"The canadian virtual observatory and the Canadian Astronomical Data
Center (CADC)"
Thursday, June 12th, 2:00PM
Bart Wakker (Univ. Wisconsin)
"The FUSE survey of OVI in and near the Milky Way"
The Far-Ultraviolet Spectroscopic Explorer (FUSE) recently made the
first extensive survey of \OVI\ in and near the Milky Way. Spectra of
100 extragalactic objects and two distant halo stars were analyzed to
obtain measures of \OVI\ absorption along paths through the Milky Way
thick disk and halo. Strong \OVI\ absorption over the velocity range
from -100 to +100 \kms\ reveals a widespread but highly irregular
distribution of \OVI, implying the existence of substantial amounts of
hot gas with T~3\tdex5 K in the Milky Way. A combination of
models involving the radiative cooling of hot fountain gas, the
cooling of supernova bubbles in the halo, and the turbulent mixing of
warm and hot halo gases is required to explain the presence of
\OVI. In addition, \OVI\ at velocities |v|=100 to 400 \kms\ is seen in
60\% of the sightlines. This high-velocity \OVI\ reflects a variety of
phenomena -- infalling low-metallicity gas, a tidal stream (the
Magellanic Stream), possibly outflowing fountain gas and finally Local
Group gas. The evidence for each of these phenomena will be described
and discussed.
Friday, May 16th 2003, 12:00 noon
Ron Taam (Northwestern Univ., Chicago)
Angular momentum loss and cataclysmic binary stars.
Tuesday, May 13th 2003, 3:30PM
Paul Vanden Bout (National Radio Astronomical Observatories)
Atacama Large Millimeter Array - Imaging Cosmic Dawn
ALMA has been designed for observations that range from high redshift
galaxies, to star forming regions, to the solar system. The science
potential and current status of the ALMA project will be presented.
Monday, May 12th 2003, 3:30PM
Brent Tully (Univ. of Hawaii)
Light to Mass Variations with Extragalactic Environment
There are now extensive clues from the motions of galaxies that there
are substantial variations between what we see and what we've
got. Intermediate density regimes are favorably illuminated by the
light of stars. There is much more mass per unit starlight in the
extreme environments of high and low density.
Monday, April 28th 2003, 3:30pm
Andrew Gould (Ohio State University)
From Sgr A* to the Dark Halo -- New Results on Galactic Structure
A new catalog constructed by matching Luyten's
high-proper motion stars to 2MASS allows one to
isolate for the first time a large population of
nearby halo stars. I use these to measure the
off-diagonal elements of the halo velocity ellipsoid
as well as the radial and vertical motion of the LSR.
I measure the halo binary distribution function to
separations of 50,000 AU and use this both to compare
the conditions of disk and halo star formation and
to probe the composition of the dark halo. In addition
I resolve the problem of the bulge microlensing optical
depth and propose an origin for S0-2 (the short-period
B-star companion of Sgr A*).
Tuesday, April 15th 2003, 3:30pm
Mark Wilkinson (Inst. of Astronomy, Cambridge, UK)
Dark Matter in Local Group Dwarf Spheroidal Galaxies
The local group dwarf spheroidal (dSph) galaxies are vital laboratories
for the study of dark matter as they both are extremely dark matter
dominated and sufficiently nearby that large samples of stellar radial
velocities can be obtained. In this talk I will discuss the importance of
dSph galaxies for understanding the nature of the dark matter and present
recent results from our ongoing program to investigate their internal
dynamics. Highlights include the first clear evidence for an extended dark
matter halo in the Draco dSph and a demonstration that the halo of the
Ursa Minor dSph is not cusped. The implications for Cold Dark Matter
models, tidal disruption models and MOdified Newtonian Dynamics (MOND)
will be discussed in the light of our new data.
Monday, April 14th 2003, 3:30pm
Naomi McClure-Griffiths (Australia Telescope National Facility)
HI Structure in the Milky Way: The Big and the Small
Neutral hydrogen (HI) structure exists in the Galaxy on all sizes from
sub-parsec (even AU!) scales up to kiloparsec scales. On the smallest
scales most of the structure is stochastic, whereas on the larger
scales we see deterministic structures in the form of spiral arms,
bubbles and chimneys. Despite the close proximity of the Milky Way,
we seem to know very little about how it works. In particular, we
don't fully understand how large scale deterministic structures are
formed, how they die and how they relate to the small-scale structure.
Over the past 5 years there has been renewed interest in these
questions, fuelled mainly by the International Galactic Plane Survey
(IGPS). This survey, made up of the Canadian, VLA, and Southern
Galactic Plane Surveys, is in the process of imaging the HI throughout
the Galactic Plane with sensitivity to angular scales between 2
arcminutes and several degrees. In this talk I will focus on some
recent and on-going work with the Southern portion of the IGPS on a
wide range of questions such as: what role do large scale
deterministic structures have in forming small scale structure? what
dynamical processes are observed in the rotation curve of the Galaxy?
and how does the HI scale height vary across the disk?
Monday, March 31st 2003, 3:30PM
Thierry Lanz (U. Maryland / Goddard Space Flight Center)
Properties of Massive Stars at Low Metallicity
Hot massive stars are prominent contributors to the global evolution
of their host galaxies. In order to determine the properties of
starbursts galaxies at high redshift, it is essential to understand
the physical properties and evolution of massive stars in a low
metallicity environment. O stars in the Small Magellanic Cloud offer
the best opportunity to achieve this goal. The SMC is a little evolved
galaxy having a relatively low metal content, and is sufficiently close
to obtain high-quality UV and optical spectra of individual stars.
I will present HST/STIS spectroscopy of a score of SMC O stars and
discuss the results of a NLTE model atmosphere analysis recently
completed.
Monday, March 24th 2003, 3:30pm
James Liebert (University of Arizona)
White Dwarfs from the Sloan Digital Sky Survey
Among its accomplishments, the SDSS Survey should increase the
number of spectroscopically-confirmed white dwarfs by a factor
of several. I discuss the first few papers being finalized by
the team now, including the discovery of white dwarfs showing
lines of atomic carbon and oxygen, very cool white dwarfs with
collision-induced absorption (CIA) due to molecular hydrogen,
and the project to test whether any DB-DO white dwarfs can be
found in the 30-45,000K "gap" temperature range.
Monday, March 10th 2003, 3:30pm
Margaret Turnbull (Steward Observatory)
The Habitability of Stellar Systems in the Solar Neighborhood
In preparation for the advent of the Allen Telescope Array, the SETI
Institute has the opportunity to greatly expand its former list of
~2000 targets compiled for Project Phoenix, a search for technological
signals of extraterrestrial origin. This opportunity has motivated us
to create a Catalog of Nearby Habitable Stellar Systems ("HabCat"),
which now comprises the largest part of the SETI target list. The
first step in creating HabCat was to define "habitability" criteria
for stars based on what we think we know about the survival
requirements for complex biology. The second step was translate these
criteria into observable astrophysical parameters. Third, we
assembled the necessary data by combining the parallax, variability,
multiplicity and kinematics data in the Hipparcos Catalogue with
information on spectral types, X-ray emissions, rotation, CaII H&K
activity, metallicity, and the presence of planetary companions from
several other sources. Finally, stars failing our habitability
criteria were ruled out, leaving 17,129 "habstars," including ~2200
members of binary or triple systems. I will discuss the logic behind
this entire process. This exercize makes excellent food for
astrobiological thought, as it reveals substantial gaps in our
understanding of the development of complex life on Earth and of the
basic properties of stars in the Solar Neighborhood.
Friday, March 7th 2003, 12:30pm
John Gizis (U. Delaware)
Brown Dwarfs and Magnetic Activity
I review brown dwarf discoveries using the Two Micron All-Sky Survey
(2MASS). The discovery of over one hundred L dwarfs (2200K <~ Teff <
~1400K) and dozens of T dwarfs (1400K < Teff < 800K) allows
astronomers to observe the properties of substellar objects. One
surprising area is magnetic activity. Searches for chromospheres,
coronae, spots, and flares have been performed by many different
groups at wavelengths from X ray to radio. I review the results, which
show that brown dwarf magnetic activity is qualitatively different
from that of cool dwarf stars.
