895 research outputs found
Quantum and Classical Dynamics of a BEC in a Large-Period Optical Lattice
We experimentally investigate diffraction of a Rb-87 Bose-Einstein condensate
from a 1D optical lattice. We use a range of lattice periods and timescales,
including those beyond the Raman-Nath limit. We compare the results to quantum
mechanical and classical simulations, with quantitative and qualitative
agreement, respectively. The classical simulation predicts that the envelope of
the time-evolving diffraction pattern is shaped by caustics: singularities in
the phase space density of classical trajectories. This behavior becomes
increasingly clear as the lattice period grows.Comment: 7 pages, 6 figure
Carbon-Oxygen White Dwarfs Accreting CO-Rich Matter I: A Comparison Between Rotating and Non-Rotating Models
We investigate the lifting effect of rotation on the thermal evolution of CO
WDs accreting CO-rich matter. We find that rotation induces the cooling of the
accreting star so that the delivered gravitational energy causes a greater
expansion with respect to the standard non-rotating case. The increase in the
surface radius produces a decrease in the surface value of the critical angular
velocity and, therefore, the accreting WD becomes gravitationally unbound
(Roche instability). This occurrence is due to an increase in the total angular
momentum of the accreting WD and depends critically on the amount of specific
angular momentum deposited by the accreted matter. If the specific angular
momentum of the accreted matter is equal to that of the outer layers of the
accreting structure, the Roche instability occurs well before the accreting WD
can attain the physical conditions for C-burning. If the values of both initial
angular velocity and accretion rate are small, we find that the accreting WD
undergoes a secular instability when its total mass approaches 1.4 Msun. At
this stage, the ratio between the rotational and the gravitational binding
energy of the WD becomes of the order of 0.1, so that the star must deform by
adopting an elliptical shape. In this case, since the angular velocity of the
WD is as large as 1 rad/s, the anisotropic mass distribution induces the loss
of rotational energy and angular momentum via GWR. We find that, independent of
the braking efficiency, the WD contracts and achieves the physical conditions
suitable for explosive C-burning at the center so that a type Ia supernova
event is produced.Comment: 39 pages, 22 eps-figures; accepted for publication in Astrophysical
Journa
An upper limit on the contribution of accreting white dwarfs to the type Ia supernova rate
There is wide agreement that Type Ia supernovae (used as standard candles for
cosmology) are associated with the thermonuclear explosions of white dwarf
stars. The nuclear runaway that leads to the explosion could start in a white
dwarf gradually accumulating matter from a companion star until it reaches the
Chandrasekhar limit, or could be triggered by the merger of two white dwarfs in
a compact binary system. The X-ray signatures of these two possible paths are
very different. Whereas no strong electromagnetic emission is expected in the
merger scenario until shortly before the supernova, the white dwarf accreting
material from the normal star becomes a source of copious X-rays for ~1e7 yr
before the explosion. This offers a means of determining which path dominates.
Here we report that the observed X-ray flux from six nearby elliptical galaxies
and galaxy bulges is a factor of ~30-50 less than predicted in the accretion
scenario, based upon an estimate of the supernova rate from their K-band
luminosities. We conclude that no more than ~5 per cent of Type Ia supernovae
in early type galaxies can be produced by white dwarfs in accreting binary
systems, unless their progenitors are much younger than the bulk of the stellar
population in these galaxies, or explosions of sub-Chandrasekhar white dwarfs
make a significant contribution to the supernova rate.Comment: 10 pages, 1 tabl
Tycho Brahe's 1572 supernova as a standard type Ia explosion revealed from its light echo spectrum
Type Ia supernovae (SNe Ia) are thermonuclear explosions of white dwarf stars
in close binary systems. They play an important role as cosmological distance
indicators and have led to the discovery of the accelerated expansion of the
Universe. Among the most important unsolved questions are how the explosion
actually proceeds and whether accretion occurs from a companion or via the
merging of two white dwarfs. Tycho Brahe's supernova of 1572 (SN 1572) is
thought to be one of the best candidates for a SN Ia in the Milky Way. The
proximity of the SN 1572 remnant has allowed detailed studies, such as the
possible identification of the binary companion, and provides a unique
opportunity to test theories of the explosion mechanism and the nature of the
progenitor. The determination of the yet unknown exact spectroscopic type of SN
1572 is crucial to relate these results to the diverse population of SNe Ia.
