15,744 research outputs found
Hydrogen-Poor Circumstellar Shells from Pulsational Pair-Instability Supernovae with Rapidly Rotating Progenitors
In certain mass ranges, massive stars can undergo a violent pulsation
triggered by the electron/positron pair instability that ejects matter, but
does not totally disrupt the star. After one or more of these pulsations, such
stars are expected to undergo core-collapse to trigger a supernova explosion.
The mass range susceptible to this pulsational phenomena may be as low as 50-70
Msun if the progenitor is of very low metallicity and rotating sufficiently
rapidly to undergo nearly homogeneous evolution. The mass, dynamics, and
composition of the matter ejected in the pulsation are important aspects to
determine the subsequent observational characteristics of the explosion. We
examine the dynamics of a sample of stellar models and rotation rates and
discuss the implications for the first stars, for LBV-like phenomena, and for
superluminous supernovae. We find that the shells ejected by pulsational
pair-instability events with rapidly rotating progenitors (>30% the critical
value) are hydrogen-poor and helium and oxygen-rich.Comment: 14 pages, 2 figure
Launch vehicle wind and turbulence response by nonstationary statistical methods
Nonstationary adjoint algorithm for determining launch vehicle flight loads due to winds and turbulenc
Multidimensional Simulations of Rotating Pair Instability Supernovae
We study the effects of rotation on the dynamics, energetics and Ni-56
production of Pair Instability Supernova explosions by performing rotating
two-dimensional ("2.5-D") hydrodynamics simulations. We calculate the evolution
of eight low metallicity (Z = 10^-3, 10^-4 Zsun) massive (135-245 Msun) PISN
progenitors with initial surface rotational velocities 50% that of the critical
Keplerian value using the stellar evolution code MESA. We allow for both the
inclusion and the omission of the effects of magnetic fields in the angular
momentum transport and in chemical mixing, resulting in slowly-rotating and
rapidly-rotating final carbon-oxygen cores, respectively. Increased rotation
for carbon-oxygen cores of the same mass and chemical stratification leads to
less energetic PISN explosions that produce smaller amounts of Ni-56 due to the
effect of the angular momentum barrier that develops and slows the dynamical
collapse. We find a non-monotonic dependence of Ni-56 production on rotational
velocity in situations when smoother composition gradients form at the outer
edge of the rotating cores. In these cases, the PISN energetics are determined
by the competition of two factors: the extent of chemical mixing in the outer
layers of the core due to the effects of rotation in the progenitor evolution
and the development of angular momentum support against collapse. Our 2.5-D
PISN simulations with rotation are the first presented in the literature. They
reveal hydrodynamic instabilities in several regions of the exploding star and
increased explosion asymmetries with higher core rotational velocity.Comment: 31 pages, 23 figures, accepted for publication in the Ap
Hidden Dirac Monopoles
Dirac showed that the existence of one magnetic pole in the universe could
offer an explanation of the discrete nature of the electric charge. Magnetic
poles appear naturally in most grand unified theories. Their discovery would be
of greatest importance for particle physics and cosmology. The intense
experimental search carried thus far has not met with success. I proposed a
universe with magnetic poles which are not observed free because they hide in
deeply bound monopole--anti-monopole states named monopolium. I discuss the
realization of this proposal and its consistency with known cosmological
features. I furthermore analyze its implications and the experimental
signatures that confirm the scenario.Comment: Comments: 15 pages, 3 figure
Phase field modeling of electrochemistry II: Kinetics
The kinetic behavior of a phase field model of electrochemistry is explored
for advancing (electrodeposition) and receding (electrodissolution) conditions
in one dimension. We described the equilibrium behavior of this model in [J. E.