Monday, March 3rd 2003, 3:30pm
Peter Ward (U. Washington - Seattle)
Impacts, threats, and the concept of a Galactic Habitable Zone.
The paradigm-changing discovery that large body impact could and did
cause at least one mass extinction (the 65 million year old K/T
event, caused by the Chicxulub impact) in the Earth's geological past
(Alvarez et al, 1980) radically transformed the field of
Paleobiology, and for the first time brought together the
astronomers, earth scientists, atmospheric scientists, and
paleontologists who have gone on to form the nascent field of
Astrobiology. Subsequent to this discovery it was theorized that all
of the larger mass extinctions during the 500 million year long Age
of Animals (the Phanerozoic Era) were impact caused, leading David
Raup to famously theorize that we do not inhabit a "safe planet".
New research over the last decade, however, suggests that the K/T
event may be the only of the so called "Big Five" mass extinctions
to have been impact caused, and now this has led to a revision of the
Impact Kill Curve hypothesis. In this talk I will show this new
evidence for the Permian and end-Triassic mass extinctions that seems
to falsify impact hypotheses as to their cause, and suggest how this
new view has partially led to a concept of a "Galactic Habitable
Zone" (Gonzales, Brownlee and Ward, 2001), defined by metallicity
and impact threat. I will conclude with musings about how the
collision of our galaxy with Andromeda could radically change the
solar system environment.
Monday, February 24th 2003, 3:30pm
Monika Kress (U. Washington)
T.B.D.
Tuesday, February 18th 2003, 3:30PM
L.H. Kuznetz (Nat. Space Biomedical Research Inst.)
On the Existence and Stability of Liquid Water on the Surface of Mars
Today
The recent discovery of high concentrations of hydrogen just below the
surface of Mars' polar regions by Mars Odyssey has enlivened the
debate about past or present life on Mars. The prevailing assumption
prior to the discovery was that the liquid water essential for its
existence is absent. That assumption was based largely on the
calculation of heat and mass transfer coefficients or theoretical
climate models. This research uses an experimental approach to
determine the feasibility of liquid water under martian conditions,
setting the stage for a more empirical approach to the question of
life on Mars. Experiments were conducted in three parts: Liquid
water's existence was confirmed by droplets observed under martian
conditions in part 1; the evolution of frost melting on the surface of
various rocks under martian conditions was observed in part 2; and the
evaporation rate of water in Petri dishes under Mars-like conditions
was determined and compared with the theoretical predictions of
various investigators in part 3. The results led to the conclusion
that liquid water can be stable for extended periods of time on the
martian surface under present-day conditions.
Friday January 17, 2003, from 2:00PM to 6:00PM
"Big Apple" Astrophysics Colloquium Series
Major Terrestrial Impacts: Scientific Modelling and Visualization
INVITED SPEAKERS:
Jay Melosh (Lunar and Planetary Laboratory, University of Arizona)
"An Overview of Major Terrestrial Impact Events"
Elizabetta Pierazzo (Planetary Science Institute, Tucson)
"Modelling the KT Cratering Event"
Galen Gisler (Los Alamos National Laboratory)
"Thermal Radiation from Impact-Generated Plumes"
Jay Melosh (Lunar and Planetary Laboratory, University of Arizona)
"Ejecta Distribution and Global Fallout from the KT Impact"
Robin Canup (Southwest Research Institute, Boulder)
"Modelling the Lunar-Forming Impact"
(The Big Apple colloquium will be held in the Linder Theatre.)
Thursday, February 6th 2003, 3:30PM
Ethan Vishniac (Johns Hopkins)
The Persistence of a Strong Turbulent Cascade to Small Scales
Observations of the ISM seem to show a strong turbulent cascade
extending from scales of tens of parsecs down to 10^8 cm. This
turbulent power spectrum is very roughly consistent with what one
would expect for incompressible hydrodynamic turbulence, in spite of
the fact that the ISM is compressible and magnetized. Even odder, we
see no clear sign that neutral damping has a dramatic effect on the
cascade, in spite of its expected role as a major sink of kinetic
energy. I will discuss an approximate theory of turbulence in a
partially ionized medium which explains these results, and the current
state of numerical simulations designed to test these ideas.
Monday, February 3rd 2003, 3:30PM
Mario Livio (Space Telescope Science Institue)
Cosmology and Life
I will review the current cosmological model and how it reflects on
the emergence of intelligent life in the universe. I will discuss the
question whether our universe is somehow "fine-tuned" for life. I will
examine the potential existence of an ensemble of universes, and the
possibility that the constants of nature vary with time. Finally, I
will examine critically a powerful argument that claims that
extraterrestrial intelligent life is exceedingly rare.
Monday, December 9th 2002, 3:30PM
Joanne Attridge (M.I.T.)
LOFAR: The Low Frequency Array
The Low Frequency Array (LOFAR) is a powerful new telescope
being designed for the 10-240 MHz frequency range. It will
improve both angular resolution and sensitivity in this range
by two orders of magnitude, effectively opening a new window
on the Universe. LOFAR also breaks new technological ground.
The array will be a fully digital instrument with no moving
parts, and will be able to observe in multiple widely separated
distances at the same time, with full sensitivity in each
direction. I will introduce LOFAR to those who may not be familiar
with it, present the current status of the design, and provide an
overview of LOFAR's exciting scientific potential. Examples of the
major science areas to be targeted by LOFAR include the cosmological
epoch of reionization, propagation studies of the interstellar medium,
the study of supernova remnants and pulsars, planetary science,
monitoring for transient radio sources, coronal mass ejections, and
ionospheric tomography.
November 18th 2002, 3:30PM
Lee Samuel Finn (Penn State Univ.)
Gravitational Wave Phenomenology
In physics, phenomenologists are those brave souls who interpret
observations in terms of existing theory(s), build models to
comprehend observations where theory has not yet tread, and bring to
the analysis of data and the design of experiments an understanding of
how existing theories can be tested or their consequences
illuminated. Gravitational wave detectors are rapidly reaching a
sensitivity that will allow observation to confront theory. As that
goal is reached, a community of gravitational wave phenomenologists is
emerging, eager to tackle the interpretive challenges that these new
observations will pose. What are those challenges? Testing general
relativity - Einstein's theory of gravity - for starters, and
exploring the astrophysics of compact objects, including the gamma-ray
burst model and the population of compact binary systems in our galaxy
and beyond. In this seminar we will discuss the character of
gravitational waves, including what they are and how they are
generated, and then examine several examples of how gravitational wave
observations can be used as an astronomical tool and to explore the
fundamental physics of gravity.
November 11th 2002, 12:00 noon
Mike Rich (U.C.L.A.)
Is there an Intermediate Mass Black Hole in the core of G1?
Spectroscopy with the Hubble Space Telescope has recently
revealed unusual kinematics in the cores of two globular clusters,
M15 and G1. In the case of G1, 3-integral models indicate that
a best fit occurs with a central compact mass of 20,000 Solar Masses.
In the case of M15, the presence or absence of a black hole is
more controversial. I will discuss the results for G1,
and report on recently announced results for a peculiar central
M/L in NGC 6752. Finally, I will examine whether there is any
additional evidence to support the existence of intermediate mass
black holes and I will consider how they relate to their supermassive
cousins in Galactic nuclei.
Monday, October 28th 2002, 3:30PM
Matthew Kenworthy (U. Cincinnati)
A needle by a burning haystack: Looking for the reflected light of an
extrasolar planet from the ground.
The discovery of large extrasolar planets (ESPs) in short period orbits
has opened up the possibility of direct detection by reflected light.
The expected fraction for the transiting planet HD 209458b is about
0.02%, requiring photometry that challenges modern limits. We present
data from a secondary eclipse of HD 209458b taken with an imaging
transmission grating, and discuss the factors which limit ground-based
detections with this method.
