88 research outputs found
A Binary Scenario for the Formation of Strongly Magnetized White Dwarfs
Since their initial discovery, the origin of isolated white dwarfs (WDs) with
magnetic fields in excess of 1 MG has remained a mystery. Recently, the
formation of these high-field magnetic WDs has been observationally linked to
strong binary interactions incurred during post-main-sequence evolution.
Planetary, brown dwarf or stellar companions located within a few AU of
main-sequence stars may become engulfed during the primary's expansion off the
main sequence. Sufficiently low-mass companions in-spiral inside a common
envelope until they are tidally shredded near the natal white dwarf. Formation
of an accretion disk from the disrupted companion provides a source of
turbulence and shear which act to amplify magnetic fields and transport them to
the WD surface. We show that these disk-generated fields explain the observed
range of magnetic field strengths for isolated, high-field magnetic WDs.
Additionally, we discuss a high-mass binary analogue which generates a
strongly-magnetized WD core inside a pre-collapse, massive star. Subsequent
core-collapse to a neutron star may produce a magnetar.Comment: To appear in the Proceedings of the 2nd International Symposium on
Strong Electromagnetic Fields and Neutron Stars, Varadero, Cub
Dimensional Dependence of the Hydrodynamics of Core-Collapse Supernovae
The multidimensional character of the hydrodynamics in core-collapse
supernova (CCSN) cores is a key facilitator of explosions. Unfortunately, much
of this work has necessarily been performed assuming axisymmetry and it remains
unclear whether or not this compromises those results. In this work, we present
analyses of simplified two- and three-dimensional CCSN models with the goal of
comparing the multidimensional hydrodynamics in setups that differ only in
dimension. Not surprisingly, we find many differences between 2D and 3D models.
While some differences are subtle and perhaps not crucial to understanding the
explosion mechanism, others are quite dramatic and make interpreting 2D CCSN
models problematic. In particular, we find that imposing axisymmetry
artificially produces excess power at the largest spatial scales, power that
has been deemed critical in the success of previous explosion models and has
been attributed solely to the standing accretion shock instability.
Nevertheless, our 3D models, which have an order of magnitude less power on
large scales compared to 2D models, explode earlier. Since we see explosions
earlier in 3D than in 2D, the vigorous sloshing associated with the large scale
power in 2D models is either not critical in any dimension or the explosion
mechanism operates differently in 2D and 3D. Possibly related to the earlier
explosions in 3D, we find that about 25% of the accreted material spends more
time in the gain region in 3D than in 2D, being exposed to more integrated
heating and reaching higher peak entropies, an effect we associate with the
differing characters of turbulence in 2D and 3D. Finally, we discuss a simple
model for the runaway growth of buoyant bubbles that is able to quantitatively
account for the growth of the shock radius and predicts a critical luminosity
relation.Comment: Submitted to the Astrophysical Journa
Induced Rotation in 3D Simulations of Core Collapse Supernovae: Implications for Pulsar Spins
It has been suggested that the observed rotation periods of radio pulsars
might be induced by a non-axisymmetric spiral-mode instability in the turbulent
region behind the stalled supernova bounce shock, even if the progenitor core
was not initially rotating. In this paper, using the three-dimensional AMR code
CASTRO with a realistic progenitor and equation of state and a simple neutrino
heating and cooling scheme, we present a numerical study of the evolution in 3D
of the rotational profile of a supernova core from collapse, through bounce and
shock stagnation, to delayed explosion. By the end of our simulation (420
ms after core bounce), we do not witness significant spin up of the
proto-neutron star core left behind. However, we do see the development before
explosion of strong differential rotation in the turbulent gain region between
the core and stalled shock. Shells in this region acquire high spin rates that
reach Hz, but this region contains too little mass and angular
momentum to translate, even if left behind, into rapid rotation for the full
neutron star. We find also that much of the induced angular momentum is likely
to be ejected in the explosion, and moreover that even if the optimal amount of
induced angular momentum is retained in the core, the resulting spin period is
likely to be quite modest. Nevertheless, induced periods of seconds are
possible.Comment: Accepted to the Astrophysical Journa
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