471 research outputs found
Neutron star - white dwarf mergers: Early evolution, physical properties, and outcomes
Neutron-star (NS) - white-dwarf (WD) mergers may give rise to observable
explosive transients, but have been little explored. We use 2D coupled
hydrodynamical-thermonuclear FLASH-code simulations to study the evolution of
WD debris-disks formed following WD-disruptions by NSs. We use a 19-elements
nuclear-network and a detailed equation-of-state to follow the evolution,
complemented by a post-process analysis using a larger 125-isotopes
nuclear-network. We consider a wide range of initial conditions and study the
dependence of the results on the NS/WD masses (;, respectively), WD-composition (CO/He/hybrid-He-CO) and
the accretion-disk structure. We find that viscous inflow in the disk gives
rise to continuous wind-outflow of mostly C/O material mixed with
nuclear-burning products arising from a weak detonation occurring in the
inner-region of the disk. We find that such transients are energetically weak
(ergs) compared with thermonuclear-supernovae (SNe), and are
dominated by the (gravitational) accretion-energy. Although
thermonuclear-detonations occur robustly in all of our simulations (besides the
He-WD) they produce only little energy of the kinetic energy) and
ejecta (few, with
overall low ejecta masses of . Such explosions may
produce rapidly-evolving transients, much shorter and fainter than regular
type-Ia SNe. The composition and demographics of such SNe appear to be
inconsistent with those of Ca-rich type Ib SNe. Though they might be related to
the various classes of rapidly evolving SNe observed in recent years, they are
likely to be fainter than the typical ones, and may therefore give rise a
different class of potentially observable transients.Comment: MNRAS final versio
MPICH-G2: A Grid-Enabled Implementation of the Message Passing Interface
Application development for distributed computing "Grids" can benefit from
tools that variously hide or enable application-level management of critical
aspects of the heterogeneous environment. As part of an investigation of these
issues, we have developed MPICH-G2, a Grid-enabled implementation of the
Message Passing Interface (MPI) that allows a user to run MPI programs across
multiple computers, at the same or different sites, using the same commands
that would be used on a parallel computer. This library extends the Argonne
MPICH implementation of MPI to use services provided by the Globus Toolkit for
authentication, authorization, resource allocation, executable staging, and
I/O, as well as for process creation, monitoring, and control. Various
performance-critical operations, including startup and collective operations,
are configured to exploit network topology information. The library also
exploits MPI constructs for performance management; for example, the MPI
communicator construct is used for application-level discovery of, and
adaptation to, both network topology and network quality-of-service mechanisms.
We describe the MPICH-G2 design and implementation, present performance
results, and review application experiences, including record-setting
distributed simulations.Comment: 20 pages, 8 figure
The rate of WD-WD head-on collisions in isolated triples is too low to explain standard type Ia supernovae
Type Ia supernovae (Ia-SNe) are thought to arise from the thermonuclear
explosions of white dwarfs (WDs). The progenitors of such explosions are still
highly debated; in particular the conditions leading to detonations in WDs are
not well understood in most of the suggested progenitor models. Nevertheless,
direct head-on collisions of two WDs were shown to give rise to detonations and
produce Ia-SNe - like explosions, and were suggested as possible progenitors.
The rates of such collisions in dense globular clusters are far below the
observed rates of type Ia SNe, but it was suggested that quasi-secular
evolution of hierarchical triples could produce a high rate of such collisions.
Here we used detailed triple stellar evolution populations synthesis models
coupled with dynamical secular evolution to calculate the rates of WD-WD
collisions in triples and their properties. We explored a range of models with
different realistic initial conditions and derived the expected SNe total mass,
mass-ratio and delay time distributions for each of the models. We find that
the SNe rate from WD-WD collisions is of the order of 0.1% of the observed
Ia-SNe rate across all our models, and the delay-time distribution is almost
uniform in time, and is inconsistent with observations. We conclude that SNe
from WD-WD collisions in isolated triples can at most provide for a small
fraction of Ia-SNe, and can not serve as the main progenitors of such
explosions.Comment: 13 pages, 4 figures, submitted to A&
Thermonuclear explosion of a massive hybrid HeCO white-dwarf triggered by a He-detonation on a companion
Normal type Ia supernovae (SNe) are thought to arise from the thermonuclear
explosion of massive ( M) carbon-oxygen white dwarfs (WDs),
although the exact mechanism is debated. In some models helium accretion onto a
carbon-oxygen (CO) WD from a companion was suggested to dynamically trigger a
detonation of the accreted helium shell. The helium detonation then produces a
shock that after converging on itself close to the core of the CO-WD, triggers
a secondary carbon detonation and gives rise to an energetic explosion.
