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 Astronomy Colloquia at Caltech for 2010-2011 

Colloquia are held every Wednesday during the academic year at 4:00 pm in the Cahill Hameetman auditorium. Tea and cookies are served at 3:45 pm in the Ahmanson library located on the 1st floor of Cahill.

 

 

Date

Speaker

                                                   Talk Title

SEPTEMBER

29

Lynne Hillenbrand

Caltech

Host: Sterl Phinney

New Worlds, New Horizons in Astronomy and Astrophysics

 

This talk will summarize the process and main results of the Astro2010 Decadal Survey including the state of the profession, the scientific motivation and recommended program, and the overall context of federal funding for astronomy and astrophysics

OCTOBER

6

Andrew Drake

Caltech

Host: George Djorgovski

CRTS: An Open Optical Transient Survey.

 

The Catalina Real-time Transient Survey (CRTS) is a Caltech operated optical transient survey that covers most of the Northern and Southern sky in search of transient astrophysical phenomena occurring on timescales of minutes to years. The project uses data from the Catalina Sky Survey NEO search and began real-time discovery and publication of transient events in November 2007. CRTS has found thousands of sources ranging from UV Ceti and dwarf nova outbursts to supernovae and Blazars. I will discuss the survey, the discoveries made to date, and our efforts to provide immediate open access to CRTS discoveries and historical CSS data.

13

Lars Bildsten

UCSB/KITP

Host: Sterl Phinney

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 1051 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.

20

M. Coleman Miller

University of Maryland
Host: Christian Ott

A Possible Explanation for Puzzling Properties of X-ray Bursts

 

Thermonuclear X-ray bursts, in which matter accreted by a neutron star from a stellar companion undergoes rapid and unstable nuclear burning, have become popular recently as events that can reveal the masses and radii of neutron stars and hence provide critical clues about the currently unknown state of matter in their cores.  These inferences require that the model atmosphere spectra developed for bursting neutron stars are good representations of the data.  I show, however, that for the highest-precision data from the Rossi X-ray Timing Explorer, a simple Bose-Einstein spectrum is clearly preferred to the model spectra.  A consequence of this is that although the peak luminosity of the brightest bursts probably is close to the Eddington luminosity, the surface flux is often 3-5 times Eddington and thus the luminosity is produced by 20-30% of the surface. This result requires the rethinking of many aspects of bursts and their implications for neutron star structure. 

27

Margaret Geller

CFA

Host: Judy Cohen

A Lensing Map ... A Redshift Survey

 

Redshift surveys and weak lensing maps are two powerful tools of modern cosmology. SHELS, a set of deep complete redshift surveys in the foreground of several weak lensing maps, offers independent measurements of the dark matter distribution ``imaged'' by the weak lensing maps. The comparison enables evaluation the completeness and efficiency of catalogs of massive clusters of galaxies (halos) derived from weak lensing surveys. Understanding the strengths and limitations of weak lensing is an important foundation for JDEM.

NOVEMBER

3

Phil Chang

CITA

Host: Christian Ott

Heavy Vortices and Dark Energy

 

I will discuss two problems in the fields of planet formation and cosmology.  The first problem concerns the formation of planetesimals. The formation of planetesimals starts with coagulation of dust particles up to pebbles on cm scales and ends with gravitational accretion of km-size bodies.  How these pebbles grow from cm scales to km scales remains a major unsolved problem in astrophysics. Vortices in protoplanetary disks may serve as nurseries for these planetesimals by concentrating dust in there cores, where these pebbles can directly form these km-sized bodies by gravitational collapse.  I will show that this is not the case.  Rather, the concentration of these pebbles in the cores of these vortices leads to a new instability, the heavy-core instability. I will discuss the general physics of this instability and argue that this instabilty precludes vortices from being the nurseries of planetesimal formation.

The second problem I will discuss is using stellar evolution to place constraints on the nature of dark energy.  One recent suggestion is the dark energy arises from the presence of a scalar field, i.e., a fifth force, that couples to matter like gravity.  Locally (in our high density patch of the universe) this scalar field is not observed, but it operates in the low density regions of the universe. I will show that the effect of this scalar field will modified stellar structure and evolution in these parts of the universe and how careful observations of the colors of red giants can put constraints on these theories.

10

Christopher Kochanek

Ohio State

Host: Christian Ott

Do Massive Stars Explode?

