Dept. of Astronomy Colloquia Schedule for Spring 2010
University of Texas at Austin

Jan. 19th

Edo Berger
Harvard University
"Gamma-Ray Bursts: from Progenitors to Probes"

Gamma-ray bursts (GRBs) are the most spectacular transient events in the universe,
resulting from the deaths of massive stars and possibly neutron star binary mergers.
Their extreme brightness provides a powerful probe of high redshift galaxies and
the intergalactic medium. In this talk I will show that the location of GRBs within the
star forming regions of galaxies affords an in-situ view of metal enrichment across
a wide redshift range. I will also highlight the recent discovery of the most distant
known object in the universe, a GRB at z~8.3. In the context of GRB progenitors,
I will describe a comprehensive study of the galactic and sub-galactic environments
of short GRBs, and the resulting implications for their still-unknown progenitors.

Jan. 26th

Jay Boisseau University of Texas at Austin
"TACC's Capabilities for Enabling World-Class Science"

Feb. 2nd

Erin Bonning
Yale University
"Multiwavelength Observations of Fermi Blazars"

Multiwavelength observations of blazars have entered a new era with the launch and
successful operation of the Fermi Gamma-ray Space Telescope. With concurrent
monitoring programs spanning decades of energies from radio through gamma-rays,
the characteristic double-peaked spectral energy distribution (SED) of blazars can be
observed evolving on timescales from days to months (and ultimately to years).
The low energy peak of the SED is well understood as synchrotron emission from
electrons accelerated in a relativistic jet; however, the source of the high-energy
emission is as yet poorly understood with multiple models often able to fit a given
single-epoch SED. With both spectral and time-variability information available for
a large number of sources, physical models for gamma-ray emission can be more
effectively constrained. I will discuss recent and ongoing observations of blazars
with Fermi, associated multiwavelength campaigns, and theoretical implications
of these observations.

Feb. 9th

Ravit Helled
University of California, Los Angeles
"Metallicity of the Massive Protoplanets around HR 8799
if formed by Gravitational Instability"

One possible mechanism for giant planet formation is disk instability in which the planet
is formed as a result of gravitational instability in the protoplanetary disk surrounding the
young star. The final composition of giant planets formed as a result of gravitational instability
depends on their ability to capture solid material (planetesimals) during their 'pre-collapse' stage,
when they are extended and cold, and contracting quasi-statically. The total mass that can be
captured by the protoplanets depends on the planetary mass, planetesimal size, the radial
distance of the protoplanet from the parent star, and the local solid surface density. We consider
three radial distances, 24, 38, and 68 AU, similar to the radial distances of the planets in the
system HR 8799, and planetary masses of 3, 5, 7, and 10 Jupiter masses, and estimate the
mass of heavy elements that can be accreted. We find that for the planetary masses usually
adopted for the HR 8799 system, the amount of heavy elements accreted by the planets is
small, leaving them with nearly stellar compositions.

Feb. 16th

Angela Speck
University of Missouri, Columbia (Tinsley Visiting Scholar)
"Through the Looking Glass: A Re-analysis of Silicate Dust Spectral Features"

Silicate dust plays an essential role in many astrophysical environments. The "amorphous"
silicate spectral features have been observed in our solar system, young stellar objects,
star formation regions, novae, and the diffuse and dense interstellar medium as well as in
extragalactic environments such as quasars and AGN. This dust contributes to the physical
processes inherent in star formation processes, as well as to several aspects of interstellar
processes such as gas heating and the formation of molecules. The discovery of this almost
ubiquitous ~10um silicate feature, led to many laboratory studies of potential cosmic dust
analogs attempting to determine the exact nature of this dust.

New laboratory data combined with re-analysis of observational data for asymptotic giant branch
(AGB) stars will be presented. Combined, these data present a challenge to the "common wisdom"
on what we think we know about silicates in space.

Feb. 23rd

John Johnson
California Institute of Technology
"The Golden Age of Exoplanet Spin-Orbit Measurements"

The angular momentum of the Solar System planets is remarkably well aligned with the
spin axis of the Sun. This arrangement supports the hypothesis that the planets and the Sun
formed from the same spinning disk of gas and dust. Until very recently, measurement of the
spin-orbit alignment of exoplanets had revealed that planets around other stars, most in very
close-in orbits (periods less than 10 days), also have well-aligned spin and orbit axes. However,
our recent measurements from this past year have revealed that there may be a second class
of exoplanets that inhabit very tilted orbits, and that some may even have retrograde configurations.
I'll show how spin-orbit measurements are made, and discuss how the distribution of observed
spin-orbit angles informs, and challenges, theories of planet migration.