Monday, October 21st 2002, 3:30PM
Jonathan Katz (Washington University in St.Louis)
Gamma-ray bursts and active galactic nuclei
Gamma-ray bursts and active galactic nuclei have many features in
common. Both classes of object are powered by accretion onto a
compact object, both fluctuate irregularly in intensity, and both emit
much of their energy in the form of energetic particles with a
nonthermal energy distribution. I will discuss models which unify
these two classes as the consequences of electromagnetic release of
accretional energy, and relate them to accreting black hole X-ray
sources and pulsars. The power per unit area is a crucial parameter.
Friday, October 19th 2002, 12:00 noon
Neil Trentham (Cambridge, U.K.)
The cosmic star formation history in infrared and optical galaxies
One aim of extragalactic astronomy is to determine
the cosmic star formation history, as presented in the
Madau or Madau-Lilly plot. The Madau plot is made up of
two components - optical galaxies, which are easily found
in redshift surveys, and infrared galaxies. We know that
the contribution from infrared galaxies is probably the
dominant one, since the infrared background measured by
COBE is so high. But these infrared galaxies are difficult
to identify - that means that we cannot measure their
redshifts and place them on the Madau plot. In this talk
I will discuss possible methods of identifying these galaxies
using submillimeter surveys and multi-wavelength imaging of
gamma-ray burst host galaxies.
Monday, October 7th 2002, 3:30PM
Alan Stern (Southwest Research Institute, Boulder)
Modelling and Observational Constraints on the Putative Vulcanoid
Population of Our Solar System
For over a century, planetary astronomers have searched for a putative
population of asteroids inside the orbit of Mercury. This population,
referred to as the Vulcanoids, is difficult to detect, but of high value
for understanding the origin and evolution of the inner solar system, if
the objects are extant. I will describe the history of this interesting
detection problem, recent dynamical and collisional simulations
constraining the Vulcanoid population, and a pair of new searches using
spacecraft and aircraft to detect these objects against the glare of the
Sun.
Friday, October 4th 2002, 2:00 PM
Jay Farihi (UCLA)
Searching for Cool Companions to White Dwarfs
The infrared search for substellar companions to nearby white dwarfs
has been going for a little more than a decade. In the last year we
have discovered thirteen apparent faint proper motion companions. Of
the recently discovered companions, most are M dwarfs and a few are
cool white dwarfs. GD165B, discovered in 1988 as part of our program,
is still the only known companion to a white dwarf with spectral type
later than M.
Monday, September 23rd 2002, 3:30PM
Koji Mukai (NASA/Goddard Space Flight Center)
Chandra Observations of Three Cataclysmic Variables
I will present selected results from Chandra observations of three
cataclysmic variables, DQ Her, V603 Aql, and V1223 Sgr. The latter
two are grating observations, resulting in X-ray spectra of unprecedented
quality. Although analysis is still in progress, the low energy spectrum
of V1223 Sgr appears to be dominated by photoionized plasma, while V603 Aql
may be dominated by collisionally excited plasma. DQ Her, on the other
hand, was observed with ACIS-S in imaging mode to study the origin of its
soft X-ray emission: there the indication is that X-rays are scattered
in an accretion disk wind.
Wednesday, September 11th 2002, 12:00 noon
Henrik Spoon (Kapteyn Astronomical Institute, The Netherlands)
ISO and VLT Observations of Ices in Galactic Nuclei
Near and mid-infrared observations of embedded protostars
and field stars seen through dense molecular clouds have revealed
the presence of ices of CO, CO_2, H_2O, CH_4 and numerous
other solid state molecular species in the interstellar medium.
The abundance, position, width and shape of an ice absorption
feature changes appreciably depending on the temperature, the
irradiation history and the matrix in which the ice is frozen
onto grains. Hence, the study of ices provides insight in
the evolution of molecular clouds from the quiescent state all
the way to the birth of a star cluster.
The ISO mission (1995-1998) has greatly advanced the study of
the ISM. In this lunch talk I would like to present the first
detections of several well-studied ices in galactic nuclei.
Monday, September 9th 2002, 3:30 PM
Ulyana Dyudina (CALTECH)
Clouds and lightning on Jupiter, interpretation of their images and
lessons learned from a 3D light scattering model
What we see as a "surface" of Jupiter are clouds. Because they are so
well observable, clouds are one of the main sources of knowledge about
Jupiter. They tell us about complicated winds and vortices, the
atmospheric chemistry, and the energy exchange between jovian interior
and outer space. Many images of the clouds were taken in reflected
sunlight. I will talk about high-resolution images taken in different
wavelengths by Galileo orbiter. Different wavelengths probe different
depths of the atmosphere, which let us derive vertical cloud
distribution. We do this applying a statistical correlation method
(Principal Component Analysis, or EOF method) to reduce the noise and
to summarize the data such that less computations are needed to derive
the 3-dimensional cloud maps. Another novel approach to study clouds
is to use the images of lightning taken at the nightside of
Jupiter. Lightning occurs in second-from-the-top water cloud layer and
illuminates the clouds from below. The observed diffuse light spots
help us to determine what kind of clouds are located between us and
lightning. We studied these clouds by writing a 3D Monte Carlo light
scattering simulation and by comparing model-produced images with the
lightning images taken by Galileo.
Tuesday, August 20th 2002, 12:00 noon
Daisuke Kawata (Swinbourne U., Australia)
An X-ray/Optical study of elliptical galaxy formation in a LCDM
cosmological simulation
We study the chemo-dynamical evolution of elliptical galaxies and
their hot X-ray emitting gas using high-resolution cosmological
simulations. Our TREE N-body/SPH code includes radiative cooling, star
formation, supernovae feedback, and chemical enrichment. We present a
series of LCDM cosmological simulations which trace the spatial and
temporal evolution of heavy element abundance patterns in both the
stellar and gas components of galaxies. X-ray spectra of the hot gas
are constructed via the use of the Mekal plasma model, and analysed
using XSPEC with the XMM EPN response function. We examine the
abundance ratios weighted by the X-ray emission at different
redshifts, and discuss the metal enrichment history of elliptical
galaxy X-ray halos.
Monday, August 19th 2002, 3:30pm (15:30)
Fabian Heitsch (Joint Institute for Laboratory Astrophysics)
How fast is Magnetic Reconnection?
Magnetic reconnection plays an essential role in the generation and
evolution of astrophysical magnetic fields. The best tested and most
robust reconnection theory is that of Sweet and Parker. According to
this theory, the reconnection rate scales with magnetic diffusivity
$\lambda$ as $\lambda^{1/2}$. In the interstellar medium, the
Sweet-Parker reconnection rate is far too slow to be of
interest. Thus, a mechanism for fast reconnection seems to be
required. We have studied the magnetic merging of two oppositely
directed flux systems in weakly ionized, but highly conducting,
compressible gas. In such systems, ambipolar diffusion steepens the
magnetic profile, leading to a thin current sheet. If the ion pressure
is small enough, and the recombination of ions is fast enough, the
resulting rate of magnetic merging is fast, and independent of
lambda. Slow recombination or sufficiently large ion pressure leads to
slower merging which scales with $\lambda$ as $\lambda^{1/2}$. We
derive a criterion for distinguishing these two regimes, and discuss
applications to the weakly ionized ISM and to protostellar accretion
disks.
Monday, August 19th 2002, 12:30pm (12:30)
Noam Soker (Univ. of Haifa)
Planets and Planetary Nebulae
The axisymmetrical structure of most planetary nebulae
requires their progenitor to interact with a stellar
or substellar companion. The formation of bipolar
planetary nebulae (those with large polar lobes)
require stellar companions. The nature of the companion,
and whether required at all, to the progenitors of elliptical planetary
nebulae (those with a small departure from sphericity)
is an open question. I suggest that planets may be the companion
to the progenitors of many elliptical planetary nebulae.
Friday, August 16th 2002, 12:00 noon
Morris Podolak (Tel Aviv University)
The snowline in protostellar accretion disks
I compute the energy balance on ice grains in a
protostellar disk, taking into consideration gas
heating, radiative heating, radiative cooling, and
evaporative cooling. The steady state temperature of
the grains determines where the snowline is. I will
discuss the dependence of the snowline position on
various parameters.
