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 Astronomy Colloquia at Caltech for 2012-13

Colloquia are held every Wednesday during the academic year at 4pm in the Cahill Hameetman auditorium.
Wine and cheese will be served in the Cahill Foyer from 5-6pm.





                                                   Talk Title



James Lattimer
SUNY/Kingsley Visitor
Host: Christian Ott

Have We Converged on an Understanding of the Dense Matter Equation of State?

Analyses of several recent experiments suggest that the symmetry energy of nuclear matter is now relatively well understood. The inferred symmetry energy is also very similar to that predicted by calculations of the properties of pure neutron matter. Since the behavior of the symmetry energy near the nuclear saturation density is closely coupled to the neutron star mass-radius relation, it is predicted that the radii of neutron stars with approximately 1.4 - 1.5 solar masses should lie in the range 11 to 12.5 km. This result is in substantial agreement with radii estimated from observations of quiescent neutron stars in globular clusters and photospheric radius expansion X-ray bursts. Neutron star radii together with the knowledge that the maximum neutron star mass exceeds 2 solar masses, as recent discoveries of massive neutron stars prove, can strongly constrain the properties of matter at densities several times greater than the nuclear saturation density.


Charlie Conroy
Host: Judy Cohen

From the IMF to the S-Process in Early-Type Galaxies

Our understanding of the stellar populations of unresolved galaxies is fundamentally limited by our lack of knowledge of the stellar initial mass function (IMF), and its variation with environment and epoch.  In this talk I will present a technique that provides direct measurements of the low-mass IMF (~0.1-1 Msun) in unresolved stellar systems.  When this technique is applied to high-quality optical-NIR spectra of nearby early-type galaxies, evidence is found for an IMF that is substantially more "bottom-heavy" than the Galactic IMF.   I will also discuss ongoing work aimed at measuring elemental abundances (including C, N, O, Na, Mg, Ca, Ti, Cr, Mn, Fe, Sr, and Ba)  of these galaxies, with the goal of constraining their formation histories.


Ramesh Narayan
Host: Tony Readhead

Black Hole Spin, Accretion, and Jets

An astrophysical black hole is fully described with only two parameters, its mass and its spin.  This simplicity makes the black hole a perfect laboratory for exploring both the physics of accretion flows and the mechanisms by which relativistic jets are ejected. Using a combination of numerical simulations and observational data, our group has explored the properties of accretion disks around black holes, focusing in particular on the regions of the disk near and inside the innermost stable circular orbit. These studies have enabled us to measure the spin parameters of ten stellar-mass black holes. We find an intriguing correlation between the radio power of relativistic jets and the black hole spin parameter. This is the first direct evidence that jets may be powered by black hole spin energy.


David Jewitt

Host: Nick Scoville
Under Our Very Noses: A Surprise from the Asteroid Belt

Asteroids and comets formed on opposite sides of the snow-line. They convey to us information about substantially different radial distances in the protoplanetary disk of the Sun.  We have recently discovered strange hybrid objects which orbit in the asteroid belt but whose properties are those of comets (from the much more distant and icy Kuiper belt).  I will describe these new objects and discuss what they tell us about the origin of the oceans and about dust production in the solar system. 


 Daniel Fabrycky
 University of Chicago 
 Host: John Johnson

  Planetary System Architectures from Kepler

The architecture of the Solar System is characterized by the planets
orbiting in the same direction the Sun spins.  The interpretation is the whole system arose from a spinning gas cloud, which flattened as it lost energy.  Surprisingly, although hundreds of exoplanets had been found, we are only now learning about orbital alignments, via the stringent constraints and large numbers afforded by multiply-transiting systems in NASA's Kepler mission.  I will take stock of the ~400 systems with more than one transiting planet found so far.  By comparing transit durations of planets in the same system, we can see that inclinations of planets relative to each other are on the order of 2 degrees, just like in the Solar System.  In one system of 3 planets, we can see the planets crossing starspots, and the detailed pattern shows that the planetary orbits and stellar spin are oriented in the same direction.  Finally, I describe the six known circumbinary transiting planets.  The dramatic time variations seen in these systems also encodes their nearly flat architecture, as well as giving crucial constraints for formation theories in a dynamically harsh environment.



