432 research outputs found
Iron Opacity and the Pulsar of Supernova 1987A
Neutron stars formed in Type II supernovae are likely to be initially
obscured by late-time fallback. Although much of the late-time fallback is
quickly accreted via neutrino cooling, some material remains on the neutron
star, forming an atmosphere which slowly accretes through photon emission. In
this paper, we derive structure equations of the fallback atmosphere and
present results of one-dimensional simulations of that fallback. The atmosphere
remaining after neutrino cooling becomes unimportant (less than the Compton
Eddington limit) is only a fraction of the total mass accreted (10^-8 of the
accreted mass or 10^-9 solar masses.) Recombined iron dominates the opacity in
the outer regions leading to an opacity 1000-10,000 times higher than that of
electron scattering alone. The resultant photon emission of the remnant
atmosphere is limited to 1/1000th the Compton Eddington Luminosity. The
late-time evolution of this system leads to the formation of a photon-driven
wind from the accretion of the inner portion of the atmosphere, leaving, for
most cases, a bare neutron star on timescales shorter than a year. The
degenerate remnant of 1987a may not be a black hole. Instead, the fallback
material may have already accreted or blown off in the accretion-driven wind.
If the neutron star has either a low magnetic field or a low rotational spin
frequency, we would not expect to see the neutron star remnant of 1987a.Comment: 15 pages text + 8 figures, accepted by Ap
Magnetorotational Instability in Liquid Metal Couette Flow
Despite the importance of the magnetorotational instability (MRI) as a
fundamental mechanism for angular momentum transport in magnetized accretion
disks, it has yet to be demonstrated in the laboratory. A liquid sodium
alpha-omega dynamo experiment at the New Mexico Institute of Mining and
Technology provides an ideal environment to study the MRI in a rotating metal
annulus (Couette flow). A local stability analysis is performed as a function
of shear, magnetic field strength, magnetic Reynolds number, and turbulent
Prandtl number. The later takes into account the minimum turbulence induced by
the formation of an Ekman layer against the rigidly rotating end walls of a
cylindrical vessel. Stability conditions are presented and unstable conditions
for the sodium experiment are compared with another proposed MRI experiment
with liquid gallium. Due to the relatively large magnetic Reynolds number
achievable in the sodium experiment, it should be possible to observe the
excitation of the MRI for a wide range of wavenumbers and further to observe
the transition to the turbulent state.Comment: 12 pages, 22 figures, 1 table. To appear in the Astrophysical Journa
Post-Pancreatoduodenectomy Outcomes and Epidural Analgesia: A 5-Year Single Institution Experience
Introduction
Optimal pain control post-pancreatoduodenectomy is a challenge. Epidural analgesia (EDA) is increasingly utilized despite inherent risks and unclear effects on outcomes.
Methods
All pancreatoduodenectomies (PD) performed from 1/2013-12/2017 were included. Clinical parameters were obtained from retrospective review of a prospective clinical database, the ACS NSQIP prospective institutional database and medical record review. Chi-Square/Fisher’s Exact and Independent-Samples t-Tests were used for univariable analyses; multivariable regression (MVR) was performed.
Results
671 consecutive PD from a single institution were included (429 EDA, 242 non-EDA). On univariable analysis, EDA patients experienced significantly less wound disruption (0.2% vs. 2.1%), unplanned intubation (3.0% vs. 7.9%), pulmonary embolism (0.5% vs. 2.5%), mechanical-ventilation >48hrs (2.1% vs. 7.9%), septic shock (2.6% vs. 5.8%), and lower pain scores. On MVR accounting for baseline group differences (gender, hypertension, pre-operative transfusion, labs, approach, pancreatic duct size), EDA was associated with less superficial wound infections (OR 0.34; CI 0.14-0.83; P=0.017), unplanned intubations (OR 0.36; CI 0.14-0.88; P=0.024), mechanical ventilation >48 hrs (OR 0.22; CI 0.08-0.62; P=0.004), and septic shock (OR 0.39; CI 0.15-1.00; P=0.050). EDA improved pain scores post-PD days 1-3 (P<0.001). No differences were seen in cardiac or renal complications; pancreatic fistula (B+C) or delayed gastric emptying; 30/90-day mortality; length of stay, readmission, discharge destination, or unplanned reoperation.
Conclusion
Based on the largest single institution series published to date, our data support the use of EDA for optimization of pain control. More importantly, our data document that EDA significantly improved infectious and pulmonary complications
What Can the Accretion Induced Collapse of White Dwarfs Really Explain?
The accretion induced collapse (AIC) of a white dwarf into a neutron star has
been invoked to explain gamma-ray bursts, Type Ia supernovae, and a number of
problematic neutron star populations and specific binary systems. The ejecta
from this collapse has also been claimed as a source of r-process
nucleosynthesis. So far, most AIC studies have focussed on determining the
event rates from binary evolution models and less attention has been directed
toward understanding the collapse itself. However, the collapse of a white
dwarf into a neutron star is followed by the ejection of rare neutron-rich
isotopes. The observed abundance of these chemical elements may set a more
reliable limit on the rate at which AICs have taken place over the history of
the galaxy.
