Cloud is determined by comparing the workflow provenance at three levels i.e., workflow structure, execution infrastructure and workflow outputs. The experimental evaluation shows that the implemented approach can detect changes in the provenance traces and the outputs produced by the workflow
Three-month follow-up after severe COVID-19 infection: are chest CT results associated with respiratory outcomes and respiratory recovery in COVID-19 patients?
Background: CT Severity Score (CT-SS) can be used to assess the extent of severe coronavirus disease 19 (COVID-19) pneumonia. Follow-up CT-SS in patients surviving COVID-19-associated hyperinflammation and its correlation with respiratory parameters remains unknown. This study aims to assess the association between CT-SS and respiratory outcomes, both in hospital and at three months after hospitalization. Methods: Patients from the COVID-19 High-intensity Immunosuppression in Cytokine storm Syndrome (CHIC) study surviving hospitalization due to COVID-19 associated hyperinflammation were invited for follow-up assessment at three months after hospitalization. Results of CT-SS three months after hospitalization were compared with CT-SS at hospital admission. CT-SS at admission and at 3-months were correlated with respiratory status during hospitalization and with patient reported outcomes as well as pulmonary- and exercise function tests at 3-months after hospitalization. Results: A total of 113 patients were included. Mean CT-SS decreased by 40.4% (SD 27.6) in three months (P 80%pred vs. 14.3 (3.2) in those with DLCO Pathophysiology and treatment of rheumatic disease
On the importance of the wind emission to the optical continuum of OB supergiants
Thermal wind emission in the form of free-free and free-bound emission is
known to show up in the infrared and radio continuum of hot and massive stars.
For OB supergiants with moderate mass loss rates and a wind velocity
distribution with \beta = 0.8...1.0, no influence of the wind to the optical
continuum, i.e. for \lambda < 1 micron, is expected. Investigations of stellar
and wind parameters of OB supergiants over the last few years suggest, however,
that for many objects \beta is much higher than 1.0, reaching values up to 3.5.
We investigate the influence of the free-free and free-bound emission on the
emerging radiation, especially at optical wavelengths, from OB supergiants
having wind velocity distributions with \beta > 1. For the case of a
spherically symmetric, isothermal wind in local thermodynamical equilibrium
(LTE) we calculate the free-free and free-bound processes and the emerging wind
and total continuum spectra. We localize the generation region of the optical
wind continuum and especially focus on the influence of a \beta-type wind
velocity distribution with \beta > 1 on the formation of the wind continuum at
optical wavelengths. The optical wind continuum is found to be generated within
about 2 R_* which is exactly the wind region where \beta strongly influences
the density distribution. We find that for \beta > 1, the continuum of a
typical OB supergiant can indeed be contaminated with thermal wind emission,
even at optical wavelengths. The strong increase in the optical wind emission
is dominantly produced by free-bound processes.Comment: 8 pages, 12 figures, accepted for publication in A&
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