Fred Lo

Host: Tony Readhead

The Mega-maser Cosmology Project and NRAO

The Mega-maser Cosmology Project (MCP) aims to determine the Hubble Constant, Ho, to ~3 percent accuracy by measuring the angular diameter distance to "mega-maser galaxies" that harbor circum nuclear Keplerian disks exhibiting powerful water maser emission.  Under the MCP, mega-maser search using the GBT and mapping using the VLBA and GBT have now identified 10 mega-maser galaxies with distances ranging from 30 Mpc to 140 Mpc, well in the Hubble flow.  A 10% angular diameter distance determination to NGC6264 at 150 Mpc has been demonstrated.  A by-product of the MCP is precise mass determination of the supermassive black-holes (BH) in the mega-maser galaxies.  The mega-maser BH mass results appear to be inconsistent with the putative M-sigma relationship between black holes and galactic bulges. The status of the MCP will be described.  The current status and recent science of ALMA and other NRAO facilities will also be presented.


Samaya Nissanke
Host: Tony Readhead

Hearing and seeing the violent universe

The strongest gravitational waves (GWs) in the universe are expected to arise from violent events such as the mergers of neutron star or black hole binaries. Decades of theoretical and experimental efforts could result in the first direct detection of GWs within the decade. Subsequent measurements should offer us an unprecedented view of strong-field gravity in action. A subset of neutron star binary mergers could be associated with a transient electromagnetic (EM) counterpart, and EM follow-up using radio,
optical and high-energy facilities of these events are critical for improving our understanding of the physics underlying strong-field gravity astrophysics, and compact objects. I will discuss the astrophysical insights that we can learn from a EM+GW measurement and the challenges that lie ahead in pinpointing neutron star binary mergers using networks of GW interferometers and multiwavelength EM telescopes and arrays.



     28   Nick Cowan
  Host: Ranga-Ram Chary
Divining the Climate of Exoplanets

The vast majority of known planets do not orbit the Sun.  Exoplanets are being discovered at an    unprecedented rate, but we know precious little about them aside from mass, incident stellar flux, and sometimes bulk density.  To understand what these worlds are like we need to know their climate.  I will describe what we have learned so far from observations of short-period, gas giant planets using state of-the-art telescopes. Extending these studies to Earth-like planets will most likely require expensive space missions, and it is worth studying which measurement suites best constrain simple climate models.  I will describe how the changing color and brightness of a terrestrial exoplanet, at both visible and thermal wavelengths, can be used to constrain its surface conditions and climate.



Fiona Harrison
Host: Ranga-Ram Chary

What Goes on Near Black Holes?  First Results from NuSTAR

The Nuclear Spectroscopic Telescope Array (NuSTAR) mission, launched on June 13, 2012, is the first space-based focusing high-energy X-ray telescope.  NuSTAR operates in the band from 4 -- 79~keV, extending the sensitivity of focusing far beyond the ~10 keV high energy cutoff achieved by any previous X-ray telescope. NuSTAR's first science targets included the black hole at the center of the Milky Way and its surroundings, massive black holes in AGN, as well as stellar mass black holes. I will present preliminary results from these observations, highlighting the new information provided by sensitive high-energy X-ray spectroscopy.






Julie McEnery
Host: Jennifer Siegal-Gaskins

Exploring the Extreme Universe with the Fermi Gamma-ray Space Telescope

Following its launch in June 2008, high-energy gamma-ray observations by the Fermi Gamma-ray Space Telescope have unveiled over 1000 new sources and opened an important and previously unexplored window on a wide variety of phenomena. These have included the discovery of an population of pulsars pulsing only in gamma rays; the detection of photons up to 10s of GeV from gamma-ray bursts, enhancing our understanding of the astrophysics of these powerful explosions; the detection of hundreds of active galaxies; a measurement of the high energy cosmic-ray electron spectrum which may imply the presence of nearby astrophysical particle accelerators; and constraints on phenomena such as supersymmetric dark-matter annihilations and exotic relics from the Big Bang.  Continuous monitoring of the high-energy gamma-ray sky has uncovered numerous outbursts from active galaxies and the discovery of transient sources in our galaxy. In this talk I will describe the current status of the Fermi observatory, review the science highlights and discuss future opportunities with Fermi.


Scott Tremaine
IAS Princeton
Host: Richard Ellis
Are planetary systems flat?