In this paper, we present a thorough study of the collapse of a massive white
dwarf in 1- and 2-dimensions and determine the amount and composition of the
ejected material. We discuss the importance of the input physics (equation of
state, neutrino transport, rotation) in determining these quantities. These
simulations affirm that AICs are too baryon rich to produce gamm-ray bursts and
do not eject enough nickel to explain Type Ia supernovae (with the possible
exception of a small subclass of extremely low-luminosity Type Ias). Although
nucleosynthesis constraints limit the number of neutron stars formed via AICs
to <0.1% of the total galactic neutron star population, AICs remain a viable
scenario for forming systems of neutron stars which are difficult to explain
with Type II core-collapse supernovae.Comment: Latex File, aaspp4 style, 18 pages total (5 figures), accepted by Ap
Optical and near-IR observations of SN 1998bw
SN 1998bw, especially after the discovery of GRB 030329/SN 2003dh, seems to
be the equivalent of the Rosetta stone for the SN/GRB connection. In this paper
I review optical and near IR observations that have been carried out for this
uncanny object, which has probably confirmed suspicions and ideas originally
formulated in the early seventies of last century.Comment: 9 pages, 7 figures. Invited review to the IAU Colloquium n. 192,
SUPERNOVAE: ten years of SN 1993J, Valencia (Spain
Poynting Jets from Accretion Disks
The powerful narrow jets observed to emanate from many compact accreting
objects may arise from the twisting of a magnetic field threading a
differentially rotating accretion disk which acts to magnetically extract
angular momentum and energy from the disk. Two main regimes have been
discussed, {\it hydromagnetic outflows}, which have a significant mass flux and
have energy and angular momentum carried by both the matter and the
electromagnetic field and, Poynting outflows, where the mass flux is negligible
and energy and angular momentum are carried predominantly by the
electromagnetic field. Here we consider a Keplerian disk initially threaded by
a dipole-like magnetic field and we present solutions of the force-free
Grad-Shafranov equation for the coronal plasma. We find solutions with Poynting
jets where there is a continuous outflow of energy and toroidal magnetic flux
from the disk into the external space. This behavior contradicts the commonly
accepted ``theorem'' of Solar plasma physics that the motion of the footpoints
of a magnetic loop structure leads to a stationary magnetic field configuration
with zero power and flux outflows.
In addition we discuss recent magnetohydrodynamic (MHD) simulations which
establish that quasi-stationary collimated Poynting jets similar to our
Grad-Shafranov solutions arise from the inner part of a disk threaded by a
dipole-like magnetic field. At the same time we find that there is a steady
uncollimated hydromagnetic outflow from the outer part of the disk. The
Poynting jets represent a likely model for the jets from active galactic
nuclei, microquasars, and gamma ray burst sources.Comment: 7 pages, 2 figures, for Proc. of 20th Texas Symposiu
Collapsars - Gamma-Ray Bursts and Explosions in "Failed Supernovae"
Using a two-dimensional hydrodynamics code (PROMETHEUS), we study the
continued evolution of rotating massive helium stars whose iron core collapse
does not produce a successful outgoing shock, but instead forms a black hole.
We study the formation of a disk, the associated flow patterns, and the
accretion rate for disk viscosity parameter, alpha ~ 0.001 and 0.1. For the
standard 14 solar mass model the average accretion rate for 15 s is 0.07 solar
masses per second and the total energy deposited along the rotational axes by
neutrino annihilation is (1 - 14) x 10**51 erg, depending upon the evolution of
the Kerr parameter and uncertain neutrino efficiencies. Simulated deposition of
this energy in the polar regions results in strong relativistic outflow - jets
beamed to about 1.5% of the sky. The jets remain highly focused, and are
capable of penetrating the star in 5 - 10 s. After the jet breaks through the
surface of the star, highly relativistic flow can commence. Because of the
sensitivity of the mass ejection and jets to accretion rate, angular momentum,
and disk viscosity, and the variation of observational consequences with
viewing angle, a large range of outcomes is possible ranging from bright GRBs
like GRB 971214 to faint GRB-supernovae like SN 1998bw. X-ray precursors are
also possible as the jet first breaks out of the star. While only a small
fraction of supernovae make GRBs, we predict that all GRBs longer than a few
seconds will make supernovae similar to SN 1998bw. However, hard, energetic
GRBs shorter than a few seconds will be difficult to make in this model.Comment: Latex, 66 pages including 27 figures (9 color), Submitted to The
Astrophysical Journal, latex uses aaspp4.sty. Figures also available at
http://www.ucolick.org/~andre
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