Friday, August 9th 2002, 12:00 noon
David Graff (Ohio State University)
Microlensing: past, present and future.
Microlensing began as a search for dark matter in the galaxy. The
dark matter search has been fascinatingly inconclusive, with an order
of magnitude too little microlensing to explain all dark matter, but
an order of magnitude too much microlensing to be explained by known
processes. I will place strong limits on the two most popular
explanations for Magellanic Cloud microlensing, LMC self lensing and
lensing by a halo population of white dwarfs. The difficulty in
interpreting microlensing stems from their strong degeneracy. In a
handful of present day events, we have been able to partially or fully
break the degeneracy in the events and place interesting limits on the
mass and kinematics of the lens and the surface profile of the source
star. I will discuss event MOA33, which will hopefully be the second
event to ever yield a complete solution, and represents the first
measurement of terrestrial parallax of any object outside the solar
system. In the near future, new satellite missions will greatly
improve the quantity and quality of data available for microlensing. A
wide field space based imager such as SNAP or GEST will generate two
orders of magnitude more microlensing events than are currently
detected, with far fine photometric precision. The SIM astrometry
satellite, to be launched in 2009, will break the microlensing
asymetry leading to a new epoch in microlensing as a tool for
precision study of the stellar structure of the galaxy.
Tuesday, August 6th 2002, 12:00 noon
Joseph Harrington (Cornell)
Physics of the Shoemaker-Levy 9 Impacts
With almost every major telescope and available spacecraft
participating, the Shoemaker-Levy 9 impacts onto Jupiter were the
most-observed events in the history of professional astronomy. Yet,
there remain many more questions than answers surrounding the
voluminous observations: Why were the debris patterns shaped as they
were? How could the impact-site temperature drop many times faster
than can be explained by thermal radiation? What caused the last of
the precursor flashes in the lightcurves, and how about the
post-impact flare or the oscillatory "bounces" observed at Palomar and
elsewhere? Why did a ring of heat expand at more than 1.5 times the
sound speed for hours after impact? Why did the lightcurves' shapes
depend so strongly on wavelength? Only about 20 theoretical papers
exist, all focussing on the early, data-scarce phases of the
events. We have divided the impacts into phases based on the
prevailing physics. Our new models extend early-phase theory to times
when good observations are available, and create synthetic observables
for detailed comparison to the data. Our models now explain all
features of the images and lightcurves, introduce no new features, and
reveal new insights into the physics of large impacts.
Tuesday, July 23rd 2002, 12:00 noon
Marcin Sawicki (Dominion Astrophysical Observatory)
Star Formation History of the Universe: The Present and the
Near Future.
We have begun to trace the evolution of the cosmic star formation rate
to redshifts when the Universe was only a fraction of its present age
and it appears that this cosmic star formation rate increases with
lookback time from z=0 to z~1 before flattening out at higher
redshifts. However, this simple picture is fraught with
uncertainties, because star formation at high redshift is shrouded in
starlight-absorbing but poorly understood interstellar dust. I will
discuss some of the biases dust introduces into our current
understanding of cosmic star formation, and how in the very near
future upcoming new facilities will allow us to circumvent the
interplay between dust and star formation, and so to construct a much
more bias-free history of cosmic star formation.
Monday, July 15th 2002, 3:30pm (15:30)
Marc Kuchner (Harvard-Smithsonian Center for Astrophysics)
Direct Imaging of Extrasolar Planets Using Band-Limited Image Masks
A space-based classical coronagraph with a "band-limited" image mask
can potentially image earth-like planets around nearby stars more
efficiently than a shaped-pupil coronagraph or a nulling
interferometer. I'll explain this optical trick and describe some of
the planned missions to search for extrasolar planets via direct
imaging.
Monday, July 1st 2002, 3:30PM
Evan Scannapieco (Osservatorio Astrofisico di Arcetri, Italy)
High Redshift Outflows and Their Feedback on Galaxy Formation
A wide range of arguments suggest that the intergalactic medium (IGM)
experienced a period of intense heating and enrichment by outflows
from starbursting dwarf galaxies. We use a three-dimensional linear
peaks model as well as hydrodynamic simulations to study the evolution
of such outflows and their feedback on galaxy formation. We find that
enrichment from these sources is likely to have been incomplete and
early, with the majority of metals ejected at redshifts above 5. Thus
dwarf-outflow models naturally reproduce the nearly constant (2 < z <
5) observed metallicity of the low column density Ly-alpha forest, an
effect of the decreasing efficiency of metal loss from larger
galaxies. We also show that IGM enrichment is intimately tied to the
properties of later-forming galaxies. Outflows strip baryons from
pre-viralized overdense regions with total masses less than 10^10
solar, reducing their number density and the overall star formation
rate, and helping to reconcile these quantities with observations.
The metallicity of the surviving < 10^10 solar mass galaxies increases
with size, but with a large scatter, reproducing the
metallicity-luminosity relation of dwarf galaxies. Finally, galaxies
greater than 10^10 solar masses form with a roughly constant initial
metallicity of 10% solar, explaining the observed lack of metal-poor
disk stars in these objects.
Thursday, June 27th 2002, 12:15PM
James Graham (U. C. Berkeley)
Extreme Adaptive Optics and its application to the
science of faint companions of nearby stars
Monday, June 24th 2002, 3:30PM
Sarah Maddison (Swinburne University of Technology, Australia)
Building Planets with Dusty-Gas
We have developed a new numerical technique for simulating dusty-gas
flows. Our unique code incorporates gas hydrodynamics, self-gravity
and dust drag to follow the dynamical evolution of a dusty-gas medium.
We have incorporated several descriptions for the drag between gas and
dust phases and can model flows with submillimetre, centimetre and
metre size "dust". We present recent calculations which follow the
evolution of the dust distribution in the pre-solar nebula.
Monday, June 17 2002, 3:30PM
Marek Kukula (Edinburgh University, Scotland)
Quasar hosts and galaxy evolution
The host galaxies of quasars in the local universe are now reasonably
well understood and are known to be large, luminous and typically
bulge-dominated systems. But at higher redshifts, where quasars
constitute a significant fraction of the massive galaxy population,
the situation is much less clear. At these redshifts quasars and their
hosts provide a window on the formation and evolution of massive
galaxies and their central black holes. This talk summarizes the
results of ground-based and Hubble Space Telescope programs to trace
the relationship between the quasar and its host galaxy from the
present day out to redshifts of around 2, when the universe was only a
third of its current age.
Monday, June 10th 2002, 3:30 PM
Mordecai-Mark MacLow (AMNH)
The Control of Star Formation by Supersonic Turbulence
The mystery of star formation in our Galaxy is why it seems to occur
so slowly in the disk. The free-fall time of giant molecular clouds
is only a few million years, and yet star formation has lasted for
billions of years. A further mystery is the observation of strongly
supersonic motions in star-forming molecular clouds that ought to
dissipate in a few crossing times in the absence of driving. Magnetic
fields have been invoked to explain both of these mysteries, through
magnetohydrostatic support against collapse, and by transforming
supersonic turbulence into dissipation-free Alfven waves. In this
talk I will show that magnetic fields do not markedly reduce the
energy dissipation rate of supersonic, trans-Alfvenic turbulence, and
so do not reduce the requirements for external driving to explain the
observed supersonic motions. However, those supersonic motions by
themselves can explain the observed low star formation rates.
Numerical simulations show that supersonic turbulence can globally
support a region against gravitational collapse. Local collapse at a
low rate will still occur for driving parameters typical of molecular
clouds, leading to the conclusion that a characteristic of global
turbulent support is likely to be isolated star formation, while a
characteristic of regions where the local density overwhelms turbulent
support is likely to be high-efficiency, clustered star formation. I
will discuss some reasons why the most likely drivers for the
turbulence, at least in disk galaxies, are likely to be the ensemble
of supernova explosions in star-forming regions of the disk, and
magnetorotational instabilities elsewhere. Finally, I will show
simulations of supernova-driven turbulence and show how it provides a
natural description of the interstellar gas.
