MIT Astrophysics Colloquia - Spring 2010

Tuesdays at 4:00 PM in the Marlar Lounge, Room 37-252
MIT Kavli Institute for Astrophysics and Space Research
70 Vassar Street, Cambridge, MA
(unless location otherwise noted)
Refreshments are served at 3:45 PM.

Sponsored by
the Astrophysics Division of the MIT Department of Physics and
the MIT Kavli Institute for Astrophysics and Space Research.

THURSDAY January 28:
The Illuminating Deaths of Massive Stars
Robert Quimby
California Institute of Technology
Host: Nergis Mavalvala

Abstract: A new class of stellar outbursts dwarfing the most powerful supernovae observed in the past century has recently been uncovered by wide field optical imaging surveys. With peak luminosities in excess of 10^44 erg/s and total radiative outputs greater than 10^51 erg, these events push the limits of conventional supernova explosion theory. It is possible that these luminous supernovae are triggered by the electron-positron pair instability, and they may thus represent local analogs of the first stellar explosions to shape the universe. Their high (restframe) ultra-violet luminosities suggest that similar events may already be detectable with existing instrumentation out to redshifts of z~4 and that they will become valued tools for selecting and studying galaxies at even larger redshifts. In this talk, I will highlight some of the key discoveries in this emerging field and the prospects for future studies.
Tuesday February 2:
Soliciting Secrets from Strong Field Spacetimes
Jeandrew Brink
California Institute of Technology
Host: Scott Hughes

Abstract: Quantitatively mapping the strong field regions around compact objects using the gravitational radiation emitted by Extreme-Mass-Ratio Inspirals has been shown to be possible in principle. In practice, however, only inspirals in the Kerr spacetime have been studied in detail. I discuss several aspects of the theoretical development required to make the problem of mapping the spacetime around generic compact objects tractable. Some recent results about particle orbits around Bumpy Black holes are highlighted.

On the Comparable-Mass-Ratio front, the field of Numerical Relativity has matured rapidly during the last five years, giving us access to highly non-linear regions of Spacetime. I use analogies from Fluid Dynamics to provide a framework for asking probing questions to further our understanding of the merger of Binary Black holes in General Relativity.

THURSDAY February 4 AT 2PM:
Beyond the M-sigma relation: The role of black holes in galaxy evolution
Jenny Greene
Princeton University
Host: Paul Schechter

Abstract: Supermassive black holes (BHs) are a ubiquitous component of nearby galaxies, and we believe that BH growth plays an important role in galaxy evolution. I discuss three projects, each designed to elucidate the role of BH growth in galaxy evolution. I begin with a new look at local BH-bulge scaling relations provided by a large sample of spiral galaxies containing megamaser disks. The resulting exquisite BH mass measurements provide strong evidence that BHs evolve differently in low-mass spiral galaxies. Second, studies of BH demographics as a function of cosmic time provide complementary constraints on the coevolution of BHs and galaxies. I discuss ongoing efforts to use active galaxies as a tracer of the BH population at high redshift. Finally, I present an exciting new sample of dual BHs uncovered with Magellan.
Tuesday February 9:
Images and spectra of an extrasolar planetary system
Bruce Macintosh
Lawrence Livermore National Laboratory
Host: Joshua Winn

Abstract: Although more than 400 extrasolar planets are now known, almost all have been detected indirectly - through radial velocity measurements or eclipses of their parent star. Direct detection - spatially resolving the planet from the star - opens up new areas of exoplanet phase space and new avenues for planet characterization. It is also extremely challenging, since a mature Jupiter-like planet is 10^9 times fainter than its host star. The promise of this approach was recently demonstrated with HST images of a planet orbiting Fomalhaut and adaptive optics images of a three-planet system orbiting the young A star HR8799. I will discuss the HR8799 system in detail, including photometry, properties of the host star, astrometry and orbital stability. We have also now obtained a spectrum of the outermost planet in the system - the coolest exoplanet ever studied spectroscopically - and these are showing an atmosphere very different from a brown dwarf.

The HR8799 planets were detectable because they are extremely young (60 Myr) and massive (5-10 Jupiter masses.) To next major step in direct detection will be dedicated instruments such as the Gemini Planet Imager (GPI). GPI will use a 4000-actuator MEMS deformable mirror, an advanced coronagraph, and nanometer-precision wavefront sensing to achieve contrasts 1-2 orders of magnitude better than any current ground or space facility. I will briefly discuss the design and scientific capabilities of GPI, which is planned to have first light in 2011 on the Gemini South 8-m telescope.