Laplace argued, correctly, that the small inclinations of planetary orbits implied that the solar system formed from a flat disk.  The evidence on whether extrasolar planetary systems are flat, however, is still ambiguous. I will describe (i) measurements of the Rossiter-McLaughlin effect in transiting planets, which show that many planetary systems have large stellar obliquities (angle between the stellar spin angular momentum and planetary orbital angular momentum); (ii) the curious phenomenon of Kozai-Lidov oscillations and their role in exciting eccentricities, inclinations, and obliquities; (iii) constraints on the flatness of planetary systems from the Kepler spacecraft and other sources; (iv) disk and high-eccentricity migration as competing mechanisms for the formation of hot Jupiters.


Dan Perley
Host: Richard Ellis

Quantifying High-Redshift Star Formation with Gamma-Ray Bursts - Promises and Perils

One of the most exciting broader applications of the study of gamma-ray bursts is in better understanding the star-formation history of the Universe, especially at the highest redshifts.  Long-duration GRBs are produced exclusively by massive, young stars, and can be detected beyond z>6 (and potentially even z>9) with small telescopes, pinpointing a SFR-selected sample of galaxies at each redshift unimpeded (in principle) by issues of gas and dust attenuation and without regard to host luminosity.  On the other hand, a GRB-selected sample may introduce new biases (such as a dependence on metallicity) that may be difficult to control for, and achieving the goal of an unbiased host-galaxy sample is much more complicated in practice---in particular with regard to the effects of dust attenuation, which can prevent accurate localization of the host.  I will discuss recent efforts to shine light upon the population of heavily dust-obscured GRBs and their host galaxies via an ongoing, multi-wavelength campaign with Keck, HST, Spitzer, and the VLA.  I will present results showing how a full accounting of these events will influence our view of the relation between the GRB and star-formation rates in the high-redshift universe, as well as conclusions involving the spatial distribution of dust within star-forming galaxies and the efficacy of common assumptions used to estimate star-formation rates in populations of faint galaxies.



Genevieve Graves
Host: Richard Ellis

Observing the Unobservable: Tracing Dark Matter Haloes and Galaxy Assembly

A fundamental challenge in cosmology and galaxy evolution is to understand how dark matter (DM) haloes influence the galaxies that form inside them.  Unfortunately, the parts of this process that we can simulate well—the growth of DM structures under the influence of gravity—cannot be observed directly, while the observable stars and gas are difficult to simulate over large scales because of the complicated physics involved.  I will present two projects that tackle different aspects of this problem in galaxy evolution.  The first is a new method for measuring weak gravitational lensing.  This method uses a photometry-only analog to the Fundamental Plane of early type galaxies in order to measure magnification due to weak lensing.  Combined with existing techniques based on gravitational shear, this method will produce the most direct measurements of dark matter haloes around ordinary galaxies, allowing us to connect observed galaxies with the dark matter haloes that host them.  The second project uses detailed, resolved galaxy kinematics and stellar population gradients in local massive galaxies to trace their assembly history.  The goal is to isolate contributions from in situ star formation, major versus minor mergers, and the possible large-scale stripping of globular clusters.  I will present a pilot study of M87, which is the first of several dozen local galaxies we ultimately aim to analyze.


Henry Ferguson
Host: Carrie Bridge
Observing Galaxy Assembly

The Cosmic Assembly Near-IR Deep Extragalactic Legacy Survey (CANDELS) is a multi-cycle observing program with the Hubble space telescope (and many other facilities) designed to   the first third of galactic evolution, from redshift z~8 to 1.5.  It is also designed to find  and measure Type Ia SNe beyond z > 1.5 and test their accuracy as standard candles for cosmology. The talk will discuss findings from the two-thirds of the survey, including observations of AGN hosts at high redshifts, and the evolution of structure of passive and star-forming galaxies.