Guyer, W. J. Boettinger, J.A. Warren, and G. B. McFadden, ``Phase field
modeling of electrochemistry I: Equilibrium'', cond-mat/0308173]. We examine
the relationship between the parameters of the phase field method and the more
typical parameters of electrochemistry. We demonstrate ohmic conduction in the
electrode and ionic conduction in the electrolyte. We find that, despite making
simple, linear dynamic postulates, we obtain the nonlinear relationship between
current and overpotential predicted by the classical ``Butler-Volmer'' equation
and observed in electrochemical experiments. The charge distribution in the
interfacial double layer changes with the passage of current and, at
sufficiently high currents, we find that the diffusion limited deposition of a
more noble cation leads to alloy deposition with less noble species.Comment: v3: To be published in Phys. Rev. E v2: Attempt to work around
turnpage bug. Replaced color Fig. 4a with grayscale 13 pages, 7 figures in 10
files, REVTeX 4, SIunits.sty, follows cond-mat/030817
The Role of Kinetic Energy Flux in the Convective Urca Process
The previous analysis of the convective Urca neutrino loss process in
degenerate, convective, quasi-static, carbon-burning cores by Barkat and
Wheeler omitted specific consideration of the role of the kinetic energy flux.
The arguments of Barkat and Wheeler that steady-state composition gradients
exist are correct, but chemical equilibrium does not result in net cooling.
Barkat and Wheeler included a "work" term that effectively removed energy from
the total energy budget that could only have come from the kinetic energy,
which must remain positive. Consideration of the kinetic energy in the
thermodynamics of the convective Urca process shows that the convective Urca
neutrinos reduce the rate of increase of entropy that would otherwise be
associated with the input of nuclear energy and slow down the convective
current, but, unlike the "thermal" Urca process do not reduce the entropy or
temperature.Comment: 16 pages, AAS LaTex, in press, Astrophysical Journal, September 20,
Vol 52
Uncertainty Relations for Positive Operator Valued Measures
How much unavoidable randomness is generated by a Positive Operator Valued
Measure (POVM)? We address this question using two complementary approaches.
First we study the variance of a real variable associated to the POVM outcomes.
In this context we introduce an uncertainty operator which measures how much
additional noise is introduced by carrying out a POVM rather than a von Neumann
measurement. We illustrate this first approach by studying the variances of
joint estimates of \sigma_x and \sigma_z for spin 1/2 particles. We show that
for unbiased measurements the sum of these variances is lower bounded by 1. In
our second approach we study the entropy of the POVM outcomes. In particular we
try to establish lower bounds on the entropy of the POVM outcomes. We
illustrate this second approach by examples.Comment: 5 pages, minor modifications and clarification
Extreme Supernova Models for the Superluminous Transient ASASSN-15lh
The recent discovery of the unprecedentedly superluminous transient
ASASSN-15lh (or SN 2015L) with its UV-bright secondary peak challenges all the
power-input models that have been proposed for superluminous supernovae. Here
we examine some of the few viable interpretations of ASASSN-15lh in the context
of a stellar explosion, involving combinations of one or more power inputs. We
model the lightcurve of ASASSN-15lh with a hybrid model that includes
contributions from magnetar spin-down energy and hydrogen-poor circumstellar
interaction. We also investigate models of pure circumstellar interaction with
a massive hydrogen-deficient shell and discuss the lack of interaction features
in the observed spectra. We find that, as a supernova ASASSN-15lh can be best
modeled by the energetic core-collapse of a ~40 Msun star interacting with a
hydrogen-poor shell of ~20 Msun. The circumstellar shell and progenitor mass
are consistent with a rapidly rotating pulsational pair-instability supernova
progenitor as required for strong interaction following the final supernova
explosion. Additional energy injection by a magnetar with initial period of 1-2
ms and magnetic field of 0.1-1 x 10^14 G may supply the excess luminosity
required to overcome the deficit in single-component models, but this requires
more fine-tuning and extreme parameters for the magnetar, as well as the
assumption of efficient conversion of magnetar energy into radiation. We thus
favor a single-input model where the reverse shock formed in a strong SN
ejecta-CSM interaction following a very powerful core-collapse SN explosion can
supply the luminosity needed to reproduce the late-time UV-bright plateau.Comment: 8 pages, 3 figure
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