THURSDAY February 11 AT 2PM:
Characterizing the Transient Sky
Daniel Kasen
University of California, Santa Cruz
Host: Saul Rappaport

Abstract: In the coming years, astronomical surveys will repeatedly scan the night sky with unprecedented depth and field of view, revealing an enormous number of supernovae and other optical transients -- many of which have not been seen before. The study of these stellar explosions is not only a vibrant field in itself, but also deeply impacts topics in cosmology, nucleosynthesis, compact objects, and the sources of gravitational waves. I discuss here advances in the theory and simulation of explosive transients as a means to interpret, classify and exploit these observations. I focus on the physics of thermonuclear -- or Type Ia -- supernova explosions, and on how to secure these as measures of cosmic expansion. Using the first set of multi-dimensional light curve calculations, I show how variations in Type Ia brightness are driven by deviations from spherical symmetry, and explain the physics of the empirical width-luminosity relation used to calibrate them. I then present predictions for two other kinds of important transients: the disruption of extremely massive stars via the electron-positron pair instability (believed to characterize the first generation of stars) and the faint optical transients from the merger of neutron stars (considered promising targets for gravitational wave observatories).
Tuesday February 16:
Understanding the Diverse Explosions of Massive Stars: Supernovae, Gamma-Ray Bursts, and their Host Galaxies
Maryam Modjaz
University of California, Berkeley
Host: Saul Rappaport

Abstract: Long-duration gamma-ray bursts (GRBs) and Type Ib/c Supernovae (SN Ib/c) are two of nature's most magnificent explosions. Both can be seen over cosmological distances, and both are products of collapsing massive stars. While GRBs launch relativistic jets, SN Ib/c are core-collapse explosions whose massive progenitors have been stripped of their hydrogen and helium envelopes. Yet for over a decade, one of the key outstanding questions in astronomy is what conditions lead to each kind of explosion in massive stars. Determining the fate of massive stars is essential for using GRBs as star formation indicators over distances up to 13 billion light-years, and for mapping the chemical enrichment history of the universe.

I will present a number of comprehensive observational studies that probe the progenitor environments, their metallicities and the explosion conditions of SN with and without GRBs. Specifically, my benchmark study on the measured metallicities of SN with and without GRBs indicates that low metallicity (less than ~1/3 solar) might be the key factor for producing SN-GRBs, providing constraints on the theoretical predictions of GRB formation. Furthermore, I will discuss SN 2008D, which was discovered serendipitously in January 2008 with the NASA Swift satellite via its X-ray emission and has generated great interest amongst both observers and theorists. I will discuss the significance of this SN, whether it harbored a jet, and its implications for the SN-GRB connection. I will conclude with an outlook on how the most promising venues of research - using both existing facilities such as Magellan and innovative SN surveys, and also upcoming large-scale surveys such as LSST - will shed light on the diverse deaths of massive stars.

THURSDAY February 18 AT 2PM:
Initial Results from the Atacama Cosmology Telescope
Jonathan Sievers
Canadian Institute for Theoretical Astrophysics, University of Toronto
Host: Paul Schechter

Abstract: The Atacama Cosmology Telescope (ACT) is a 6-meter telescope observing the Cosmic Microwave Background from 17,000 feet in the Chilean Andes. ACT has a total of three thousand bolometers observing at three frequencies (150, 220, and 270 GHz). ACT's main science goals include a high-precision measurement of the CMB angular power spectrum on arcminute scales as well as studying galaxy clusters through their impact on the CMB. I will present the results from the first full season of observations with ACT from the 150 GHz channel, including the power spectrum and cluster discoveries. I will also present a brief update on efforts to measure redshifted 21-cm emission from the Epoch of Reionization using the Giant Metre-Wave Radio Telescope.
Tuesday February 23:
The Formation Histories of Massive Galaxies
Mariska Kriek
Princeton University
Host: Paul Schechter