Richard Ellis
Host: Ranga-Ram Chary
Observations of Star-Forming Galaxies in the Reionization Era

Deep exposures with the Hubble Space Telescope (HST) have provided the primary evidence that star-forming galaxies were present in the first billion years of cosmic history. Sometime during this early period the intergalactic medium transitioned from a neutral gas to one that is fully ionized. How did this `cosmic reionization' occur and were star-forming galaxies responsible? The electron scattering optical depth inferred from cosmic microwave background observations suggest that reionization occurred sometime in the redshift interval z=20 to z=6 so probing the abundance, luminosity distribution and spectral properties of galaxies during this uncharted period holds the key to progress in addressing these fundamental questions. The Ultra Deep Field 2012 campaign represents a new series of infrared images totaling 128 orbits taken with HST's Wide Field Camera 3. Through its added depth and appropriate use of additional filters compared to earlier work, new insight has been gained into our understanding of when reionization occurred and the role of early galaxies in the process.


Mansi Kasliwal
Host: Carrie Bridge
The Bridged Gap: Transients in the Local Universe

Our framework of cosmic explosions no longer has a glaring six-magnitude luminosity "gap" between the brightest novae and faintest supernovae. We now know of at least four new classes of transients that bridge this gap: .Ia explosions, calcium-rich halo transients, intermediate luminosity red transients, and luminous red novae. As we better characterize these recent discoveries, we find that these classes represent different missing pieces in our understanding of stellar evolution: electron capture induced collapse in low mass stars, fallback onto black holes in high mass stars, helium shell detonation in ultracompact binaries, and stellar mergers. Moreover, Calcium-rich gap transients may explain the old conundrum of the observed overabundance of Calcium in clusters.  The next frontier in gap transients is uncovering elusive binary neutron star mergers; a goal which may soon be within reach with coordination between the next generation of synoptic surveys and advanced gravitational wave interferometers. I conclude with the preparation for this march towards discovering even fainter, even faster and even rarer transients in the local universe.




Robert Kirshner
Host: Assaf Horesh
Exploding stars and Cosmic Acceleration: Better Precision in the Infrared

SN Ia are the best astronomical objects for measuring cosmic distances.  They provide direct evidence for cosmic acceleration, as recently recognized by the 2011 Nobel Prize in Physics.  Large samples have been gathered to constrain the properties of dark energy, but they are beginning to hit the limits imposed by systematic errors.  Recent observations show that SN Ia are even better distance indicators when measured in near infrared bands from 1 to 2 microns.  They are more nearly standard candles and they are less affected by dust absorption.  I will show the evidence for the good behavior of SN IA in the IR and describe the RAISIN program being carried out now with PanSTARRS and the Hubble Space Telescope to use IR observations of SN IA to improve constraints on dark energy.


Emily Levesque
Univ of Colorado
Host: Lynne Hillenbrand
Massive Stars Across the Cosmos: Engines, Lighthouses, and Laboratories

Massive stars can provide valuable information on a Galactic, extragalactic, and even cosmological scale. The radiative signatures observed in HII regions and star-forming galaxies are determined by massive stellar populations. Long-duration gamma-ray bursts, produced during the core-collapse deaths of unusual massive stars, can be utilized as powerful probes of the high-redshift universe. Finally, massive star populations in the Milky Way and Local Group allow us to closely examine how these stars evolve, how their physical parameters are impacted by host environments, and how they contribute to interstellar and intergalactic enrichment through processes such as nucleosynthesis, mass loss, and supernova feedback. I will present recent research focused on the next generation of current stellar population synthesis models, detailed studies of gamma-ray burst host galaxies, and the impact of host environments on the physical properties and evolution of the Local Group massive star population. Combining examinations of distant star-forming galaxies and gamma-ray bursts with the study of nearby stellar populations in this manner will greatly improve our understanding of massive stars and their utility as tools that can directly contribute to our greater picture of the cosmos.


Stuart Littlefair
Kinsgley Visitor
Host: Gregg Hallinan
Adventures in High Time Resolution Astrophysics

Time resolved astrophysics benefits enormously from specialised instrumentation. In the ten years that the fast photometer ULTRACAM has been operating it has made many important contributions to astronomy, in fields as diverse as Solar System objects, interacting binaries, white dwarfs, brown dwarfs, black holes and pulsars. In this talk I present some of the scientific highlights from ULTRACAM, as well as other instrumentation developed at the University of Sheffield and a look at where the future of time resolved instruments might lie.