Here we report an optical spectrum of Tycho Brahe's supernova near maximum
brightness, obtained from a scattered-light echo more than four centuries after
the direct light of the explosion swept past Earth. We find that SN 1572
belongs to the majority class of normal SNe Ia. The presence of a strong Ca II
IR feature at velocities exceeding 20,000 km/s, which is similar to the
previously observed polarized features in other SNe Ia, suggests asphericity in
SN 1572.Comment: 15 pages, 3 figures - accepted for publication in Natur
Modeling core collapse supernovae in 2 and 3 dimensions with spectral neutrino transport
The overwhelming evidence that the core collapse supernova mechanism is
inherently multidimensional, the complexity of the physical processes involved,
and the increasing evidence from simulations that the explosion is marginal
presents great computational challenges for the realistic modeling of this
event, particularly in 3 spatial dimensions. We have developed a code which is
scalable to computations in 3 dimensions which couples PPM Lagrangian with
remap hydrodynamics [1], multigroup, flux-limited diffusion neutrino transport
[2], with many improvements), and a nuclear network [3]. The neutrino transport
is performed in a ray-by-ray plus approximation wherein all the lateral effects
of neutrinos are included (e.g., pressure, velocity corrections, advection)
except the transport. A moving radial grid option permits the evolution to be
carried out from initial core collapse with only modest demands on the number
of radial zones. The inner part of the core is evolved after collapse along
with the rest of the core and mantle by subcycling the lateral evolution near
the center as demanded by the small Courant times. We present results of 2-D
simulations of a symmetric and an asymmetric collapse of both a 15 and an 11 M
progenitor. In each of these simulations we have discovered that once the
oxygen rich material reaches the shock there is a synergistic interplay between
the reduced ram pressure, the energy released by the burning of the shock
heated oxygen rich material, and the neutrino energy deposition which leads to
a revival of the shock and an explosion.Comment: 10 pages, 3 figure
A Minimum Dilution Scenario for Supernovae and Consequences for Extremely Metal-Poor Stars
© 2020 The Author(s) 2020 Published by Oxford University Press on behalf of the Royal Astronomical Society.To date no metal-free stars have been identified by direct observations. The most common method of constraining their properties is searching the spectra of the most metal-poor stars for the chemical elements created in the first stars and their supernova (SN). In this approach, modelled SN yields are compared to the observed abundance patterns in extremely metal-poor stars. The method typically only uses the abundance ratios, i.e. the yields are diluted to the observed level. Following the usual assumption of spherical symmetry we compute a simple lower limit of the mass an SN can mix with and find that it is consistent with all published simulations of early chemical enrichment in the interstellar medium. For three different cases, we demonstrate that this dilution limit can change the conclusions from the abundance fitting. There is a large discrepancy between the dilution found in simulations of SN explosions in minihaloes and the dilution assumed in many abundance fits. Limiting the dilution can significantly alter the likelihood of which supernovae are possible progenitors of observed CEMP-no stars. In particular, some of the faint, very low yield SNe, which have been suggested as models for the abundance pattern of SMSS0313-6708, cannot explain the measured metal abundances, as their predicted metal yields are too small by two orders of magnitude. Altogether, the new dilution model presented here emphasizes the need to better understand the mixing and dilution behaviour of aspherical SNe.Peer reviewedFinal Accepted Versio
Consistent alpha-cluster description of the 12C (0^+_2) resonance
The near-threshold 12C (0^+_2) resonance provides unique possibility for fast
helium burning in stars, as predicted by Hoyle to explain the observed
abundance of elements in the Universe. Properties of this resonance are
calculated within the framework of the alpha-cluster model whose two-body and
three-body effective potentials are tuned to describe the alpha - alpha
scattering data, the energies of the 0^+_1 and 0^+_2 states, and the
0^+_1-state root-mean-square radius. The extremely small width of the 0^+_2
state, the 0_2^+ to 0_1^+ monopole transition matrix element, and transition
radius are found in remarkable agreement with the experimental data. The
0^+_2-state structure is described as a system of three alpha-particles
oscillating between the ground-state-like configuration and the elongated chain
configuration whose probability exceeds 0.9
Evolution of Low-Mass Population III Stars
We present the evolutionary models of metal-free stars in the mass range from
0.8 to 1.2 Msun with up-to-date input physics. The evolution is followed to the
onset of hydrogen mixing into a convection, driven by the helium flash at red
giant or asymptotic giant branch phase.
The models of mass M >= 0.9 Msun undergo the central hydrogen flash,
triggered by the carbon production due to the 3-alpha reactions.
We find that the border of the off-center and central ignition of helium core
flash falls between 1.1 and 1.2 Msun; the models of mass M <= 1.1 Msun
experience the hydrogen mixing at the tip of red giant branch while the models
of M = 1.2 Msun during the helium shell flashes on the asymptotic giant branch.
The equation of state for the Coulomb liquid region, where electron
conduction and radiation compete, is shown to be important since it affects the
thermal state in the helium core and influences the red giant branch evolution.
It is also found that the non-resonant term of 3-alpha reactios plays an
important role, although it has negligible effect in the evolution of stars of
younger populations.
We compare our models with the computations by several other sets of authors,
to confirm the good agreement except for one study which finds the helium
ignition much closer to the center with consequences important for subsequent
evolution.Comment: 23 pages, 6 figures, 2 tables, accepted for Ap
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