Department of Astronomy

Abstracts for Spring 2011 Colloquia (most recent update: February 18, 2011)

Jan. 18, 2011

Raffaella Schneider, INAF/Osservaorio Astrofisico di Roma (Tinsley Scholar)
"Dust in the Early Universe"

In the last few years mm and submm observations of high redshift quasars and galaxies
have provided a powerful way of probing the very existence and properties of dust within
1 Gyr of the Big Bang. It is generally believed that at these cosmic times dust could have
only condensed in the explosive ejecta of supernovae since intermediate-to-low mass
stars did not have time to evolve off the main sequence into the dust-condensation
stage. To account for the dust masses observed at z > 6, about 1 Msun of dust per
supernova needs to be produced, in agreement with the condensation efficiencies
predicted by supernova dust nucleation models. However, the largest dust masses
measured so far in supernova ejecta are about two orders of magnitude smaller.
Conversely, indications for supernova dust beyond z ~ 6 have recently come from
observations of dust reddening of the host galaxies of a z = 6.2 quasars and z = 6.3
gamma-ray burst.

In this talk, we will review the recent advancements made in this controversial subject.
We will present the results of recent models for dust production in supernova ejecta
and intermediate mass stars and its subsequent evolution in the interstellar medium
of high redshift galaxies. We will also discuss the implications of an early epoch of dust
formation for the cosmic transition between Population III and Population II stars.

Jan. 25, 2011

Jonathan Braithwaite, University of Bonn (Tinsley Scholar)
"Magnetic relaxation in ICM bubbles and the magnetic flux problem
in star formation"

In various astrophysical situations a volume of magnetised plasma is first stirred
by turbulent motions and then allowed to relax without outside influence. For instance,
various types of radiative star, which are observed to contain static magnetic equilibria,
have previously undergone convection. I look at the principal factors in the initial
turbulent state which determine the properties of the eventual equilibrium, presenting
also simulations of the relaxation of magnetic field in bubbles observed as x-ray cavities
in the intergalactic medium. I make predictions concerning the nature of the magnetic
field in such bubbles and how it depends on the nature of the AGN jets/outflows which
inflate them. Finally, I look at a possible solution to the so-called "magnetic flux problem"
in star formation, where a protostar must somehow lose almost all of the magnetic flux
present in its parent cloud.

Feb. 1, 2011

Peter Goldreich, California Institute of Technology
"Reading the Record of Ancient Impacts"

Debris from asteroids and comets continually bombards the Earth. Impacts by big bodies
launches ejecta on trajectories that transports it thousands of kilometers from the inpact site.
These events are recorded in discrete layers of sub-millimeter size spherules in sea floor
sediments. Larger, rarer, and interestingly shaped objects are found on land. I will describe
the physical processes that occur in energetic impacts. Emphasis will be given to the
mechanisms that determine the sizes and shapes of the ejecta and the cooling of the fireball.
An analogy with structure formation in the expanding universe will be exploited, and a
possible relation with the formation of chondrules will be explored.

Feb. 8, 2011

Kim-Vy Tran, Texas A&M University and University of Zurich
"Star Formation in Galaxy Clusters Over the Past 10 Billion Years"

Understanding how galaxies form and evolve in clusters continues to be a fundamental
question in astronomy. The ages and assembly histories of galaxies in rich clusters
test both stellar population models and hierarchical formation scenarios. Is star formation
in cluster galaxies simply accelerated relative to their counterparts in the lower density field,
or do cluster galaxies assemble their stars in a fundamentally different manner? To answer
this question, I review results from our Spitzer/MIPS Infra-Red Cluster Survey (SMIRCS;
0 < z < 1) and present first results for one of the most distant clusters yet discovered
at a look-back time of nearly 10 billion years (z = 1.62).

Feb. 11-12, 2011

McDonald Observatory Board of Visitors Meeting (Austin)

Feb. 12, 2010
1-2 PM
ACE 2.302

Daniel Jaffe, University of Texas at Austin (19th Annual Great Lecture in Astronomy)
"Exploring Newly Discovered Worlds with the Giant Magellan Telescope"