Monday, June 3rd 2002, 3:30 PM
Alex Filippenko (U. of California, Berkeley)
Stellar-Mass Black Holes in Binary Systems
An important class of binary systems has been identified in which a
low-mass secondary (companion) star orbits a probable black hole. In
most cases they were first observed in outburst as "X-ray novae."
During outburst, the radiation is emitted predominantly by the
accretion disk surrounding the compact primary. A lower limit to the
mass of the primary in a given X-ray nova can be measured when it
returns to quiescence; at that time, light from the secondary star
contributes significantly to the visible spectrum, and the secondary's
radial-velocity curve can be determined with a series of time-resolved
spectra. I will describe Keck observations that have led to the
measurement of a large minimum mass of the primary in five X-ray
novae, about half of the current sample of low-mass X-ray binaries
having compelling evidence for black holes. In some cases, we have
also been able to constrain the mass ratio and the inclination of the
system.
Monday, May 27th (memorial day) 2002, 12:00 noon
Daniel R. Altschuler (Director, Arecibo Observatory)
THE NAIC ARECIBO OBSERVATORY - A Brief History
The Arecibo Observatory in Puerto Rico was innaugurated in
1963. Almost 40 years later it is still the largest single dish
rediotelescope on Earth. After two upgrades, it continues to be a very
productive instrument. Its history and some of its major achievements
are presented.
Monday, May 13th 2002, 3:30 PM
John Ouellette (AMNH)
Blue Stragglers
Blue stragglers have represented a puzzle for stellar evolution theory
for some time: while the vast majority of the stars in a cluster
appear to be evolving in a fashion consistent with our current
understanding of stellar theory, blue stragglers stick out like the
proverbial sore thumb. Recent developments have demonstrated that
when stellar dynamics impinge upon the realm of individual stars,
blue stragglers (along with other stellar exotica) are often the
result. By comparing the evolution of the predicted dynamical
end-products to the apparent evolution of blue stragglers we can not
only learn something about blue stragglers, but also about the
dynamics within the cluster environment. I will present my attempts
at doing this, as well as discuss future directions for this effort.
Friday, May 10th 2002, 12:30 PM
Tom Abel (Harvard-Smithsonian Center for Astrophysics)
Isolated Massive First Stars
Direct ab initio numerical simulations show that modern theories of
structure formation predict isolated massive stars as the first
luminous objects in the universe. They are rapidly rotating and are
likely to form black holes at the end of their lifes. They may form
gamma ray bursts which would open a remarkably bright window to the so
far dark ages at redshifts > 7. If they form stars heavier than 140
solar masses they will explode in very bright pair instability
supernovae. The UV radiation and supernovae of the first stars unbind
all the gas of micro-galaxy they were formed and hence efficiently
enrich the intergalactic medium. I will also show animations done
recently for a Discovery Channel show which nicely illustrates these
findings.
Monday, May 6th 2002, 3:30 pm
Noaki Yoshida (Harvard-Smithsonian Center for Astrophysics)
Numerical simulations of the formation of large AND small scale
structure of the universe
I present recent results of large numerical (N-body + hydro)
simulations of structure formation. Various topics will be covered,
from clustering of galaxy clusters to the formation of molecular
hydrogen at high redshift. I also discuss often-claimed problems with
the popular CDM models and the possible alternatives and
modifications to them.
Monday, May 6th 2002, 12:30 PM
Grace Wolf-Chase (Adler Planetarium & Astronomy Museum, U. of Chicago)
Probing the Properties and Problems of Protostars
More than twenty years ago, the discovery of massive, supersonic,
bipolar outflows associated with infrared sources in molecular clouds
revolutionized the field of star formation. It's a curious fact that
bipolar outflows were one of the few astronomical discoveries that had
been completely unanticipated from theory. Early on, it was recognized
that outflows are a ubiquitous phenomenon associated with early
stellar evolution, having energy requirements so severe that they must
ultimately be powered by the release of gravitational potential energy
liberated by matter accreting onto a forming star. Much observational
and theoretical effort over the past couple of decades has focussed on
understanding isolated, low-mass star formation, but it is now clear
that most star formation occurs in groups or clusters, which severely
complicates the picture! I will discuss these complications from an
observational standpoint, describe some of our efforts to disentangle
sources in confused regions, and discuss possible implications for
inferred relationships between ejection and accretion rates.
Monday, April 22nd 2002, 3:30 PM
David R. Alves (U. Columbia)
The Distance to the Large Magellanic Cloud
The distance to the Large Magellanic Cloud (LMC) is a popular yet
controversial subject for astronomers. Uncertainty in the LMC distance
is the largest systematic error affecting astronomical determinations
of the Hubble constant. I will report on two new measurements of the
LMC distance. The first is based on the traditional standard candle
method. New optical and near-infrared (K-band) photometry of the LMC's
horizontal branch red clump yields the most precise distance estimate
to date. In terms of distance modulus, I find 18.506 +- 0.033
(random) +- 0.03 (systematic) mag. The second method is geometric,
meaning that it is independent of stellar evolution theory. The
large angular size of the LMC uniquely allows for the detection of an
apparent solid-body rotation due to its transverse motion. Analysis
of over 1000 radial velocity measurements of LMC carbon stars thus
yields the LMC transverse motion in km/sec. Proper motion studies
yield this same quantity in arcsec/year. Together these imply an LMC
distance modulus of 18.82 +- 0.29 mag. Although the error obtained is
large, this is a promising new method.
Monday, April 15th 2002, 3:30PM
Caleb Scharf (Columbia U.)
Complex, complex, clusters...
It has long been convenient to think of clusters of galaxies as being
relatively simple systems, and fair samples of the Universe. Recent
observations and theories have all but overthrown that cosy picture, and
demand more sophisticated treatments if clusters are to be used as probes
of cosmology. The amount of hot X-ray gas in the largest clusters indeed
scales with mass, but there is a tremendous scatter in the bright galaxy
population for a given mass scale, and large variations in the stellar
composition of these galaxies - some clusters exhibit strong
red-sequences, some do not. In the X-ray, data from Chandra and XMM have
revealed a host of new phenomena such as cold fronts, radio bubbles, high
metallicity `knots', and revise the properties of many established
features, such as cooling flows. In the theoretical domain there are now
at least two directly competing models for the thermodynamical history of
intra-cluster gas. Finally, we have recently discovered evidence for
gamma-ray emission associated with clusters, which may be the first direct
signature of the actual mass accretion process; seen as microwave
background photons are upscattered by relativistic electrons in
large-scale gas shocks.
Monday, April 8th 2002, 12:00 noon
Carolyn Porco (Southwest Research Institute/U. Colorado/U. Arizona)
The Shape of Rings to Come
The Grand Tour of the outer solar system by the Voyager spacecraft
in the 1980's opened a new era in the study of planetary rings,
those strikingly flat appendages encircling all the giant outer
planets. The ring systems of Jupiter, Saturn, Uranus, and Neptune
were found to have a remarkable and rich phenomenology, in some cases
shaped by physical mechanisms at work in disk systems of much larger
spatial scale. This presentation will highlight some of the major
advances in the study of ring processes over the last decade or so,
illustrating the connections (where appropriate) between these
processes and those believed to operate in the solar nebula,
extrasolar disks and the spiral galaxies. It will conclude on what
can be expected when Cassini, the next spacecraft bound for Saturn,
arrives there in the summer of 2004 and opens up the next era in the
study of planetary rings.
Monday, April 8th 2002, 3:30 PM
Eline Tolstoy (Kapteyn Institute)
The Local Group as the Key to Galaxy Evolution
I will present results of recent VLT programmes to
obtain spectroscopic abundances of individual
stars in a sample of southern dwarf spheroidal galaxies.
The detailed abundance analyses of these stars combined
with the star formation properties of these
galaxies coming from analysis of their Colour-Magnitude
Diagrams gives us an intricate picture of the
properties of these galaxies and how they have varied
with time all the way back to their epoch of formation.