We generally assume that most (>90%) of massive stars explode as supernovae and that they lack any detectable phenomenon associated with their final evolutionary phases. The experimental evidence for the first of these assumptions is weak -- the similarity of massive star formation and supernova rates probably requires that at least 50% of massive stars become SN.  The theoretical support is also week - after 50 years of effort, some theorists can sometimes make some stars explode.  In the second case, the experimental evidence appears to contradict the assumption for at least two types of SN/massive star transients - one where the star ejects an enormous amount of mass shortly before the SN, and another where the star becomes self-obscured by a dusty wind shortly before some kind of eruption.  Both of these assumptions can be experimentally checked using deep optical and mid-IR monitoring studies to characterize the progenitors.

17

Ben Zuckerman

UCLA

Host: Jessica Lu

Violent events in extrasolar planetary systems: from post-T Tauri stars to white dwarfs.

 

 24   

 Thanksgiving

NO COLLOQUIUM

December

1

Jessica Lu

Caltech

Host: Lynne Hillenbrand

 Star Formation in Extreme Environments

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. Precise IMFs are constructed by using high-precision astrometry to distinguish individual cluster stars. I will discuss the whether the measured IMFs differ for massive clusters at a range of Galactocentric radii and how they compare to the "universal" IMF established locally.

 

 

8

Aaron Parsons

UC Berkeley

Host: Shri Kulkarni

21cm Cosmology: Probing the Epoch of Reionization


The Epoch of Reionization (EoR)--the rapid ionization of the majority of the hydrogen in the universe by the light of the first stars and supermassive black holes--is perhaps the last major phase transition of our universe that remains unexplored.  First-generation experiments aiming to measure the 3-dimensional power spectrum of reionization fluctuations via redshifted 21cm emission are currently underway.  While calibration, foreground removal, and obtaining the requisite sensitivity are all challenging aspects of these efforts, early results suggest that there may imminently be a detection that significantly impacts our understanding of the dominant processes at work during this era.

I will discuss current prospects for detecting the 21cm EoR signal in the context of our recent progress with 16- and 32-antenna deployments of the Precision Array for Probing the Epoch of Reionziation (PAPER) in Green Bank, West Virginia, and the Karoo Desert of South Africa. I will also discuss a novel technique for accessing the 3-dimensional power spectrum of reionization, and the impacts of systematics and foregrounds on recent measurements.  Finally, I will present our current plans for the Hydrogen Epoch of Reionization Array (HERA) that will supersede all current efforts and enable the direct imaging of reionization structures.

15

  Neelima Sehgal

  Stanford

  Host: Judy Cohen

Cosmology from Sunyaev-Zel'dovich Galaxy Clusters Detected with the Atacama Cosmology Telescope

For the first time microwave surveys such as the Atacama Cosmology Telescope (ACT) and the South Pole Telescope (SPT) are detecting galaxy clusters through the Sunyaev-Zel'dovich (SZ) effect. Counts of galaxy clusters as a function of mass and redshift provide a powerful probe of structure growth and cosmology. I will discuss constraints on the matter power spectrum amplitude, sigma_8, and dark energy equation of state, w, from SZ cluster counts detected with ACT.

JANUARY

5

Evan Kirby

Caltech

Host: Judy Cohen

The Chemical Evolution of Milky Way Satellite Galaxies from Keck/DEIMOS Multi-Element Abundance Measurements


I will present the results of a Keck/DEIMOS spectroscopic campaign of eight Milky Way dwarf spheroidal (dSph) satellite galaxies. The primary data product is a catalog of nearly 3000 stars with spectral synthesis-based abundance measurements of Fe and the alpha elements Mg, Si, Ca, and Ti. The dSph metallicity distributions show that the histories of the less luminous dSphs were marked by massive amounts of gas loss.  From the [alpha/Fe] distributions, I will demonstrate that the early star formation histories of most dSphs was very similar and that Type Ia supernova ejecta contribute to the abundances of all but the most metal-poor ([Fe/H] < -2.5) stars. Finally, I will show the results of a numerical chemical evolution model, which reveals that the star formation history of a dSph is a strong function of its present-day luminosity, but not velocity dispersion, half-light radius, or Galactocentric distance.