Feb. 24th (Wed)

Andrew Blain
California Institute of Technology
"Discovering and Understanding the Most Luminous Galaxies"

The most extreme end of the galaxy luminosity function is revealed most clearly in far-infrared light.
By seeking the far-infrared properties of galaxies when star formation and galaxy evolution were
at their most intense, their true total power output can be revealed, along with information about
the astrophysics in the gas from which stars form and supermassive blackholes grow. I will describe
multi-wavelength observations of some of the most extreme galaxies at redshifts 2-3, and highlight
some opportunities with ALMA, and NASA's recently launched WISE MIDEX mission, that of
24th February should have imaged more than 20% of the sky from 3.3-23 microns.

Mar. 2nd

Evgenya Shkolnik
Carnegie Institution Department of Terrestrial Magnetism
"Star-Planet Interactions: The Tidal and Magnetic Influence of Hot Jupiters"

The interacting processes taking place between a giant planet orbiting its star within 10 stellar
radii (also known as a "hot Jupiter") have been getting increasing attention both observationally
and theoretically. Our work has shown that such a short-period planet can induce activity on the
upper atmosphere of its host star through both tidal and magnetic star-planet interactions (SPI).

Evidence for magnetic SPI includes a diverse array of photometric, spectroscopic and
spectropolarimetric studies. Because of the small separation (< 0.1 AU), many of the hot Jupiters
lie within the Alfven radius of their host stars, allowing direct magnetic interaction with the stellar
surface. Models show both the stellar and planetary magnetic fields are strongly affected, possibly
influencing the magnetic activity of both bodies, as well as modifying irradiation and non-thermal
and dynamical processes.

In addition, a hot Jupiter's tidal influence on its star may increase the stellar rotation rate and thus
also increase the global stellar activity level. Our recent work has shown that stars with hot Jupiters
have twice the UV emission than stars with planets in wider orbits, which is also anti-correlated
with the stellar synchronization time scales. Even though the stars with hot Jupiters are not fully
synchronized (full synchronization in most cases will take longer than the age of the Universe),
they have already undergone some increase in rotation rate, provided that the planets migrated
early on in the system's history.

Studying both tidal and magnetic star-planet interactions aids our understanding of the formation,
migration and evolution of hot Jupiters, and provides the best-available probe of exoplanetary
magnetic fields.

Mar. 23rd

Suvi Gezari Johns Hopkins University
"Synergistic Transient Searches in the Ultraviolet and Optical"

Motivated by the successful discovery of UV/optical transients with serendipitous contemporaneous
observations between the GALEX Deep Imaging Survey and the optical CFHT Legacy Deep Survey,
GALEX has begun a dedicated time domain survey that operates in parallel with the Pan-STARRS
Medium Deep Survey. With simultaneous UV and optical coverage of 7 deg^2 of sky with a cadence
of days, we have the unique ability to detect UV bright phenomena at early times, including shock
breakout and hot ejecta in core-collapse SNe, as well as probe the simultaneous UV and optical
properties of variable AGNs and flares from dormant supermassive black holes caught in the act of
tidally disrupting a star. In this talk, I will present our transient detection and classification methods
and the first results of our survey.

Mar. 30th

Genevieve Graves
University of California, Berkeley
"The 2D Galaxy Population: Mass-Dependent Star Formation,
Stochastic Assembly-Driven Quenching"

To zeroth order, galaxies form a one-dimensional family in which their observed properties -
morphology, color, metallicity, environment, central black hole masses - all scale with their mass.
This makes it difficult to disentangle the critical properties that determine their evolutionary history
because everything correlates with everything else. Examining where this 1D sequence breaks down
can provide essential clues to galaxy evolution. I present observational evidence that galaxy
star formation histories form a 2D family, such that galaxies of the same mass today start out with
similar star formation histories but shut down star formation ("quench") at different times. A parallel
study using semi-analytic models of galaxy evolution suggests that these differences are due to
their underlying halo mass assembly histories. Indeed, the observed 2D family of galaxy star formation
histories seems to be a generic result of a mass-threshold for quenching (whether the quenching process
is driven by stellar mass, halo mass, or central black hole mass), given a paradigm of mass-dependent
evolution coupled with stochastic hierarchical assembly.