Wednesday, April 3rd 2002, 12:00 noon
Akimi Fujita (Columbia U./AMNH)
Cosmological Feedback From Dwarf Starburst Galaxies
We present the results from the study of cosmological feedback from
dwarf starburst galaxies, in the form of (1) kinetic/thermal energy
and metals and (2) UV radiation. The study is based on the models of
stellar winds and repeated supernova explosions in the galaxies, using
a hydrodynamic code, ZEUS-3D. (1) The cooling of gas in cosmologically
perturbed regions is computed with evolving dark matter potentials
based on a spherical collapse model. We test the effect of ram
pressure of infalling gas on the bubble evolution, and compute how
much of the energy and metals produced in starbursts can be carried
out of the halo gravitational potentials. We discuss the roles of such
feedback in galaxy formation and the enrichment of the IGM. (2) We
solve the radiative transfer problem of stellar radiation through
dwarf starburst galaxies and test the effects of the shells of
swept-up ISM and galactic outflows on the escape of ionizing
radiation. We compare our results with the local observations and
discuss the role of high-z dwarf galaxies on the reionization of the
universe.
Monday, March 25th 2002, 3:30 PM
Russ Makidon (Space Telescope Science Institute)
Realistic AO Imaging Simulations: Providing Science Drivers for New
Instrumentation
Developing new instrumentation for an observatory requires detailed
knowledge both of the telescope and the scientific targets of interest.
Understanding the individual characteristics of a telescope's system is
key to maximizing the effectiveness of any new instrument or observing
technique. In this talk, we report on a set of tools developed to
simulate realistic images from astronomical observatories employing
adaptive optics (AO). Our main focus is the AEOS 3.67 m telescope and its
941-actuator AO system, the largest such system in use today. We make use
of the AEOS telescope and AO system parameters to produce a series of
realistic simulated broadband images, and refine those simulations by
comparing them with AEOS I-band image data. We then extend these
simulations to the H-band, and provide realistic expectations for
observational studies with a diffraction-limited coronagraph on AEOS
optimized for the near-infrared.
Monday, March 18th 2002, 12:00 noon
Debra Wallace (Georgia State University)
Street-lights and Smoke-Detectors: Stellar Early Warning Systems
Hot massive stars guide our journey to explore the Universe in much
the same way as street-lights illuminate a path to direct one's way on
Earth. Observable at great distances due to their intrinsic
brightness, their use as calibrators enables us to derive the
morphology, chemical yields, Mass-Luminosity Relationship, Initial
Mass Functions, and star-formation rates in regions of our own and
nearby galaxies, extra-galactic Super Star-forming Clusters (SSCs),
and star-forming regions of high red-shift, distant
galaxies. Wolf-Rayet (WR) stars, as the last evolutionary phase of
these massive stars, serve as the smoke-detectors of the supernovae to
come. Only in these objects can one study the immediate precursor's of
one of the Universe's most energetic explosive events. As the
cornerstone of so much research effort, the need to fully understand
massive stars is crucial and requires study across the electromagnetic
spectrum. I will discuss ongoing efforts to quantify these stars,
their environments, and their evolution. In doing so, I will address
my (and my collaborators) contributions to this effort via our
ground-breaking work using the Hubble Space Telescope. We have
resolved WR stars at unprecedented resolution using the Wide Field and
Planetary Camera II, the Space Telescope Imaging Spectrograph, and the
Fine Guidance Sensor 1R to discover and quantify previously unknown
companions and clusters. These high resolution observations are
essential to provide a true census of the number and astrophysical
parameters of massive stars in confined environments where they often
occur, and to understand the effects of nearby companions on massive
star evolutionary paths.
Monday, March 18th 2002, 3:30 PM
Michael Norman (Center for Astrophysics and Space Sciences U. of
California, San Diego)
Numerical Simulations of High Redshift Star Formation in Dwarf
Galaxies
We present first results from three-dimensional hydrodynamic
simulations of the high redshift formation of dwarf galaxies.
The simulations use an Eulerian adaptive mesh refinement
technique to follow the non--equilibrium chemistry of
hydrogen and helium with cosmological initial conditions drawn
from a popular Lambda-dominated cold dark matter model. We
include the effects of reionization using a uniform radiation field, a
phenomenological description of the effect of star formation and, in
a separate simulation, the effects of stellar feedback. The results
highlight the anticipated effects of photo-ionization on the collapse of
galaxies with virial temperatures of approximately 10^4 K.
Dwarf sized dark matter halos that collapse prior to reionization
are able to form stars. Halos of similar mass that assemble after
reionization do not form stars by redshift of three. The dwarfs that
form stars show large variations in their gas content
because of photo-ionization as well as stellar feedback effects.
Monday, March 11th 2002, 3:30 PM
Martin Bureau (Columbia University)
ENVIRONMENT, RAM PRESSURE, AND SHELL FORMATION IN HoII
Neutral hydrogen VLA D-array observations of the dwarf irregular
galaxy HoII, a prototype galaxy for studies of shell formation, are
presented. The large-scale HI morphology is reminiscent
of ram pressure and is unlikely caused by interactions. A case is
made for intragroup gas in poor and compact groups like the M81
group, to which HoII belongs. Numerous shortcomings of the supernova
explosions and stellar winds scenario to create the shells in HoII
are highlighted, and it is suggested that ram pressure may be able
to reconcile the observations available.
Monday, March 4 2002, 1:30 PM
Michael Strauss (Princeton)
HIGH-REDSHIFT QUASARS FROM THE SLOAN DIGITAL SKY SURVEY AND THE
EPOCH OF REIONIZATION
The Sloan Digital Sky Survey, a multiband imaging and spectroscopic
survey of the high-latitude sky, has been very successful in
discovering high-redshift quasars, including four with z > 5.7. I
discuss the ways in which these objects have been selected, and
spectroscopic observations at z=6.3 that show the Gunn-Peterson
trough, indicative of a neutral intergalactic medium. Thus the
universe underwent a phase transition from mostly neutral to mostly
ionized at z~6, about 1 billion years after the Big Bang.
Monday, February 25th 2002, 12:00 noon
Richard Kron (University of Chicago)
SLOAN DIGITAL SKY SURVEY: SOME RECENT RESULTS
The SDSS has scanned roughly 2500 square degrees in the Northern
Galactic Cap, and has obtained 240,000 spectra (ordinary galaxies,
"luminous red galaxies," quasars, and a few stars). Some of these
data have already been released, much of the rest will be released in
January 2003, and data yet to be obtained will be released
subsequently. In the meantime, several research undertakings have
gone beyond the original motivation for the survey (large-scale
structure). This talk will illustrate the potential of the SDSS data
products with some recent results, specifically: observations of the
highest-redshift quasars; a large statistical sample of asteroids; the
structure of the Draco dwarf galaxy; and statistical properties of
early-type galaxies.
Tuesday, February 19th 2002, 3:30 PM
Andrew Hamilton (University of Colorado)
Black Hole Flight Simulator
What does it really look like if you fly into a black hole?
Do you see the entire future of the universe pass before your eyes?
Can you fly through a black hole into another universe?
I will use an interactive Black Hole Flight Simulator currently
under development in collaboration with the Denver Museum for
Nature and Science to reveal answers to these questions and others.
Monday, February 11th 2002, 3:30 PM
Orsola DeMarco (AMNH)
WHAT ARE WOLF-RAYET CENTRAL STARS TRYING TO TELL US?
Over the last 20 years, the peculiar composition of ~15% of all central
stars of planetary nebula, the Wolf-Rayet central stars, has been explaned
with a variety of scenarios. The born-again scenario was thought viable,
until Asymptotic Giant Branch (AGB) convection with overshoot provided a
simpler alternative scenario for their evolution. The ISO discovery that
most of these stars exhibit oxygen- as well as carbon-rich circumstellar
dust, came as a surprise. Double dust chemistry is observed (and
theoretically understood) to be a very rare phenomenon in post-AGB
stars, accounting for few percent of ALL central stars. The fact that
almost all the Wolf-Rayet central stars exhibit the double dust chemistry
suggests that an altogether alternative scenario might apply. Within the
framework of binary evolution, the possibility is being investigated that
a low mass main sequence star, brown dwarf or planet enters the AGB
envelope and can account for the observations.