However, most studies of such scenarios have been done in one or two
dimensions, and/or did not consider self-consistent models for the accretion
and the He-donor. Here we make use of detailed 3D simulation to study the
interaction of a He-rich hybrid HeCO WD with a more
massive CO~WD. We find that accretion from the hybrid
WD onto the CO~WD gives rise to a helium detonation. However, the helium
detonation does not trigger a carbon detonation in the CO~WD. Instead, the
helium detonation burns through the accretion stream to also burn the helium
shell of the donor hybrid HeCO-WD. The detonation of its massive helium shell
then compresses its CO core, and triggers its detonation and full destruction.
The explosion gives rise to a faint, likely highly reddened transient,
potentially observable by the Vera Rubin survey, and the high-velocity () ejection of the heated surviving CO~WD companion.
Pending on uncertainties in stellar evolution we estimate the rate of such
transient to be up to of the rate of type Ia SNe.Comment: 14 pages, 10 figures, accepted by MNRAS, comments welcom
The demographics of neutron star - white dwarf mergers: rates, delay-time distributions and progenitors
The mergers of neutron stars (NSs) and white dwarfs (WDs) could give rise to
explosive transients, potentially observable with current and future transient
surveys. However, the expected properties and distribution of such events is
not well understood. Here we characterize the rates of such events, their delay
time distribution, their progenitors and the distribution of their properties.
We use binary populations synthesis models and consider a wide range of initial
conditions and physical processes. In particular we consider different
common-envelope evolution models and different NS natal kick distributions. We
provide detailed predictions arising from each of the models considered. We
find that the majority of NS-WD mergers are born in systems in which
mass-transfer played an important role, and the WD formed before the NS. For
the majority of the mergers the WDs have a carbon-oxygen composition (60-80%)
and most of the rest are with oxygen-neon WDs. The rates of NS-WD mergers are
in the range of 3-15% of the type Ia supernovae (SNe) rate. Their delay time
distribution is very similar to that of type Ia SNe, but slightly biased
towards earlier times. They typically explode in young 0.1-1Gyr environments,
but have a tail distribution extending to long, Gyrs-timescales. Models
including significant kicks give rise to relatively wide offset distribution
extending to hundreds of kpcs. The demographic and physical properties of NS-WD
mergers suggest they are likely to be peculiar type Ic-like SNe, mostly
exploding in late type galaxies. Their overall properties could be related to a
class of rapidly evolving SNe recently observed, while they are less likely to
be related to the class of Ca-rich SNe.Comment: updated version: accepted for publication in A&
Saying no to an European constitution? Dutch revolt, Enigma or pragmatism
FDR Trias Europea: de verhoudingen tussen de overheidsmachten in de EU en de lidstaten in een bewegend constitutioneel landschap -- ou
Performance-based financing as a health system reform : mapping the key dimensions for monitoring and evaluation
Peer reviewedPublisher PD
Using bundle embeddings to predict daily cortisol levels in human subjects
BACKGROUND: Many biological variables sampled from human subjects show a diurnal pattern, which poses special demands on the techniques used to analyze such data. Furthermore, most biological variables belong to nonlinear dynamical systems, which may make linear statistical techniques less suitable to analyze their dynamics. The current study investigates the usefulness of two analysis techniques based on nonlinear lagged vector embeddings: sequentially weighted global linear maps (SMAP), and bundle embeddings. METHODS: Time series of urinary cortisol were collected in 10 participants, in the morning ('night' measurement) and the evening ('day' measurement), resulting in 126 consecutive measurements. These time series were used to create lagged vector embeddings, which were split into 'night' and 'day' bundle embeddings. In addition, embeddings were created based on time series that were corrected for the average time-of-day (TOD) values. SMAP was used to predict future values of cortisol in these embeddings. Global (linear) and local (non-linear) predictions were compared for each embedding. Bootstrapping was used to obtain confidence intervals for the model parameters and the prediction error. RESULTS: The best cortisol predictions were found for the night bundle embeddings, followed by the full embeddings and the time-of-day corrected embeddings. The poorest predictions were found for the day bundle embeddings. The night bundle embeddings, the full embeddings and the TOD-corrected embeddings all showed low dimensions, indicating the absence of dynamical processes spanning more than one day. The dimensions of the day bundles were higher, indicating the presence of processes spanning more than one day, or a higher amount of noise. In the full embeddings, local models gave the best predictions, whereas in the bundles the best predictions were obtained from global models, indicating potential nonlinearity in the former but not the latter. CONCLUSIONS: Using a bundling approach on time series of cortisol may reveal differences between the predictions of night and day cortisol that are difficult to find with conventional time-series methods. Combination of this approach with SMAP may especially be useful when analyzing time-series data with periodic components
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