Less than two decades ago, mankind knew about only one planetary system: our own.
Since the discovery of the first extrasolar planets in 1995, a furious effort by the astronomical
community has led to the discovery of hundreds of extrasolar planets. Many of these systems
look very different from our solar system, having massive planets close to the parent stars
while ours are far from the Sun, and/or planets in highly eccentric orbits, where most orbits
in the Solar System are quite circular. The high contrast between the brightness of the parent
stars and the faintness of the planets makes it very hard to obtain concrete information about
the planets themselves: How and when do they form? How massive and how dense are they?
How hot are they? Do they have atmospheres and what are these atmospheres made of?
A new generation of large ground-based telescopes, including the Giant Magellan Telescope
, in which UT Austin and Texas A&M are partners, will afford us the first direct measurements
that can address all of these questions for exoplanets approaching the size and mass of planets
like the Earth but residing in star systems many light years away. GMT in particular, with its superb
high-contrast imaging and its Innovative capabilities for infrared spectroscopy, will be able to
move us from an era of detection to one of real exploration. I will discuss the exciting future of
extrasolar-planet studies and how it builds on the solid foundation of the current exoplanet
research that forms an important part of the current program in Texas.

Feb. 15, 2010

George F. R. Ellis, University of Cape Town
"Is Inhomogeneity Important in Cosmology?"

The real universe is comprised of a cosmic web of structures, with vast walls and voids on
larger scales and mostly empty space on smaller scales. Is this significant for cosmology,
or can we ignore it and used the standard linearized Robertson-Walker models with impunity?
There are three ways inhomogeneities can affect the values we assign cosmological parameters
on the basis of astronomical observations. Firstly there may be back-reaction from small scale
inhomogeneities to large scale dynamics; this almost certainly occurs, but is probably not important,
although there are some caveats to that statement. Secondly the observational properties of
a universe with major voids can be significantly different from those of a smoothed out model:
this has the potential to cause significant re-evaluation of the interpretation of the supernova
observations. Thirdly a large-scale underdensity, with our Galaxy somewhere near the centre,
can mimic the apparent acceleration of the universe, indeed the observations can be explained
by inhomogeneous models with no dark energy present. These models are philosophically
unpopular, but philosophy will have to give way to observational tests that can check whether
the Copernican Principle is indeed satisfied or not. Recent claims that these models have been
observationally disproved are overstated: the required perturbation calculation have not yet
been properly deployed.

Feb. 16, 2010
3:00 pm

Charles Steidel, California Institute of Technology (Tinsley Visiting Professor)
"Exploration of the Circum-Galactic Medium at High Redshifts"

During the peak epoch of galaxy formation, the intergalactic medium is both the source of
gas fueling star formation in forming galaxies, as well as the waste dump for the products
of star formation and black hole accretion that are not retained by galaxies. By studying
the "Circum-Galactic Medium", the region within a few hundred physical kpc of forming
galaxies, one can begin to constrain the flow of baryons into and out of galaxies. At present,
there is a puzzling discrepancy between observations and theoretical expectations whose
resolution may be key to unraveling the aspects of galaxy formation that are least well-understood.

Feb. 22, 2010

Jessica Lu, California Institute of Technology
"Clarifying our View of Star Formation in Extreme Environments with Adaptive Optics"

The formation of stars is a fundamental astrophysical process; and yet we still debate whether
it varies with environment. Milky Way young star clusters range in mass over four orders of
magnitude; but, the best-studied star forming regions, such as Taurus and Orion, represent
only a small range of initial conditions at the low mass end. Young star clusters with masses
greater than 10,000 solar masses are promising targets for determining whether the initial mass
function (IMF) that results from the star formation process is universal or depends on environment.
Such clusters are challenging observational targets as they require high spatial resolution at infrared
wavelengths and are heavily contaminated by field stars. I present results from a Keck adaptive
optics study of several massive young star clusters in the Milky Way, including around the
supermassive black hole at the Galactic Center. Precise IMFs are constructed by using high-precision
astrometry and spectroscopy to distinguish individual cluster stars. I will discuss whether the measured
IMFs differ for massive clusters at a range of Galactocentric radii and how they compare to the
"universal" IMF established locally.

Mar. 1, 2010

Robert Kraft, University of California/Lick Observatory
"Chemical Abundances in the Oldest Galactic Stars:
Globular Clusters vs. the Halo Field"

This is a review of the quest to determine the chemical abundances of the oldest stars in the Galaxy,
the members of the halo field and globular cluster populations. From forty years of spectroscopic studies,
it appears that cluster and field stars are not chemically identical but differ significantly especially in
the behavior of O, Na, Mg and Al. We explore both deep mixing and primordial abundance variations
as explanations for these differences, and find that both are needed. The unusual abundance ratios
of the Mg isotopes found in some clusters points to the importance of a primordial "pollution" scenario
involving the output of intermediate mass AGB (asymptotic mass branch) stars. We discuss briefly
scenarios for the formation of the Galactic halo.