Tuesday, January 29th 2002, 12:00 noon
Stan Owocki (Bartol Research Institute, University of Delaware)
The Rocket Science of Launching Stellar Disks
Certain subclasses of hot, luminous stars -- the so-called Be stars -- are
characterized strong hydrogen Balmer line-emission thought to originate in
a circumstellar disk. Unlike the accretion disks found in protostellar or
binary-mass-exchange systems, the evolved, isolated nature of Be stars
means their disks must form by expelling material from the star itself.
Using analogies with launching rockets from earth, I will discuss the key
issues for determining the mechanism(s) for propelling surface material
into an orbiting disk. In particular, I will describe my own recent
simulations of a Radiatively Driven Orbital Mass Ejection (RDOME) model for
Be disk formation.
Monday, January 28th 2002, 3:30 PM
Luc Dessart (University College London)
A THEORETICAL STUDY OF SMALL AND LARGE SCALE VARIABILITY OF HOT STAR
SPECTRA BASED ON RADIATION HYDRODYNAMICS SIMULATIONS
Hot star winds are powered by the scattering and absorption of stellar
continuum photons by optically thick lines of ions. This so-called line
driving is inherently very instable and leads to the formation of shocks
and subsequently structure in the wind outflow, supposedly at the origin
of the optical emission line variability observed in hot star spectra.
In this seminar, I will discuss our theoretical means to better understand
the physics of line driving, the dynamics of the outflow and the diverse
characteristics of hot star wind structures.
In Practice, our theoretical approach is based on radiation hydrodynamics
simulations of the radiative instability coupled with a simplistic
radiative transfer calculation. Because the physics of line driving is
non-local, the computing of the radiative force is a very costly task,
forcing simulations to remain one dimensional. In an attempt to simulate
a three dimensional wind, we implemented a patch method, which assumes the
wind is made of independent rays whose structure can be described by one
dimensional radiation hydrodynamics simulations. I will discuss what
we have learned following this working hypothesis, the successes and
failures, and how we wish in the future to relax some of our
assumptions for better consistency.
If time permits, I will also present recent investigations of the large
scale emission line variability observed in hot star spectra, assuming it
is caused by co-rotating interaction regions in the stellar outflow.
These arise from the photospheric perturbation of a line driven wind in a
rotating hot star, leading to slowly advecting density compressions. I
will show that such features can be observed by means of spectrocopic
monitoring and/or interferometric measurements with VLTI. Small and
large scale variability in hot star spectra represent excellent tools
for understanding the wind dynamics, the wind structure and the
characteristics of the underlying star hidden by the very optically
thick stellar wind.
Wednesday, December 19th 2001, 12:00 noon
Dani Maoz (Columbia University and Tel-Aviv University)
DISTANT SUPERNOVAE: THE (OTHER) WONDERFUL THINGS YOU CAN DO WITH THEM
"Apart from their current use as standard candles, distant supernovae
(SNe) have many other applications. The type-II-SN rate vs. redshift
is an independent probe of the Universe's star formation history, and
the type-Ia-SN rate carries clues to the SN-Ia formation process.
The SN rate in clusters is important for understanding the high metal
content of the intra-cluster medium, and intergalactic SNe can trace
the population of intergalactic stars. Lensed SNe behind clusters can
can probe the cluster potentials and measure the star formation rates
of the lensed galaxies, while the natural magnification of clusters
may reveal the most distant SNe yet. I will show results from a
program to search for SNe in and behind rich galaxy clusters,
concentrating on the discovery of six SNe at z = 0.2-1.0, in an
archival study of deep HST cluster exposures. We use these SNe to
derive the first measurements of the SN-Ia rate in clusters and of the
faint (I < 27 mag) field-SN counts.
Reference: astro-ph/0109089.
Tuesday, December 18th 2001, 3:30 PM
Glenn Schneider (Steward Observatory, University of Arizona)
DUST-BUSTERS: WHEN WORLDLETTS COLLIDE
"Current theories of disk/planet evolution suggest an epoch of
planet-building in nascent circumstellar environments through the
formation and growth of embryonic bodies. During this formative epoch
(likely from approximately 1-10 million years), remnant gaseous
protostellar disks metamorphose into dusty environments arising from
the collisional erosion of agglomerating planetesimals and as gaseous
atmospheres accrete onto giant planet cores. The circumstellar regions
then become dominated by a non-primordial population of debris which
may be shaped and sculpted through dynamical interactions with
evolving co-orbital planetary-mass bodies. Structures within debris
disks, which now may be studied in detail via differential
coronagraphy with the Hubble Space Telescope second generation
instruments, may implicate the existence of such bodies. For stars
within 100 pc of the Earth, Jovian mass planets still hot from their
recent formation may be directly imaged in the near-infrared at Kuiper
belt like distances from their parent stars. I will discuss the direct
evidence, from HST disk and companion imaging surveys, for the
existence of recently-born extra-solar planets outside of the domain
of detection where exoplanets have been previously inferred from
radial velocity surveys."
Monday, December 17th 2001, 3:30 PM
James D. Lowenthal (University of Massachusetts)
THE FAINTEST RADIO GALAXIES
"Sensitive radio maps of "blank sky" made with the Very Large Array
reveal faint sources at the 10-micro-Jansky flux level. What are
these faintest of radio galaxies? Keck optical and near-IR imaging
and spectroscopic followup of about 50 micro-Jansky sources reveals
that some are high-redshift QSOs, but most are luminous, star-forming
disk galaxies at moderate redshift (z_med=0.75). HST imaging reveals
that many of these systems are in fact interacting and merging
starbursts. The radio emission appears to be powered by the star
formation process, thus providing a way for the evolution of
star-forming galaxies to be tracked in the radio, where dust
obscuration is negligible."
Monday, December 10th 2001, 3:30 PM
Christine Thurl (Wesleyan University)
DID OMEGA-CENTAURI FORM IN A MERGER OF GLOBULAR CLUSTERS?
Recent photometric studies of the globular cluster $\omega$~Cen have
revealed evidence for multiple stellar populations. Possible scenarios
for the formation of such an object include: (i) multiple star
formation epochs within a single object; and (ii) mergers of distinct
low-mass, single population systems. In this talk I will present my
research work done with Dr. Kathryn V. Johnston at Wesleyan
University. We examine merger scenarios of spherical stellar systems
by estimating the likelihood of merging of such systems in different
environments using analytic extensions to N-body simulations. We apply
these results to today's Milky Way Galaxy environment and find that
mergers are unlikely to have occurred during a Galactic
lifetime. Mergers are much more probable in a dwarf galaxy environment
such as the Fornax dwarf spheroidal galaxy.
Monday, November 19th 2001, 3:30 PM
Marc Hemsendorf (Rutgers University)
GALACTIC BLACK HOLES AND BROWNIAN MOTION
First results from simulations studying the effect of Brownian motion
of the central mass on a cusp are presented. N-body simulations of
cuspy stellar systems around supermassive black holes represent a very
challenging computational task. The wide range of dynamical timescales
combined with large particle numbers required here leads us to the
implementation of a systolic force calculation into a Hermite direct
force block time-step scheme. With this new algorithm, we are able to
handle particle numbers as large as 1,000,000 on the parallel
computers like Cray T3Es. The systolic algorithm could also be useful
for computing clusters in conjunction with GRAPE. This talk will be
less technical than this abstract suggests.
Tuesday, November 13th 2001, 12:00 noon
Crystal Martin (Caltech)
METAL EJECTION IN DWARF STARBURST WINDS
"I will present Chandra observations of NGC 1569 and discuss the
chemical evolution of this galaxy. Implications for the enrichment of
the intergalactic medium will also be discussed."
