2,293 research outputs found
FBOTs and AT2018cow following electron-capture collapse of merged white dwarfs
We suggest that fast-rising blue optical transients (FBOTs) and the brightest
event of the class AT2018cow result from an electron-capture collapse to a \NS\
following a merger of a massive ONeMg white dwarf (WD) with another WD. Two
distinct evolutionary channels lead to the disruption of the less massive WD
during the merger and the formation of a shell burning non-degenerate star
incorporating the ONeMg core. During the shell burning stage a large fraction
of the envelope is lost to the wind, while mass and angular momentum are added
to the core. As a result, the electron-capture collapse occurs with a small
envelope mass, after years. During the formation of a neutron
star as little as of the material is ejected at the
bounce-off with mildly relativistic velocities and total energy few ergs. This ejecta becomes optically thin on a time scale of days -
this is the FBOT. During the collapse, the neutron star is spun up and magnetic
field is amplified. The ensuing fast magnetically-dominated relativistic wind
from the newly formed neutron star shocks against the ejecta, and later against
the wind. The radiation-dominated forward shock produces the long-lasting
optical afterglow, while the termination shock of the relativistic wind
produces the high energy emission in a manner similar to Pulsar Wind Nebulae.
If the secondary WD was of the DA type, the wind will likely have of hydrogen; this explains the appearance of hydrogen late in the
afterglow spectrum. The model explains many of the puzzling properties of
FBOTs/AT2018cow: host galaxies, a fast and light anisotropic ejecta producing a
bright optical peak, afterglow high energy emission of similar luminosity to
the optical, and late infra-red features.Comment: arXiv admin note: text overlap with arXiv:1709.0222
The Evolution of the Type Ia Supernova Luminosity Function
Type Ia supernovae (SNe Ia) exhibit a wide diversity of peak luminosities and
light curve shapes: the faintest SNe Ia are 10 times less luminous and evolve
more rapidly than the brightest SNe Ia. Their differing characteristics also
extend to their stellar age distributions, with fainter SNe Ia preferentially
occurring in old stellar populations and vice versa. In this Letter, we
quantify this SN Ia luminosity - stellar age connection using data from the
Lick Observatory Supernova Search (LOSS). Our binary population synthesis
calculations agree qualitatively with the observed trend in the >1 Gyr-old
populations probed by LOSS if the majority of SNe Ia arise from prompt
detonations of sub-Chandrasekhar mass white dwarfs (WDs) in double WD systems.
Under appropriate assumptions, we show that double WD systems with less massive
primaries, which yield fainter SNe Ia, interact and explode at older ages than
those with more massive primaries. We find that prompt detonations in double WD
systems are capable of reproducing the observed evolution of the SN Ia
luminosity function, a constraint that any SN Ia progenitor scenario must
confront.Comment: Accepted for publication in ApJL. Minor changes to previous version
for clarity. Data used to construct the observational CDFs in Figure 4 are
available in an ancillary fil
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
Binary black hole mergers from field triples: properties, rates and the impact of stellar evolution
We consider the formation of binary black hole mergers through the evolution
of field massive triple stars. In this scenario, favorable conditions for the
inspiral of a black hole binary are initiated by its gravitational interaction
with a distant companion, rather than by a common-envelope phase invoked in
standard binary evolution models. We use a code that follows self-consistently
the evolution of massive triple stars, combining the secular triple dynamics
(Lidov-Kozai cycles) with stellar evolution. After a black hole triple is
formed, its dynamical evolution is computed using either the orbit-averaged
equations of motion, or a high-precision direct integrator for triples with
weaker hierarchies for which the secular perturbation theory breaks down. Most
black hole mergers in our models are produced in the latter non-secular
dynamical regime. We derive the properties of the merging binaries and compute
a black hole merger rate in the range (0.3- 1.3) Gpc^{-3}yr^{-1}, or up to
~2.5Gpc^{-3}yr^{-1} if the black hole orbital planes have initially random
orientation. Finally, we show that black hole mergers from the triple channel
have significantly higher eccentricities than those formed through the
evolution of massive binaries or in dense star clusters. Measured
eccentricities could therefore be used to uniquely identify binary mergers
formed through the evolution of triple stars. While our results suggest up to
~10 detections per year with Advanced-LIGO, the high eccentricities could
render the merging binaries harder to detect with planned space based
interferometers such as LISA.Comment: Accepted for publication in ApJ. 10 pages, 6 figure
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&
Detecting hierarchical stellar systems with LISA
A significant fraction of stars are members of gravitationally bound
hierarchies containing three or more components. Almost all low mass stars in
binaries with periods shorter three days are part of a hierarchical system. We
therefore anticipate that a large fraction of compact galactic binaries
detected by the Laser Interferometer Space Antenna (LISA) will be members of
hierarchical triple or quadruple system. The acceleration imparted by the
hierarchical companions can be detected in the gravitational wave signal for
outer periods as large as 100 years. For systems with periods that are shorter
than, or comparable to, the mission lifetime, it will be possible to measure
the period and eccentricity of the outer orbit. LISA observations of
hierarchical stellar systems will provide insight into stellar evolution,
including the role that Kozai-Lidov oscillations play in driving systems
towards merger.Comment: 15 pages, 14 figure
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