 Rennan Barkana

 Tel Aviv
 Host: Richard Ellis

  Detecting the First Stars at Redshift 20:

  Understanding the formation and evolution of the first stars andgalaxies is one of the most exciting frontiers in astronomy. Since the
  universe was filled with neutral hydrogen at early times, the most promising method for observing the epoch of the first stars is using
  the prominent 21-cm spectral line of hydrogen. Current observational efforts are focused on a cosmic age of 500 million years,
  with earlier times considered much more challenging. We show that stars from a much earlier era may be observable as a result of a 
  recently noticed effect of different motions of the dark matter and the baryons in the early universe. We produce simulated maps of the
  first stars and show that these relative motions significantly enhance large-scale fluctuations and produce prominent structure on 
  the scale of a degree in the 21-cm intensity distribution. The particular signature of this structure should make it easier to confirm the
  existence of million solar-mass halos at early times.




Hans-Walter Rix
Host: Richard Ellis
How the Milky Way Built its Disk

If we could map the overall structure of the Milky Way's stellar disk, looking only at stars of a given age or abundance, it would be a direct empirical route to delineating how a typical galaxy disk was built. We have solved the problem of how to look at our own Galaxy this way, using large ground-based spectroscopic surveys. I will show how the overall radial and vertical structure of the Milky Way's disk changes as a function of stellar abundances, which may serve as proxies for stellar ages. Through comparison with disk formation simulations, I will also discuss what this may mean for the build-up of the Milky Way disk and its evolution. Slicing the Milky Way's disk into sets of such stellar'mono-abundance populations' is also proving a powerful way to constrain the Galactic potential.


Alex Szalay
Johns Hopkins University

Kingsley Visitor
Host: George Djorgovski

Redshift Space Distortions of the Galaxy Distribution

The talk will present results from analyzing the SDSS Main Galaxy Survey. We will discuss the impact of using unusual geometries in computing the galaxy correlation function. We show that redshift-space distortions can substantially sharpen the BAO peak in directions close to the line of sight. We also demonstrate a detection of Baryon Acoustic Oscillations even in this relatively shallow galaxy sample.


Armin Rest
Host: Ranga-Ram Chary
An Astronomical Time Machine: Light Echoes from Historic Supernovae and Stellar Eruptions

Tycho Brahe's observations of a supernova in 1572 challenged the dogma that the celestial realm was unchanging. 440 years later we have once again seen the light that Tycho saw: some of the light from the 1572 supernova is reflected off dust and is only now reaching Earth. These light echoes, as well as ones detected from other historic events like Cas A and Eta Carinae's Great Eruption, give us a very rare opportunity in astronomy: direct observation of the cause (the explosion/eruption) and the effect (the remnant) of the same astronomical event. Furthermore, in some cases we can compare light echoes at different angles around a remnant, and thus investigate possible asymmetry in the explosion. I will discuss how the unprecedented view at these exciting events with light echoes allows us to unravel some of their secrets.


Steven Tingay
Curtin Institute, Western Australia
Host: Gregg Hallinan

  The Murchison Widefield Array: the low frequency Precursor for the
  Square Kilometre Array

  I will describe the Murchison Widefield Array (MWA), a new low frequency (80 - 300 MHz) interferometer that
  consists of 128 aperture array "tiles" with a maximum baseline of 3 km.  The MWA is located in the pristine
  environment of remote Western Australia, one of the two sites that will host the Square Kilometre Array (SKA),
  and is the only low frequency Precursor for the SKA (one of three official Precursors).  The science mission for
  the MWA includes: the search for redshifted HI signals from the Epoch of Reionisation (EoR); studies of  fast
  and slow transients and variable objects; large-scale galactic and extragalactic continuum surveys (full Stokes);
  and studies of solar, heliospheric and ionospheric phenomena.  Key characteristics that enable the suite of MWA
  science goals include its very wide field of view (~600 sq. deg. corresponding to primary beam FWHM at 150
  MHz), flexible signal processing backend, and massive data archive capability.  I'll describe the completion of
  the construction phase of the instrument, its capabilities, the science mission, results from science 
  commissioning and the commencement of full science operations expected from July 2013.  I will briefly
  describe the central role the  MWA will play during the SKA pre-construction phase (2013 - 2016), particular to
  the development of the low frequency SKA