12

Geoffrey Bower

UC Berkeley

Host: Tony Readhead

Wide Field Radio Transient Surveys

The time domain of the radio wavelength sky has been only sparsely explored. Nevertheless, serendipitous discovery and results from limited surveys indicate that there is much to be found on timescales from nanoseconds to years and at wavelengths from meters to millimeters. These observations have revealed unexpected phenonmena such as rotating radio transients and coherent pulses from brown dwarfs. Additionally, archival studies have revealed an unknown class of radio transients without radio, optical, or high-energy hosts. The new generation of centimeter-wave radio telescopes such as the Allen Telescope Array (ATA) will exploit wide fields of view and flexible digital signal processing to systematically explore radio transient parameter space, as well as lay the scientific and technical foundation for the Square Kilometer Array. Known unknowns that will be the target of future transient surveys include orphan gamma-ray burst afterglows, radio supernovae, tidally-disrupted stars, flare stars, and magnetars. While probing the variable sky, these surveys will also provide unprecedented information on the static radio sky. I will present results from three large ATA surveys (the Fly’s Eye survey, the ATA Twenty CM Survey (ATATS), and the Pi GHz Survey (PiGSS)) and several small ATA transient searches.  Finally, I will discuss the landscape and opportunities for future instruments at centimeter wavelengths.

19

Meg Urry

Yale

Host: Judy Cohen

 Supermassive Black Hole Growth and Galaxy Evolution


The growth of black holes over billions of years releases energy that may quench star formation (“feedback”). Tracing the history of black hole growth out to very high redshifts with multiwavelength surveys, we find that most AGN are heavily obscured and that obscuration is more common in the young Universe and in low-luminosity AGN. By studying the host galaxies of AGN, we see evidence at z~1 that AGN may help quench star formation, even though at z~0 star formation in the local Universe appears to turn off well before AGN reach their peak brightness. Finally, we find an intriguing dependence of AGN activity on host galaxy morphology, which may hint at AGN triggering mechanisms.

26

Garth Illingworth

UCSC

Host: Christian Ott

Galaxies in the Epoch of Reionization: Galaxy Buildup in the First Gyr


The extraordinarily-deep HST WFC3/IR data taken in 2009 and 2010 as part of the HUDF09 program has dramatically expanded our insight into the galaxy population at z~7-8-9, and given us constraints on galaxies at z~10. Well over 100 z~7-8 galaxies have now been detected from the deep HUDF09 data and the wider-field ERS data. These samples have enabled us to undertake an analysis of the luminosity functions to much lower luminosities.  We have used the very deep data to derive the contribution to the luminosity density and the star formation rate from lower luminosity galaxies to as faint as ~0.05L* from redshift 4-7.  We have incorporated recent results on the contribution of ULIRGS and sub-mm star-forming galaxies to evaluate the relative contributions to the star formation rate density of these massive objects and the lower luminosity, lower mass UV-detected galaxies.  The HST WFC3/IR and Spitzer IRAC data over the HUDF suggests that z~7-8 galaxies include earlier populations that reach back to z>10. I will discuss the advances that have resulted from combining these recent deep HST and the deep Spitzer observations in establishing the star formation rate density and the mass density buildup in the first 1-2 Gyr.

FEBRUARY

2

Tori Hoehler

NASA Ames

Host: Judy Cohen

Physical and chemical toeholds for exoplanet bioastronomy


If a search for exoplanet life were mounted today, the likely focus would be to detect oxygen (or ozone) in the atmosphere of a water-bearing rocky planet orbiting roughly 1AU from a G-type star.  This appropriately conservative and practical default is necessary in large part because biological input on the question of where and how to look for life has progressed little beyond a purely empirical reliance on the example of terrestrial biology.  However, the apparent and expected diversity of exoplanetary environments that may be accessible to observation provides motivation to reevaluate habitability and biosignatures in broader terms.  Consideration of the core properties of life in reference to fundamental chemical and physical principles may serve to significantly constrain the search space for exoplanet bioastronomy, without purely empirical reference the specifics of terrestrial life.  Potential physical and chemical “toeholds” for consideration of the liquid water + oxygen paradigm will be suggested, as specific examples of this approach.

9

Marcella Carollo

ETH Zurich

Host: Peter Capak

Group behavior: The case of galaxies


I will present observational results that indicate a significant role of the group environment on the evolution of galaxies. Numerical simulations and models cast light on the possible physical causes and on how the evolution of baryons is tied to that of the dark matter.