Abstract: Recent studies have demonstrated that the galaxy population at z=2-3 shares characteristics with today's galaxies: massive galaxies are predominantly red, a color-density relation was already in place, and massive quiescent galaxies form a red sequence in color-mass space. However, the high-redshift universe far from resembles the local universe. Massive galaxies at z~2.5 do not seem to represent a Hubble sequence as their structures and morphologies are different from their local analogs. Furthermore, the space density of massive galaxies still has to grow significantly, implying that many local early-type galaxies assemble or form at later times. In my talk I will discuss these similarities and differences, and their implications for our understanding of the physical processes that govern galaxy formation and evolution.
THURSDAY February 25 AT 2PM:
Nonlinear tides in close binary systems
Nevin Weinberg
University of California, Berkeley
Host: Saul Rappaport

Abstract: Tides strongly influence the evolution and fate of a wide variety of binary systems, from extrasolar planets to compact object binaries. Despite the subjects long history, there remain major gaps in our understanding of tidal evolution, with many of the observed orbital properties of close binaries still not well understood. In this talk I will describe ongoing work that attempts to go beyond the standard linear theory treatment of tides. Focusing on solar-type stars with short-period stellar or planetary (hot Jupiter) companions, I will describe how nonlinear interactions dramatically modify the excitation and damping of resonantly excited internal waves in stars. I will show that the linear approximation used in previous investigations fails over much of the parameter space in which it is employed and discuss how nonlinear effects influence the orbital properties of solar-type binaries and the orbital decay of hot Jupiters. I will conclude with a discussion of future applications of this nonlinear treatment of tides, including the role of nonlinear tidal interactions during the inspiral of compact object binaries (double white dwarf and double neutron star binaries).
predominantly red, a color-density relation was already in place, and massive quiescent galaxies form a red sequence in color-mass space. However, the high-redshift universe far from resembles the local universe. Massive galaxies at z~2.5 do not seem to represent a Hubble sequence as their structures and morphologies are different from their local analogs. Furthermore, the space density of massive galaxies still has to grow significantly, implying that many local early-type galaxies assemble or form at later times. In my talk I will discuss these similarities and differences, and their implications for our understanding of the physical processes that govern galaxy formation and evolution.
Tuesday March 2:
Planet-star interactions and exoplanet magnetic fields
Caleb Scharf
Columbia University
Host: Scott Hughes

Abstract: A small percentage of normal stars harbor giant planets that orbit within a few tenths of an astronomical unit. At such distances the potential exists for significant tidal and magnetic field interaction, resulting in energy dissipation. I will discuss some new results from an examination of the high-energy environment of planet-hosting stars - that show evidence for a remarkable correlation between planet mass and stellar X-ray luminosity. Interpretation of these results is fraught with potential systematics and observational biases, however they appear to hold up to scrutiny. Optimistically simple models then yield an estimate of the relative magnetic field strength of planets from 1 to 10 Jupiter masses that seems to match the predictions of geodynamo scaling laws.
Tuesday March 9:
Understanding the Star Formation Rate
Mark Krumholz
UC Santa Cruz
Host: Scott Hughes

Abstract: Stars are the engines of the Universe: nuclear reactions within them are the only significant source of non-gravitational power in the cosmos and the source of all heavy elements. However, the process by which stars form remains poorly understood, and one mystery in particular stands out: why is star formation so slow? In many galaxies the bulk of the interstellar medium does not participate in star formation, and in all galaxies even those clouds that are active form stars at a rate of only ~1% of their mass per dynamical time. Any successful theory of cosmic evolution must be able to explain these facts, and be able to predict how the star formation process changes with galactic environment and over cosmological time. In this talk I discuss progress toward a physical theory of star formation capable of meeting these requirements.
Tuesday March 16:
Black hole Accretion in the Nearby Universe: Evidence for Down-Sizing
Elena Gallo
Massachusetts Institute of Technology
Host: Scott Hughes