Juna Kollmeier
Host: Nick Scoville

RR Lyrae Variables: Getting Out of the Galaxy

RR Lyrae stars are once again poised to play a critical role in launching humanity out of the solar system and Milky Way and into the local cosmos. The "era of precision cosmology" has
given rise to new tension between the Hubble constant derived from Planck+Standard Cosmology (67) and the local Cepheid-based determination (72), whose difference is larger than the formal errors.  Before inferring
"new physics" from this conflict, the Cepheid result must be independently checked.  Several advances in astronomical techniques now permit RR Lyrae stars to play this role.  I report new work toward calibrating
RR Lyraes at the 2% level via statistical parallax and argue that they are likely to be
exquisite individual distance indicators in the infrared.  Present and future large telescopes should enable precision tests of Cepheid distances to at least a dozen galaxies with a range of properties, the only completely independent test presently imaginable.


Chung Pei Ma
UC Berkeley
Kingsley Visitor
Host: Lynne Hillenbrand

Supermassive Black Holes in Giant Elliptical Galaxies

New kinematic data and modeling efforts in the past few years have substantially expanded and revised dynamical measurements of black hole masses at the centers of nearby galaxies.  I will describe an ongoing project to measure some of the most massive black holes in giant elliptical galaxies using 10 meter class telescopes, integral field spectrographs, and adaptive optics.  I will present updated scaling relations between the black hole mass and host galaxy properties such as the stellar velocity dispersion and bulge mass, and discuss the implications of these correlations for the formation and evolution of massive galaxies.


Hilke Schlichting
Host: Nick Scoville

Planet formation in the Kepler Era

I will review the standard scenario for planet formation, which was motived to explain our Solar System, and show that planet formation likely proceeded differently for many planetary systems discovered to date by Kepler. In addition, I will present a new model for orbital resonance. I will show that it explains the surprisingly small number of near commensurate orbits in multi-planet systems and accounts for the deficit of planet pairs at exact resonance and for the corresponding excess of pairs with slightly greater separation.


  Ranga-Ram Chary
  Host: Judy Cohen

  CSI-Los Angeles: The Growth of Distant Galaxies and Reionization
 The discovery of an unusual population of galaxies with extremely strong nebular line emission has come as a
  panacea to several problems in galaxy evolution. These objects, classified as Halpha emitters, were first
  detected in the z>5 Universe in deep Spitzer imaging, where they dominate the spectroscopically confirmed
  star-forming galaxy population. Since then, they have been re-discovered in the local Universe in the Sloan  
  Digital Sky Survey where they constitute 0.04% of the spectroscopic sample, and at intermediate redshift

  using  Hubble/WFC3 grism surveys. Their ubiquity in the distant Universe and the unusual properties of their
  stellar population make them strong candidates for being progenitors of massive galaxies at z~2 and for being
  the primary sources responsible for reionization. I will present our work in identifying these galaxies, insights
  into the mechanism for fueling star-formation in these objects and their role in reionization. The merits of using
  multiwavelength observations in obtaining an improved understanding of high redshift galaxy populations will
  in particular be highlighted.

     29   Christy Tremonti
  Host: Carrie Bridge
  Gone with the Wind?  Galactic winds and star formation quenching

  One of the defining features of the last 8-10 billion years of cosmic history is the
  emergence of a red sequence of non-star-forming galaxies. In spite of considerable
  theoretical and observational work, there is still no consensus on what causes blue
  galaxies to stop forming stars and transition onto the red sequence.  To gain insight  into
  the processes at work, we have been studying samples of galaxies in the midst of star
  formation quenching.  We have identified post-starburst galaxies spectroscopically at
  z~0.1, 0.4, and 0.6 from SDSS-I and SDSS-III/BOSS.  I will discuss the relationship
  between post-starbursts and galaxy mergers,  and the role of AGN and star-formation
  driven galactic winds in truncating star formation.



Mark Reid
Host: Tony Readhead

First Results on the Structure of the Milky Way from the BeSSeL Survey

Using the VLBA, we are measuring distances to massive star forming regions by trigonometic parallax.  We routinely achieve parallax accuracy of ~10 micro-arcseconds, which corresponds to 10% distance accuracy at 10 kpc!  The first results from the Bar and Spiral Structure Legacy (BeSSeL) Survey will be presented.  These measurements directly address the nature of the spiral structure, size, rotation speed, and mass of the Milky Way.

Information for Speakers


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Updated: 1/29/13 gma