16

Nick Kaiser

IfA Hawaii

Host: Peter Capak

Early Science from the Pan-STARRS PS1 Telescope

The 1.8m Pan-STARRS PS1 telescope on Haleakala, Maui features a 1.4 billion pixel CCD camera and 7 square degree field of view.  It has been in full-time operation for a little over a year, carrying out surveys to map the entire sky visible from Hawaii in 5 pass-bands (g,r,i,z & y) as well as deeper surveys of selected areas of sky. These surveys are designed to facilitate studies in 12 key projects, ranging from asteroids in the inner solar system to testing cosmology with observations of distant galaxies. In this talk, I describe the design of the system, the system performance and the status of the surveys. Early science results include galaxy counts, a Hubble diagram from supernovae, galactic structure and mapping of dust in the galaxy and, in the solar system, new Kuiper belt objects, comets and potentially hazardous asteroids. I conclude with an update on the status of the second telescope, due to join PS1 next year.

23

Fred Adams

Michigan
Host: Hillenbrand/Cohen



Constraints on the Birth Environment of the Solar System

Most stars -- and hence most solar systems -- form within groups and clusters.  The first objective of this talk is to explore how these star forming environments affect solar systems forming within them. The discussion starts with the dynamical evolution of young clusters with N = 100 - 3000 members. We use N-body simulations to study how evolution depends on system size and initial conditions.  Multiple realizations of equivalent cases are used to build up a robust statistical description of these systems, e.g., distributions of closest approaches and radial locations.  These results provide a framework from which to assess the effects of clusters on solar system formation. Distributions of radial positions are used in conjunction with UV luminosity distributions to estimate the radiation exposure of circumstellar disks. Photoevaporation models determine the efficacy of radiation in removing disk gas and compromising planet formation. The distributions of closest approaches are used in conjunction with scattering cross sections to determine probabilities for solar system disruption. The result of this work is a quantitative determination of the effects of clusters on forming solar systems. The second objective of this talk is to use these results to place constraints on the possible birth environments for our solar system.

MARCH

 

2

Norbert Christlieb

University of Heidelberg

Host: Judy Cohen
The metallicity distribution function of the Milky Way halo, as determined from the Hamburg/ESO Survey.
The metallicity distribution functions (MDFs) of the components of the Galaxy provide information on their chemical enrichment history, and the MDFs are key observables of models Galactic chemical evolution (GCE). I will report on determinations of the MDF of the Galactic halo by means of statistically complete samples of metal-poor stars selected with quantitative criteria from the Hamburg/ESO objective-prism survey (HES). I will present comparisons with the MDFs of dwarf galaxies as well as with predictions of GCE models, and give an outlook to on upcoming wide-field surveys for metal-poor stars.

9

Mark Krumholz
UCSC
Host: Hillenbrand

 The Formation of Massive Stars

I discuss our current understanding of how massive stars form, with attention to three important problems. First, under what circumstances is it possible for a self-gravitating cloud to avoid fragmenting into numerous small stars, and instead form a single massive one? Second, why is it that massive stars are all binaries, and what determines the properties of those binaries? Third, how are massive stars able to continue accreting despite the enormous radiation pressure that they exert on the dusty gas around them? I present provisional solutions to all of these problems, and discuss prospects for future work.

16

Dimitrios Giannios
Princeton
Host: Judy Cohen

Structure and radiation of relativistic jets

Collimated, relativistic outflows have been observed or inferred to originate from supermassive black holes in the centers of galaxies, solar-mass compact objects in binaries and gamma-ray bursts. A theoretical paradigm for jet formation has been developed since the late 1970s that can account in a unifying manner for all these sources. This paradigm envisions that jets come from rotating objects (neutron stars, black-hole ergospheres, or accretion disks) with strong magnetic fields that extract their rotational energy.

Despite recent progress in the field, we still lack a coherent connection between the jet dynamics and the electromagnetic radiation emitted by these sources. The guiding theme that I propose for such a connection is the dissipation of magnetic energy in the jet. This key process connects the aspects of acceleration of jets, their interaction with the external medium, and radiation mechanisms.

    23

SPRING BREAK

NO COLLOQUIUM

    30

Jacob Bean
Harvard
Host: Shri Kulkarni
The Small Star Opportunity to Find and Characterize Habitable Planets

One of the most exciting aspects of the field of exoplanets is the push towards the detailed study of habitable planets. Although most attention in this area is focused on Sun-like stars and has a time-horizon of decades, low-mass stars offer an opportunity for the detection and atmospheric characterization of such planets in the near-future. I will describe work to overcome the technical challenges to making these kinds of observations for planetary systems around low-mass stars. I will also present results from two studies that utilize the power of low-mass stars: a planet search sensitive to potentially habitable planets and the first atmospheric characterization of a "super-earth" type planet. I will conclude with a look ahead at how similar observations with future instruments could yield the detection of biologically relevant molecules in the atmosphere of a potentially habitable planet around a low-mass star by the end of the decade.