Abstract: An issue of fundamental importance in understanding the galaxy-black hole connection is the duty cycle of accretion. If black holes are indeed ubiquitous in galactic nuclei, little is known about the frequency and intensity of their activity, the more so at the low-mass/low-luminosity end. I will present new results from AMUSE-Virgo, a Chandra survey of (formally) inactive early type galaxies in the Virgo cluster. Out of 100 objects, 32 show a nuclear X-ray source, including 6 hybrid nuclei which also host a massive nuclear cluster as visible from archival HST images. After carefully accounting for contamination from nuclear low mass X-ray binaries based on the shape and normalization of their X-ray luminosity function, we conclude that between 24-34% of the galaxies in our sample host a X-ray active super-massive black hole. This sets a firm lower limit to the black hole occupation fraction in nearby bulges within a cluster environment. At face value, the active fraction is found to increase with host stellar mass. However, taking into account selection effects, we find that the average Eddington-scaled X-ray luminosity scales with black hole mass to the power -0.62, with an intrinsic scatter of 0.46 dex. This finding can be interpreted as observational evidence for `down-sizing' of black hole accretion in local early types, that is, low mass black holes shine relatively closer to their Eddington limit than higher mass objects. As a consequence, the fraction of active galaxies, defined as those above a fixed X-ray Eddington ratio, decreases with increasing black hole mass.
Tuesday March 23:
Spring Break: No Colloquium
Tuesday March 30:
Detecting Extrasolar Planets, Plants and Beaches
Edwin Turner
Princeton University
Host: Scott Hughes

Abstract: Following an introductory overview of the scientific goals of optical/NIR imaging of extrasolar terrestrial planets, work on some related technical problems in coronagraphic optics, terrestrial planet characterization using diurnal light curves, Earthshine observations of the our planet's diffuse scattered light and speculative, but potentially practical, techniques for the direct detection of extrasolar plants and coastlines will be presented.
WEDNESDAY April 7: PLEASE NOTE UNUSUAL DATE!
High-z Galaxy Formation in the Standard Cosmology
Avishai Dekel
Hebrew University, Jerusalem
Host: Ed Bertschinger

Abstract: Using hydro-cosmological simulations and analytic modeling, we attempt solid predictions for the formation of massive galaxies at high redshift within the LCDM cosmology. The emerging new picture highlights the formation of galaxies at the nodes of the cosmic web. These galaxies are steadily fed by cold streams along dark-matter filaments, which are observable in Lyman-alpha. The streams, including a smooth component and merging galaxies, penetrate through hot gas in dark-matter halos to form extended, turbulent, rotating disks and bulges. The intense gas input drives a self-regulated, violent gravitational disk instability with transient features and giant clumps, where stars form rapidly. The violent instability induces quick migration to the center, complementing mergers as a mechanism for spheroid formation. The clumpy disks evolve in steady state for a few Gyr, where star formation and bulge growth are governed by the cosmological streaming. Star formation is suppressed in small halos by stellar feedbacks and in massive halos by gravitational quenching and AGN feedback --- these processes pose interesting open questions. We propose that the development of disks versus spheroids, and the opposite phenomena of star formation and quenching, are all determined by the streams' intensity and degree of clumpiness. The role of major mergers in star formation and quenching is secondary. This picture seems consistent with recent observations of galaxies at high redshifts.
SPECIAL COLLOQUIUM MONDAY April 12 AT 3 PM:

(Note: Although not part of our "normal" colloquium series, the following special seminar is likely of enough interest that we advertise it alongside our regular colloquia.)

Differential Rotation and Convection in the Sun
Steven A. Balbus
Ecole Normale Superieure, Paris
Host: Benjamin Weiss (EAPS)

Abstract: The differential rotation profile of the solar convection zone, apart from inner and outer boundary layers, can be reproduced with remarkable accuracy if the isorotation contours correspond to the characteristics associated with the classical vorticity equation. The analytic form of these characteristics is surprisingly simple. The very existence of these characteristics, however, requires that there be a formal functional relationship involving the entropy and the angular velocity. In this talk, I will precisely specify this relationship, and explain how it naturally arises from the dynamics of convection and differential rotation. Direct support for the entropy/angular velocity relationship can be adduced from published numerical simulations of the solar convection zone. More generally, the theory can also be used diagnostically for the internal entropy structure of the solar convective zone, and may be extended to fully convective stars.
Tuesday April 13:
Big science from little stars: What we can learn from the Volkswagens of the Milky Way
Andrew West
Boston University
Host: Scott Hughes