Mar. 8, 2010

Jeffrey A. Newman, University of Pittsburgh
"Title: TBA"

Abstract: to come.

Mar. 15, 2011

Spring Break: 14-18 March: No talk scheduled.

Mar. 22, 2011

Lars Bildsten, UCSB/Kavli Institute for Theoretical Physics (Tinsley Visiting Professor)
"Diverse Energy Sources for Supernovae"

The theoretical community is beginning to appreciate (and predict) the potential diversity of explosive
outcomes from stellar evolution while the supernovae surveys are finding new kinds of supernovae.
I will speak about two such new supernovae. The first are ultraluminous core collapse supernovae
with radiated energies approaching 10^51 ergs. I will present our recent work that explains these
events with late-time energy deposition from rapidly rotating, highly magnetized neutron stars:
magnetars. I will close with our theoretical work on helium shell detonations on accreting white
dwarfs that predict a new class of supernovae; called ".Ia's". The first such candidate may well
have been found by the Palomar Transient Factory.

Mar. 24, 2011

Lars Bildsten, UCSB/Kavli Institute for Theoretical Physics (Tinsley Visiting Professor)
"Exploding Stars!"

Stars explode once every second in the Universe, often becoming brighter than their home galaxies.
Though most remain undiscovered by astronomers, recently enhanced capabilities to scan the skies
now detect over 10 per day. This has revealed new modes of explosions, some much brighter than
we expect, and some much fainter. After describing the common outcomes, I will focus on the exciting
new discoveries and their novel theoretical interpretations.

Mar. 29, 2011

Pawan Kumar, University of Texas at Austin
"Recent Advances in our Understanding of Enigmatic Gamma-ray Bursts"

After a brief summary of key observational properties of gamma-ray bursts an overview of the recent
progress in our understanding of these explosions will be provided. Data obtained by the Swift satellite
has answered several long standing questions, and also added new puzzles. Swift has shed some
light on the workings of the "central engine" in these bursts, and last year it discovered a burst at
a redshift of 8.25 (when the universe was 625 million years old) which is one of the most distant objects
we know today (detection of a galaxy at z~10 was reported in January this year). The Fermi satellite
(launched in June 2008) has provided excellent data covering a very broad energy band --- 8 keV to
~300 GeV --- that has clarified some aspects of these bursts, and also added a few new mysteries.

Apr. 5, 2011

Dimitar D. Sasselov, Harvard-Smithsonian Center for Astrophysics
"Title: TBA"

Abstract: to come.

Apr. 12, 2011

Caryl Gronwall, Pennsylvania State University
"The Physical Properties of Lyman-alpha Emitters from z=2 to 3"

Lyman-alpha emitters (LAEs) are low-mass, nearly dust-free objects that represent galaxies "in the act"
of formation. We believe that LAEs at redshifts of 2 to 3 are the progenitors of present-day L* galaxies.
We have used the Mosaic camera of the CTIO 4-m telescope to conduct a deep, narrow-band survey
of Ly-alpha Emitting Galaxies (LAEs) in the Extended Chandra Deep Field-South over the redshift
ranges 3.08 < z < 3.15 and 2.04 < z < 2.08. Our survey covers 0.28 square degrees down to a limiting
line flux of about 1.5E-17 ergs/cm2/s at z=3.1. Our LAE sample now consists of several hundred
candidate line-emitters. By combining our narrow-band data with archival broadband photometry from
MUSYC (the Multiwavelength Survey by Yale-Chile), we have been able to determine the photometric
properties of these samples including their luminosity functions, equivalent widths, colors, star-formation
rates, and dust properties of these objects as well as the evolution of these properties from z=3 to 2.
We have also measured the stellar populations using the combined optical and infrared photometry
from archival Spitzer data as well as their morphological properties using Hubble Space Telescope
imaging. I will also discuss future prospects for studying Ly-alpha emitters using HETDEX.

Apr. 19, 2011

Aristotle Socrates, Institute for Advanced Study, Princeton
"Title: TBA"

Abstract: to come.

Apr. 26, 2011

Ethan Vishniac, McMaster University, Ontario, Canada
"Title: TBA"

Abstract: to come.

May 3, 2011

Speaker, Affiliation TBD
"Title: TBA"

Abstract: to come.

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