Astrophysics Science Division Colloquium Series
Schedule: October - December 2006
Astrophysics Science Division Colloquium Series
Schedule: October - December 2006
Through the courtesy of the speakers since 2004,
most presentations are available on line.
Future schedules:
|
Past schedules:
2006, Second Quarter
2006, First Quarter
2005, Fourth Quarter
2005, Third Quarter
2005, Second Quarter
2005, First Quarter
2004, Fourth Quarter
2004, Third Quarter
2004, Second Quarter
2004, First Quarter
2003, Fourth Quarter
|
Time: 3:45 pm (Meet the Speaker at 3:30 pm) -
Location: Bldg 21, Room 183 -
unless otherwise noted.
To view the abstract of a seminar, click on the title.
Resolving the Formation of Protogalaxies
John Wise
Stanford
Thursday, October 12, 2006
Time - 3:45pm; Location - Bldg 2/Rm 8
Abstract
Numerous cosmological hydrodynamic studies have addressed the
formation of galaxies. Here we expand the standard model of galaxy
formation to include molecular hydrogen cooling and primordial stellar
feedback with a suite of cosmological Eulerian adaptive mesh
refinement simulations that resolve the Jeans length by at least 16
cells. We gradually introduce molecular hydrogen cooling, radiative
transfer, metal enrichment, and radiative backgrounds to determine the
importance of each process. In simulations that consider the standard
galaxy formation model with only hydrogen and helium cooling,
gravitationally unstable central objects with masses more than 10^5
solar masses within a radius of 1 pc form within dark matter halos
~10^8 solar masses. These cores do not fragment down in sub-solar
scales and could form a massive black hole. We also observe that no
rotationally supported disk forms before this central collapse. Then
we introduce molecular hydrogen cooling while suppressing the residual
electron fraction in order to restrict our focus to protogalaxies.
Here protogalactic halos with masses greater than ~8 x 10^6 solar
masses at z ~ 20 can cool and collapse. Due to the exponential nature
of Press-Schechter formalism, this corresponds to an order of
magnitude increase in protogalaxy number density at z = 20, and galaxy
formation may start earlier than previously thought. Next we consider
radiative feedback from primordial stars using time dependent adaptive
ray tracing that is solved self-consistently with the hydrodynamics,
chemistry, and radiative cooling. This technique retains the time
derivative and is photon conserving. We follow more than 20
primordial stars as they photo-ionize their host halos and the
surrounding few kpc and expel all baryons from their dark matter
halos. The increased electron fraction in the relic HII regions of
primordial stars causes molecular hydrogen to become more abundant and
increases the number of primordial stars by a factor of a few. The
dynamical and thermal feedback from primordial stars affect the
protogalaxy in several ways, e.g. decreasing the baryon content,
skewing the angular momentum distribution to higher values, increasing
the temperature of accreting gas, and enriching the IGM with the first
metals. Our results highlight the importance of the inclusion of
primordial stars and molecular hydrogen cooling in high redshift
galaxy formation models.
|
Searching for double beta decay with the Enriched Xenon Observatory
Carter Hall
Department of Physics, University of Maryland
Thursday, October 19, 2006
Time - 11am; Location - Bldg 2/Rm 8
Abstract
Neutrinoless double beta decay has recently become a top priority for
the global experimental neutrino physics program. Double beta decay has
the potential to resolve the scale of the neutrino mass spectrum, and is
also the only practical tool we have for understanding the particle/anti-
particle nature of the neutrino. The Enriched Xenon Observatory (EXO)
collaboration is developing sensitive searches for the double beta decay
of Xenon-136. Our first experiment, EXO-200, will be the largest double
beta decay experiment ever attempted by an order-of-magnitude, and is
rapidly being constructed. We are also pursuing R&D to realize a system
to tag the daughter barium nucleus of the decay using the techniques of
single-ion spectroscopy. This seminar will summarize the current status
of our work.
|
Radiation transfer in neutron star magnetospheres: the binary
pulsar and magnetars
Maxim Lyutikov
University of British Columbia
Tuesday, October 24, 2006
Abstract
This talk will be separated into two parts.
First, I will discuss eclipses in the binary pulsar system PSR
J0737-3039A/B, where the faster pulsar A is eclipsed once per orbit.
A simple model of eclipses based on synchrotron absorption on closed
field lines of the slower pulsar B reproduces the complicated observed
light curve down to intricate details and provides a direct proof of the
long-standing assumption of dipolar magnetic fields of neutron stars.