Abstract: M dwarfs are the smallest stars in the Galaxy that can still fuse hydrogen. They can be 10 times less massive, twice as cool and over 1000 times dimmer than the Sun. Yet despite their diminutive physical properties, M dwarfs make up ~70% of all of the stars in the Milky Way and have main sequence lifetimes that exceed trillions of years (according to theoretical models). Their dominance in the Galaxy makes M dwarfs excellent tracers of both the structure and evolution of the local Milky Way. In addition, these little stars have intense stellar flares and strong magnetic fields that allow us to probe their interiors and may have important consequences for the habitability of planets that orbit them. I will present results from several M dwarf studies that have come out of the Sloan Digital Sky Survey (SDSS) and represent the largest photometric and spectroscopic M dwarf samples ever assembled (many tens of millions and over 50,000 stars respectively). Specifically, I will highlight work that has used the unprecedented statistical power of SDSS to examine the structure and kinematics of M dwarfs in the Milky Way as well as the nature of their magnetic fields (and subsequent activity) and what we can learn about the ages of low-mass stars.
Tuesday April 20:
The most massive objects in the Universe
Daniel Holz
Los Alamos National Laboratory
Host: Scott Hughes

Abstract: Our Universe has a finite observable volume, and therefore within our Universe there is a unique most massive object. This object will be a supercluster of galaxies. The distribution of the highest mass objects in the Universe is a potentially powerful test of the standard Lambda-CDM cosmological model, probing non-Gaussianity and the behavior of gravity on large scales. Theoretical studies of the growth of structure can robustly predict the highest mass objects. Furthermore, it is now possible to observe volume-limited samples of high-mass clusters. Preliminary results are inconclusive, with XMMU J2235.3-2557 being roughly 4 sigma inconsistent with Lambda-CDM, while the most massive clusters in the recent SPT catalog are 2-sigma inconsistent. Future surveys will probe much larger volumes, and the most massive object in the Universe may be identified within the next decade.
Tuesday April 27:
Pushing Einstein's Boundaries: Gravitational Approaches to the Challenges of Modern Cosmology
Mark Trodden
University of Pennsylvania
Host: Scott Hughes

Abstract: Einstein's general theory of relativity (GR) is one of the most successful and well-tested physical theories ever developed. Nevertheless, modern cosmology poses a range of questions, from the smallest scales to the largest, that remain currently unresolved by GR coupled to the known energy and matter contents of the universe. This raises the logical possibility that GR may require modification on the relevant scales.

I will discuss the status of some modern approaches to alter GR to address cosmological problems. We shall see that these efforts are extremely theoretically constrained, leaving very few currently viable approaches. Meanwhile, observationally, upcoming missions promise to constrain allowed departures from GR in exciting new ways, complementary to traditional tests within the solar system.

Tuesday May 4:
Astrophysical Measurement of the Equation of State of Neutron Star Matter
Feryal Ozel
University of Arizona
Host: Scott Hughes

Abstract: Neutron stars are the densest objects in the universe and may contain hyperon-dominated matter, condensed mesons, or even deconfined or strange quark matter. Because of their low temperatures and high chemical potentials, the physical conditions in their interiors differ greatly from the dense conditions of the early universe or those achieved at hadron colliders. This region of the QCD phase diagram can best be probed through astrophysical observations that measure the masses and radii of neutron stars. I will discuss how we can break degeneracies in the measurements of neutron star properties by combining recent developments in our understanding of their atmospheres with observations of multiple spectroscopic phenomena from X-ray binaries. I will present unique measurements of the masses and radii of three neutron stars in low-mass X-ray binaries and show the equations of state of neutron star matter that are compatible with these observations. These measurement constrain, for the first time, the pressure of cold matter above nuclear saturation density and offer tantalizing evidence for new degrees of freedom at ultrahigh densities.
Tuesday May 11:
Radio Millisecond Pulsars: The Gifts that Keep on Giving
Scott Ransom
National Radio Astronomy Observatory
Host: Scott Hughes

Abstract: There are about 2000 pulsars known, and while all of them as neutron stars are fascinating objects, the best and most exciting science comes from a very small percentage (~1%) of exotic objects, most of which are millisecond pulsars (MSPs). Within the past 5-6 years, many such systems have been uncovered, and they are providing a wealth of science. In this talk I'll briefly cover 3 recent results on exotic MSPs. The first is the discovery of a true millisecond pulsar (MSP) in an eccentric orbit around a likely main sequence star in the Galactic disk (systems like that "shouldn't" exist, and in this case the pulsar is also quite massive at ~1.7 Msun). The second is the discovery of a very bright radio MSP which appears to have just been created and is still in transition from its low-mass x-ray binary stage -- a missing-link in the MSP creation process. And the third is a large crop of new (and unexpected!) MSPs uncovered by searching unassociated gamma-ray sources from Fermi.

This page is maintained by Scott Hughes