APRIL

6 

Adam Burgasser
UCSD
Host: Jessica Lu

Cool Star Science with the FIRE Spectrograph


The Folded-port InfraRed Echellette (FIRE) has recently been commissioned on the Magellan 6.5m Baade Telescope. This single object, near-infrared spectrometer simultaneously covers the 0.85-2.45 μm window in both cross-dispersed (λ/∆λ ≈ 6000) or prism-dispersed (λ/∆λ ≈ 250-350) modes. FIRE’s compact configuration, high transmission optics and high quantum efficiency detector provides considerable sensitivity in the near-infrared, making it an ideal instrument for studies of cool stars and brown dwarfs. In this talk I present a sample of some of the first cool star science results with FIRE based on commissioning and science verification observations, including: accretion and jet emission in the low-mass T Tauri star TWA 30B; radial and rotational velocities of T-type brown dwarfs; evidence of increased cloudiness in young, planetary-mass objects; discovery and characterization spectra of very low-temperature WISE brown dwarf discoveries; and a stringent limit on the near-infrared brightness for what appears to be the first ~300 K "room temperature" brown dwarf.

13

Martin Haehnelt

Cambridge
Host: W. Sargent/Cohen

Probing  galaxies and the IGM with Lyman-alpha
absorption and emission.

I will give an overview of  the use of QSO absorption spectra to study the matter distribution, the metagalactic UV background  and the physical state of the IGM. I will further discuss  how the spectral shape and spatial distribution of Lyman-alpha emission act as powerful diagnostic of the kinematics of the galactic winds in DLA/LLS host galaxies and LBGs.

20

Harvey Tananbaum

CHANDRA Center

Host: Lynne Hillenbrand

Chandra's First Decade (Plus) of Discovery
(13th annual Greenstein Lecturer)


NASA's Chandra X-ray Observatory has provided an unprecedented view of the Universe at x-ray wavelengths since its launch in July 1999. Chandra's spectacular images and detailed spectra of astrophysical systems ranging from solar system objects to distant galaxies and clusters shed (x-ray) light on diverse topics such as stellar formation and demise, black hole-galaxy-cluster interactions, and properties of dark matter and dark energy. After a brief overview and status report on the Observatory, the focus of the talk will be on selected scientific highlights with emphasis on recent Chandra results.

27

Brian Mason
NRAO
Host: Sunil Golwala

High Resolution Imaging of the Sunyaev-Zel'dovich Effect in Galaxy Clusters

Galaxy clusters are the most massive gravitationally bound objects in the universe and, as such, are extensively used as cosmological probes. These systems are usually assumed to be relaxed & spherical to simplify the analyses. High angular resolution X-ray and, now, Sunyaev-Zeldovich Effect (SZE) measurements often reveal striking deviations from equilibrium. These signatures convey information about the physical  processes in, and history of, the Intra-Cluster Medium, and can bias and  degrade results from  large scale SZE surveys. The GBT is now engaged in a systematic survey of SZ clusters at high angular resolution using MUSTANG. I will present results from this ongoing campaign, describe our plans for further observations, and discuss future directions.

MAY

4

Michael Shull

University of Colorado

Host: Christian Ott

Recent Results from the Cosmic Origins Spectrograph on Hubble

The Cosmic Origins Spectrograph (COS) installed on the Hubble Space Telescope provides access to high-quality ultraviolet spectra of the intergalactic medium (IGM), galactic halos, quasars, hot stars, accretion disks, and helium reionization.   I will describe new COS results on the thermal state, metallicity, and spatial distribution of the IGM at redshifts z < 0.5 and the epoch of reionization in helium at z = 2.5-3.3. We are also characterizing the ionizing (EUV) continua of quasars at z < 1.  I will also describe the Colorado group's numerical simulations of the IGM, and the dynamical and radiative feedback from galaxy formation, using the adaptive-mesh N-body hydrodynamic code Enzo.  These simulations help to understand the effects of IGM shock-heating, metal-transport, radiative cooling, and photoionization of  the "Cosmic Web" during large-scale structure formation and galaxy outflows. 