Monday, November 5th 2001, 3:30 PM
Alan Hirshfeld (University of Massachusetts)
STELLAR PARALLAX FROM BRAHE TO BESSEL
"Between the late 1500s and early 1800s, there were numerous
failed attempts to measure the parallaxes of stars and thereby gauge
the scale of the cosmos. The talk chronicles these early attempts and
highlights the circumstances that led to the first successful
parallax measurement by Bessel in 1838."
Tuesday, October 30th 2001, 12:00 noon
Mike Fall (Space Telescope Science Institute)
FORMATION AND DISRUPTION OF GLOBULAR STAR CLUSTERS
"This talk will review the theory and observations
relating to the formation and disruption of globular
star clusters. The emphasis will be on understanding
why these objects have a characteristic mass ~ 10^5
M_sun. Much of the talk will cover new observations
of young star clusters in the Antennae galaxies and
theoretical modelling of the disruption of clusters
by the author and his collaborators."
Monday, October 29th 2001, 3:30 PM
Mike Fall (Space Telescope Science Institute)
FORMATION OF GALACTIC DISKS
"This colloquium will review the theory of the formation
of galactic disks (Fall & Efstathiou) and make comparisons
with observations at low and high redshifts. The emphasis
will be on understanding the origin and evolution of the
angular momentum of galaxies, which determines the most
basic properties of the disks. Some of the problems the
theory has encountered will be reviewed, as well as
some of the most likely solutions."
Monday, October 29th 2001, 12:00 noon
Jane Charlton (Pennsylvania State University)
CHARTING METAL RICH GASEOUS ENVIRONMENTS INSIDE AND OUTSIDE OF GALAXIES
"There is a diverse population of structures and environments in the
universe that have in common the ability to produce MgII absorption in
the spectra of quasars. This talk presents a census of the population
including: 1) the mysterious weak MgII absorbers, some of which are
metal-rich star forming pockets outside of galaxies, perhaps related
to Population III star clusters or invisible cold dark matter
mini-halos; 2) classic strong MgII absorbers, which are a result of
the combined ISM, coronae, and high velocity clouds of luminous
galaxies, in some cases shaped by the superwind phenomenon; and 3)
damped Lyman-alpha absorbers, which may be produced by lines of sight
that happen to pass through tiny, dense star-forming regions in a
variety of different galaxy environments. Once the quasar absorption
line code is broken, a morphological classification of gaseous
environments can be constructed, and evolution can be charted in
detail."
Tuesday, October 23rd 2001, 3:30 PM
Vikram Dwarkadas (Bartol Research Institute)
THE EVOLUTION OF SUPERNOVA REMNANTS IN CIRCUMSTELLAR WIND-BLOWN BUBBLES
"Core-collapse Supernovae evolve from massive stars (> 8 solar
masses). The evolution of the resulting supernova remnant in the
interstellar medium has been well studied. However mass loss from massive
stars can result in strong stellar winds that substantially modify the
medium around the star. As the supernova shock wave expands in this
stellar wind-bubble, its evolution will be quite different from that in
the interstellar medium. This can make a considerable difference to the
X-ray, optical and radio emission from the remnant. I will review the
basic aspects of the evolution of supernova remnants in wind-blown
bubbles. The case of SN 1987A, as well as a computation of a 35 solar
mass star from its birth to the evolution of the remnant, will be
discussed."
Monday, October 15th 2001, 3:30 PM
Jarred Hurley (American Museum of Natural History)
THE STELLAR LABORATORY
"The rich environment of a star cluster provides an ideal laboratory
for the study of self-gravitating systems. It also provides important
tests for stellar evolution theory and the formation of exotic stars
and binaries. The recent availability of the GRAPE-6 special purpose
hardware, with its 1 Tflops performance, has finally provided the
capability of producing a direct model of a globular cluster. I will
outline a state-of-the-art N-body code that includes a comprehensive
treatment of stellar and binary evolution. Highlights of the AMNH
N-body/GRAPE program will then be given. These include the
modification of stellar populations, such as blue stragglers and close
double-WD binaries, by dynamical interactions and the possibility of
free-floating planets existing in a star cluster."
Monday, September 24th 2001, 3:30PM
Todd J. Henry (Harvard-Smithsonian Center for Astrophysics)
WHO ARE YOUR NEIGHBORS AND HOW MUCH DO THEY WEIGH?
"The sample of stellar neighbors within 10 parsecs of the Sun will be
explored. The RECONS (Research Consortium of Nearby Stars) effort to
discover new nearby stars and to characterize this fundamental sample
will be discussed. Discoveries from an effort in Chile to reveal
members of your neighborhood will be highlighted, as well as recent
results from HST that are used to map out the mass-luminosity relation
for your lightweight neighbors. Empirical determinations of the
luminosity and mass functions, which have direct bearing on the total
mass of the galactic disk, will be presented."
Monday, September 17th 2001, 12:00 noon
Eric Gawiser (University of California, San Diego)
TOWARDS A COMPLETE PICTURE OF DAMPED LYMAN ALPHA SYSTEMS
"Damped Lyman alpha absorption systems (DLAs) contain most of the
neutral hydrogen in the high-redshift universe and are the likely
progenitors of typical galaxies like the Milky Way. Nonetheless, it
is not yet clear if DLAs are massive galaxies or low-mass
protogalaxies, or whether they are similar to the Lyman break galaxies
or represent a separate population. I will present results from our
observational program designed to clarify the DLA picture. Neither
the fraction of critical density comprised by neutral hydrogen nor the
metallicity of the universe as probed by DLAs evolves from z=4 to z=2.
Dust extinction does not appear to bias these measurements
significantly. The cosmic star formation rate measured in DLAs is
comparable to that of Lyman break galaxies although considerably
weaker on a system-by-system basis. Finally, I will describe an
ongoing effort to determine the mass of DLAs by studying their
cross-correlation with Lyman break galaxies."
Monday, August 27th 2001, 12:00 noon
Jack J. Lissauer (NASA Ames Research Center)
PLANET FORMATION AND THE CHARACTERISTICS OF EXTRASOLAR PLANETS
"An overview of current theories of planetary growth, emphasizing the
formation of extrasolar planets, is presented. Models of planet
formation are based upon observations of the Solar System, extrasolar
planets, and young stars and their environments. Terrestrial planets
are believed to grow via pairwise accretion until the spacing of
planetary orbits becomes large enough that the configuration is stable
for the age of the system. Giant planets begin their growth like
terrestrial planets, but if they become massive enough before the
protoplanetary disk dissipates, then they are able to accumulate
substantial amounts of gas. These models predict that rocky planets
should form in orbit about most single stars. It is uncertain whether
or not gas giant planet formation is common, because most
protoplanetary disks may dissipate before solid planetary cores can
grow large enough to gravitationally trap substantial quantities of
gas. A potential hazard to planetary systems is radial decay of
planetary orbits resulting from interactions with material within the
disk. Planets more massive than Earth have the potential to decay the
fastest, and may be able to sweep up smaller planets in their path.
The implications of the giant planets found in recent radial velocity
searches for the abundances of habitable planets are discussed."
Suggested Readings:
Lissauer, J.J., 1993. "Planet Formation" Ann. Rev. Astron. Astrophys.
31, 129-174.
Lissauer, J.J., 1999. "How Common are Habitable Planets?" Nature 402,
C11-C14.
Wuchterl, G., T. Guillot and J.J. Lissauer, 2000. "Giant Planet
Formation" Protostars and Planets IV, V. Mannings, A.P. Boss and
S.S. Russell, eds. (Tucson: University of Arizona Press), 1081-1109.
Tuesday, August 21st 2001, 12:00 noon
D. J. Pisano (University of Wisconsin)
COMPANIONS TO ISOLATED GALAXIES AND THEIR IMPLICATIONS
FOR GALAXY FORMATION
"I will discuss the results from an HI survey searching for the
gaseous remnants of the galaxy formation process around 41 extremely
isolated galaxies. In particular, I will focus on the properties of the
isolated galaxies and their companions and the implications of the survey
results for models of galaxy formation."
