681 research outputs found
Flame Evolution During Type Ia Supernovae and the Deflagration Phase in the Gravitationally Confined Detonation Scenario
We develop an improved method for tracking the nuclear flame during the
deflagration phase of a Type Ia supernova, and apply it to study the variation
in outcomes expected from the gravitationally confined detonation (GCD)
paradigm. A simplified 3-stage burning model and a non-static ash state are
integrated with an artificially thickened advection-diffusion-reaction (ADR)
flame front in order to provide an accurate but highly efficient representation
of the energy release and electron capture in and after the unresolvable flame.
We demonstrate that both our ADR and energy release methods do not generate
significant acoustic noise, as has been a problem with previous ADR-based
schemes. We proceed to model aspects of the deflagration, particularly the role
of buoyancy of the hot ash, and find that our methods are reasonably
well-behaved with respect to numerical resolution. We show that if a detonation
occurs in material swept up by the material ejected by the first rising bubble
but gravitationally confined to the white dwarf (WD) surface (the GCD
paradigm), the density structure of the WD at detonation is systematically
correlated with the distance of the deflagration ignition point from the center
of the star. Coupled to a suitably stochastic ignition process, this
correlation may provide a plausible explanation for the variety of nickel
masses seen in Type Ia Supernovae.Comment: 14 pages, 10 figures, accepted to the Astrophysical Journa
Theoretical Modeling of the Thermal State of Accreting White Dwarfs Undergoing Classical Novae
White dwarfs experience a thermal renaissance when they receive mass from a
stellar companion in a binary. For accretion rates < 10^-8 Msun/yr, the freshly
accumulated hydrogen/helium envelope ignites in a thermally unstable manner
that results in a classical novae (CN) outburst and ejection of material. We
have undertaken a theoretical study of the impact of the accumulating envelope
on the thermal state of the underlying white dwarf (WD). This has allowed us to
find the equilibrium WD core temperatures (T_c), the classical nova ignition
masses (M_ign) and the thermal luminosities for WDs accreting at rates of
10^-11 - 10^-8 Msun/yr. These accretion rates are most appropriate to WDs in
cataclysmic variables (CVs) of P_orb <~ 7 hr, many of which accrete
sporadically as dwarf novae. We have included ^3He in the accreted material at
levels appropriate for CVs and find that it significantly modifies the CN
ignition mass. We compare our results with several others from the CN
literature and find that the inclusion of ^3He leads to lower M_ign for
>~ 10^-10 Msun/yr, and that for below this the particular author's
assumption concerning T_c, which we calculate consistently, is a determining
factor. Initial comparisons of our CN ignition masses with measured ejected
masses find reasonable agreement and point to ejection of material comparable
to that accreted.Comment: 14 pages, 11 figures; uses emulateapj; accepted by the Astrophysical
Journal; revised for clarity, added short discussion of diffusio
The Detonation Mechanism of the Pulsationally-Assisted Gravitationally-Confined Detonation Model of Type Ia Supernovae
We describe the detonation mechanism comprising the "Pulsationally Assisted"
Gravitationally Confined Detonation (GCD) model of Type Ia supernovae SNe Ia.
This model is analogous to the previous GCD model reported in Jordan et
al.(2008); however, the chosen initial conditions produce a substantively
different detonation mechanism, resulting from a larger energy release during
the deflagration phase. The resulting final kinetic energy and nickel-56 yields
conform better to observational values than is the case for the "classical" GCD
models. In the present class of models, the ignition of a deflagration phase
leads to a rising, burning plume of ash. The ash breaks out of the surface of
the white dwarf, flows laterally around the star, and converges on the
collision region at the antipodal point from where it broke out. The amount of
energy released during the deflagration phase is enough to cause the star to
rapidly expand, so that when the ash reaches the antipodal point, the surface
density is too low to initiate a detonation. Instead, as the ash flows into the
collision region (while mixing with surface fuel), the star reaches its
maximally expanded state and then contracts. The stellar contraction acts to
increase the density of the star, including the density in the collision
region. This both raises the temperature and density of the fuel-ash mixture in
the collision region and ultimately leads to thermodynamic conditions that are
necessary for the Zel'dovich gradient mechanism to produce a detonation. We
demonstrate feasibility of this scenario with three 3-dimensional (3D), full
star simulations of this model using the FLASH code. We characterized the
simulations by the energy released during the deflagration phase, which ranged
from 38% to 78% of the white dwarf's binding energy. We show that the necessary
conditions for detonation are achieved in all three of the models.Comment: 22 pages, 8 figures; Ap
Predictive policing in an Australian context: assessing viability and utility
Studies in the United States and Europe have demonstrated that burglary and vehicle crime exhibit consistent patterns, supporting the application of crime prediction techniques to proactively deploy police resources to reduce incidents of crime. Research into whether these techniques are applicable in an Australian context is currently limited.