Apr. 6th

Anders Johansen Leiden Observatory (Tinsley Visiting Scholar)
"From Pebble to Planetesimal"

In the early stages of planet formation dust grains collect to form km-sized planetesimals. Growth
beyond a few cms is frustrated by collisional shattering and rapid radial drift of pebbles and rocks.
I will present my work on the dynamics of pebbles, rocks, and boulders in gaseous protoplanetary
disks. The loose drag force coupling with the gas causes particles to pile up in large scale
geostrophic gas pressure bumps which arise in turbulence driven by the magnetorotational
instability. The local density of rocks and boulders becomes high enough to allow a gravitational
contraction into gravitationally bound clusters with masses comparable to solid objects of several
hundred kilometers in diameter. As 100-km scale protoplanets may form in an environment dense
in dust and pebbles, the accretion of small solids is important for their further growth. I show that
accretion of pebbles in a gaseous environment is very efficient and that the process leads to
prograde spin up of the growing protoplanets, in agreement with the trend for prograde rotation
displayed by the largest asteroids.

Apr. 13th

Adam Burrows Princeton University (Tinsley Visiting Professor)
"A View of the Frontier: Exoplanet Research at the Crossroads"

Approximately 500 exoplanets, mostly giant planets in the Jovian mass range, but also more
than 70 "Neptune-mass" planets, have been detected orbiting stars in the solar neighborhood.
More than 15% of these are transiting their primaries and these have collectively yielded a wealth
of structural and physical information. A number of these exoplanets have recently been detected
directly by the Spitzer infrared space telescope, NICMOS on HST, Kepler, and CoRoT in secondary
eclipse and primary transit. These constitute the first remote-sensing data of extrasolar worlds,
and JWST stands in the wings to push this science to the next level. I will present the embryonic
theory of such irradiated giant planets and discuss what we have learned about their atmospheres,
radii, and compositions. I will also address what we may soon learn from the variety of ground-based
and space-based telescopes being planned or already under development as the exploding field
of exoplanets accelerates into its future.

Apr. 20th

Dara Norman National Optical Astronomy Observatory (NOAO)
"A Perspective on Quasar Fueling from LRG-QSO Correlation Functions"

Public data from the 2dF quasar survey and 2dF/Sloan Digital Sky Survey LRG and QSO, with
their vast reservoirs of spectroscopically located and identified sources, afford us the chance to
more accurately study their real-space correlations in the hopes of identifying the physical
processes that trigger quasar activity. We have used these two public databases to measure
the projected cross-correlation, omega p , between quasars and luminous red galaxies. We find
the projected two-point correlation to have a fitted clustering radius of r_0 = 5.3 +/- 0.6 and a slope
gamma = 1.83 +/- 0.42 on scales from 0.7 to 27 h -1 Mpc. We attempt to understand this strong
correlation by separating the LRG sample into two populations of blue and red galaxies. We
measure at the cross-correlation with each population. We find that these quasars have a stronger
correlation amplitude with the bluer, more recently star-forming population in our sample than
the redder passively evolving population. We compare this result to published work on hierarchical
models. The stronger correlation of bright quasars with LRGs that have undergone a recent burst
of star formation suggests that the physical mechanisms that produce both activities are related
and that minor mergers or tidal effects may be important triggers of bright quasar activity and/or
that bright quasars are less highly biased than faint quasars.

Apr. 27th

Raffaella Schneider INAF/Osservotorio Astrofisico di Arcetri
"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.

May. 4th

Emily Freeland Texas A&M University
"Bend-Double Radio Sources as Probes of Intergalactic Gas in Galaxies:
Implications for Missing Baryons and Galaxy Evolution"

As the most common environment in the universe, groups of galaxies are likely to contain
a significant fraction of the missing baryons in the form of intergalactic gas. The density
of this gas is an important factor in the effectiveness of ram pressure stripping on the evolution
of galaxies in these systems. I will discuss the intergalactic gas densities that we measure
using radio galaxies with bent jets. I will also present evidence that the group environment
is altering the atomic hydrogen content of dwarf galaxies through ram pressure stripping.

Last Modified: May 20, 2010