In a second part I will describe a model of non-thermal emission from
magetars due to resonant cyclotron scattering of surface radiation in
their magnetospheres. Applying the model to anomalous X-ray pulsar
1E 1048.1--5937 gives a fit just as good as less physically motivated
"black body plus powerlaw" spectra and allows one to determine
density and temperature of the magnetospheric plasma.
|
An Overview of Suzaku Results on Active Galaxies
James Reeves
GSFC / Johns Hopkins University
Tuesday, October 31, 2006
Abstract
I give an overview of early observational results from Suzaku on AGN.
One of the primary goals of the AGN program was to utilize the excellent
broad high energy bandpass of Suzaku in order to accurately study the
iron K line and Compton reflection component in many AGN, while removing
the ambiguity present in modeling the iron line profile. The Suzaku data
show that both narrow and broad velocity components of the iron line are
present in many Seyfert galaxies, while the Compton reflection component
is detected with the HXD in several AGN. The observations show that at
least part of the iron line and reflection hump appears to originate
from the innermost accretion disk, with the narrow iron line core likely
arising from distant matter (such as the molecular torus). Studies of
the broad band spectral variability with Suzaku is revealing that in
many AGN, the emission is least variable at higher energies. This may
be attributed to the reflection component appearing to be less variable
than the primary continuum emission. Gravitational light bending from
near a Kerr black hole offers one explanation for the relative constancy
of the reflection spectrum, although dilution of the variability by a
distant reprocessor may be important in some AGN.
|
Triggering Short Gamma-Ray Bursts
Enrico Ramirez-Ruiz
IAS, Princeton
Tuesday, November 14, 2006
Abstract
Although they were discovered more than 30 years ago, short gamma-ray
bursts are still a mystery. All that we can be confident about is that
they involve compact objects and relativistic plasma. Current ideas and
prospects are briefly reviewed. There are, fortunately, several new
observations that could help clarify the issues.
|
Radio Detection of Ultra-High Energy Neutrinos with the ANITA Experiment
Jason Link
NASA GSFC/USRA
Tuesday, November 21, 2006
Abstract
The ANITA (Antarctic Impulsive Transient Antenna) is an Antarctic NASA
long duration balloon mission which will 'listen' for the impulsive
radio chirp of ultra-high energy neutrinos interacting in the ice
sheet. The payload is poised to launch any day now from Mc Murdo base
in Antarctica. In this colloquium I'll discuss ultra-high energy
neutrinos, Askaryan pulses, ANITA instrumentation and hardware,
scientific results from the prototype ANITA-LITE flight, Antarctic
penguins, and expected results for the full ANITA flight.
|
Hubble Space Telescope Observations of the Light Echo Around
V838 Monocerotis
Howard E. Bond
Space Telescope Science Institute
Tuesday, December 5, 2006
Abstract
A "light echo" is one of the rarest and most beautiful phenomena in
astronomy. The most spectacular light echo in history is occurring
now around the previously unknown star V838 Monocerotis, and is being
imaged regularly by the Hubble Space Telescope.
A light echo is created when light from a sudden stellar brightening
spreads out into space and illuminates nearby interstellar dust.
Because of the detour that the light takes in going out to the dust,
scattering off it, and then traveling to the Earth, the light arrives
months or years after the light from the star itself.
The light echo around V838 Mon, which had a sudden outburst in early
2002, leads to a direct geometric determination of the distance to the
star through a novel technique based on polarimetric imaging. The
resulting distance of 20,000 light-years shows that V838 Mon was
temporarily one of the brightest stars in the entire Milky Way. Its
outburst was of a type not seen before, thus posing a severe puzzle to
stellar theoreticians. The puzzle is deepened by our recent discovery
that V838 Mon belongs to a very young stellar cluster, lying at the
outer edge of our galaxy.
Perhaps more important to the non-specialist, however, the images of
the light echo are among the most stunning obtained to date by the
Hubble Space Telescope.
|
Not Your Grandmother's HII Regions: An X-ray Tour of Massive
Star-forming Regions
Leisa K. Townsley
Penn State University
Tuesday, December 12, 2006
Abstract
Chandra and XMM-Newton are providing remarkable new views of massive
star-forming regions, revealing all stages in the life cycle of
high-mass stars and their effects on their surroundings. We will tour
several such regions, highlighting physical processes that characterize
the life of a cluster of massive stars, from deeply-embedded cores too
young to have established an HII region to superbubbles so large that
they shape our views of galaxies. Along the way we see that X-ray
observations reveal hundreds of pre-main sequence stars accompanying
the massive stars that power great HII region complexes. The most
massive stars themselves are often anomalously hard X-ray emitters;
this may be a new indicator of close binarity. These complexes are
sometimes suffused by diffuse X-ray structures, signatures of
multi-million-degree plasmas created by fast O-star winds. In older
regions we see the X-ray remains of the deaths of massive stars that
stayed close to their birthplaces, exploding as cavity supernovae
within the superbubbles that these clusters created.
|
Jerry Bonnell