Using crime data from the Queensland Police Service, this study assessed the presence of spatio-temporal patterns in burglary, theft of motor vehicle and theft from motor vehicle offences in three distinct local government areas. After establishing the presence of spatiotemporal clustering, the forecasting performance of two predictive algorithms and a retrospective crime mapping technique was evaluated.
Forecasting performance varied across study regions; however, the prediction algorithms performed as well as or better than the retrospective method, while using less data. The next step in evaluating predictive policing within Australia is to consider and design effective tactical responses to prevent crime based on the forecast locations and identified patterns
X-Ray Emission from Young Stars in the Massive Star Forming Region IRAS 20126+4104
We present a ks Chandra observation of the IRAS20126+4104 core
region. In the inner two X-ray sources were detected, which
are coincident with the radio jet source I20S and the variable radio source
I20Var. No X-ray emission was detected from the nearby massive protostar I20N.
The spectra of both detected sources are hard and highly absorbed, with no
emission below keV.
For I20S, the measured keV count rate was ctsks.
The X-ray spectrum was fit with an absorbed 1T APEC model with an energy of
kTkeV and an absorbing column of Ncm.
An unabsorbed X-ray luminosity of about ergs
was estimated. The spectrum shows broad line emission between 6.4 and 6.7\,
keV, indicative of emission from both neutral and highly ionized iron. The
X-ray lightcurve indicates that I20S is marginally variable; however, no flare
emission was observed.
The variable radio source I20Var was detected with a count rate of
ctsks but there was no evidence of X-ray variability. The
best fit spectral model is a 1T APEC model with an absorbing hydrogen column of
Ncm and a plasma energy of kT = 6.0keV.
The unabsorbed X-ray luminosity is about ergs.Comment: 17pages, 4 figures to appear in Astronomical Journa
A Chandra/ACIS Study of 30 Doradus I. Superbubbles and Supernova Remnants
We present an X-ray tour of diffuse emission in the 30 Doradus star-forming
complex in the Large Magellanic Cloud using high-spatial-resolution X-ray
images and spatially-resolved spectra obtained with the Advanced CCD Imaging
Spectrometer aboard the Chandra X-ray Observatory. The dominant X-ray feature
of the 30 Doradus nebula is the intricate network of diffuse emission generated
by interacting stellar winds and supernovae working together to create vast
superbubbles filled with hot plasma. We construct maps of the region showing
variations in plasma temperature (T = 3--9 million degrees), absorption (N_H =
1--6 x 10^{21} cm^{-2}), and absorption-corrected X-ray surface brightness (S_X
= 3--126 x 10^{31} ergs s^{-1} pc^{-2}). Enhanced images reveal the pulsar wind
nebula in the composite supernova remnant N157B and the Chandra data show
spectral evolution from non-thermal synchrotron emission in the N157B core to a
thermal plasma in its outer regions. In a companion paper we show that R136,
the central massive star cluster, is resolved at the arcsecond level into
almost 100 X-ray sources. Through X-ray studies of 30 Doradus the complete life
cycle of such a massive stellar cluster can be revealed.Comment: 42 pages, 15 bitmapped figures, 4 tables; accepted to A
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