11

Lars Hernquist
CfA

Host: Nick Scoville

Collective Origin of Spiral Structure in Disk Galaxies

It is now nearly 50 years since spiral structure in galaxies was hypothesized to originate from density waves propagating through a shearing disk.  However, the nature of this process remains uncertain. Relevant theories range from interpreting spiral arms as long-lived density waves to their being produced stochastically in response to gravitational perturbations.  In this talk, I examine the latter possibility, where spiral arms are seeded by density fluctuations orbiting within a disk.  Using high-resolution simulations, I study the response of a thin, differentially rotating disk of stars to a population of perturbers.  Individually, each perturber excites a wake locally in the distribution of stars around it.  When sufficient numbers of these perturbers are present, they collectively amplify to yield large-scale patterns that resemble those in flocculent and intermediate spiral galaxies.  Combining the N-body experiments with simple analytic arguments, I develop a theory for spiral structure based on the collective effects of swing amplification.  The model makes numerous testable predictions, making it possible to finally test the theory that spiral arms are stochastic in nature.

18

Mike Fitzgerald

UCLA

Host: Lynne Hillenbrand

 Adaptive Optics Imaging of Circumstellar Debris Disks

Disks of dusty solid material around nearby stars are useful as tracers of planet formation.  The dust in these systems is created by the erosion of larger parent bodies, and these freshly created grains are subject to forces that alter their trajectories around the star.  This dust is detectable by its thermal emission or via scattered starlight.  The spatial distribution of dust grains reflects both the distribution of parent bodies and the the forces perturbing grain orbits.  High-contrast imaging of circumstellar debris disks can be used to infer the presence of extrasolar planets and to constrain their orbital properties.  I will present Keck adaptive optics coronagraphic imaging of circumstellar debris systems and their connections to planet formation.  I will also discuss future prospects for jointly imaging planets and their associated scattered-light dust disks with the Gemini Planet Imager.

25

Brian Metzger
Princeton

Host: Christian Ott

Electromagnetic Transients from Coalescing Compact Binaries (& Other Exotica)

Sensitive, wide-field surveys are revolutionizing our understanding of transient astrophysical phenomena across the electromagnetic (EM) spectrum.  In this talk I will illustrate the discovery potential of transient surveys in the coming decade by describing three examples of "known unknowns", i.e. events that we are confident occur in Nature, but for which an unambiguous EM counterpart has not yet been identified.  The first example is the inspiral and merger of two neutron stars (NSs) or a NS and a black hole (BH).  These events may be responsible for short duration gamma-ray bursts (GRB) and are among the most promising sources of gravitational waves for detection with Advanced LIGO.  I will show that ejecta from the merger produces heavy radioactive elements via r-process nucleosynthesis, the decay of which subsequentially powers an optical transient lasting about 1 day.  Next, I will describe a model for accretion following the tidal disruption of a white dwarf (WD) by a NS or BH.  I will demonstrate that densities and temperatures in the disk are sufficiently high to burn the WD material into increasingly heavier elements at sequentially smaller radii.  Outflows from the disk may result in a week-long supernova-like transient, powered by the decay of radioactive Nickel.  The final "known unknown" I will discuss is the tidal disruption of a star by a supermassive BH.  Although these events have long been predicted to be accompanied by a thermal flare, powered by the accretion of bound stellar debris, I will describe the observational consequences if a fraction of the accretion power is channeled into a relativistic jet.  The recently-discovered transient Swift J164449.3+573451 may be the first example of a tidal disruption event viewed "face on", in analogy to the blazar geometry of normal AGN.

JUNE

 

1

Michael Liu
Hawaii
Host: Jessica Lu

Studying Young Gas-Giant Planets Directly
 

Direct detection is rapidly becoming a key method to study gas-giant extrasolar planets. Direct photometry, spectroscopy, and astrometry of planets around young (<~1 Gyr) stars are now possible using the largest ground-based telescopes equipped with adaptive optics. Akin to the early growth of radial velocity and transit studies, direct measurements of the first discoveries have revealed a surprising diversity in the exoplanet population. We discuss how these initial results have challenged our conceptions of gas-giant planet formation and properties. To move forward from these exciting individual discoveries to well-defined statistical samples, we are currently carrying out a 3-year observing campaign using the Near-Infrared Coronagraphic Imager (NICI) on the Gemini-South Telescope. NICI typically achieves about 1-2 magnitudes better contrast compared to previous planet-finding efforts. We describe the Campaign's goals, design, performance, and results. The NICI Planet-Finding Campaign represents the largest and most sensitive imaging survey to date for massive (~1 Mjup) planets around other stars.


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