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Research Summary - Theory Group
2001-2002
Studies regarding interstellar dust include a possible solution to
the G Dwarf problem as the effect of stellar atmosphere pollution by
interstellar dust material. The effects of interactions between dust
grains in the solar F corona and particles and fields of coronal mass
ejections was considered. Also studied was the coupling of dust grains
to interstellar gas through plasma wave emission from the spinning
dust particles.
White Dwarfs
Genetic algorithm models of pulsating white dwarfs were used to
put constraints on the rate of burning from carbon to oxygen,
one of the key nuclear reactions in advanced stellar evolution.
Supernovae
Sophisticated radiation hydrodynamic Models of Type Ia supernovae
were constructed to better understand the ignition, propagation,
and explosion of these supernova. Particular attention was paid to
models of near infrared emission to better constrain the explosion
physics and the use of this type of supernova to constrain the dark energy.
The nature of rotating, magnetic, collapsing stars was explored
with the conclusion that large magnetic fields could be
routinely created. These fields could, in turn, produce magnetic
jets that could explode the star.
Supermassive Black Holes
QSO emission-line widths were used to to examine the relation between
black hole mass and velocity dispersion as a function of redshift and
to extend the relationship to larger masses.
Gamma-ray bursts
The behavior was studied of "structured" bursts in which the collimated
flow of highly relativistic jets has a Lorentz factor that varies
with angle. Numerical models showed that structured jets tended to
expand laterally at slower than the sound speed. The rate of
decline of the afterglow light curve tends to steeper once the
core of the jet is fully observable despite Doppler beaming. Structured
jets tend to give better fits to the afterglow light curves of some
bursts.
Models of radiative acceleration were constructed to help establish
that GRB021004 represented the explosion of this gamma-ray burst
within the wind of a massive star.
Cosmology
Adaptive Smoothed Particle Hydrodynamics (ASPH), the new and improved
version of SPH, was applied to a number of cosmological problems.
Gas dynamical simulations were performed of the photoevaporation of cosmological
dark matter minihalos that were overtaken by the ionization fronts that swept
through the the intergalactic medium during reionization. The cosmological model
was one dominated by dark energy and dark matter. These computations included
the effects of radiative transfer.
In the standard Cold Dark Matter theory of structure formation,
virialized minihalos form in abundance at high redshift (z > 6), during
the cosmic "dark ages." Calculations showed that the hydrogen in these
minihalos, the first nonlinear baryonic structures to form in the universe,
is mostly neutral and sufficiently hot and dense to emit strongly at
the 21-cm line.
The gravitational lensing properties of cosmological halos was computed.
Using the multiple-lens plane algorithm, the effect of gravitational
lensing on light propagation for 43 different COBE-normalized Cold Dark
Matter models were computed, the largest cosmological parameter survey
ever done in this field.
A completely analytic treatment was made of cosmological fluctuations whose
wavelength is small enough to come within the horizon well before the energy
densities of matter and radiation become equal. This analysis yields a
simple formula for the conventional transfer function T(k) at large
wavenumber k, which agrees very well with computer calculations of T(k).
It also yields an explicit formula for the microwave background multipole
coefficient Cl at very large l.
Astrobiology
The scattering of incident X-ray and gamma-ray radiation as might
be typical of a stellar flare, supernova, or gamma-ray burst was
computed as it passed through an exoplanet atmosphere. Much of
the incident energy arrives at the ground in the form of biologically-active
auroral ultraviolet due to excitation of molecules by secondary
electrons.
Habitable zones were computed around the demographically common
dMe stars.
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