MIT Astrophysics Brown Bag Lunch Series: Spring 2017

Mondays at 12:05 PM in 37-187 (unless otherwise noted)
MIT Kavli Institute for Astrophysics and Space Research
70 Vassar Street, Cambridge, MA

The Brown Bag lunch is a forum for visiting astronomers/astrophysicists to speak about their research. Talks begin at 12:05 and speakers should plan 40 minutes of material, to leave room for questions during and after the presentation. If you are visiting MIT and would like to give a lunch talk, you may either contact the organizers directly or have your local colleagues arrange a time. The present organizers of the series are Michael McDonald and Paul Torrey.


Wednesday January 11 -- note day of the week

Reverberation Mapping of AGN Accredition Disks

Michael Fausnaugh
Ohio State

Reverberation mapping is a well-established method for measuring the masses of super-massive black holes in AGN.  This technique can also be used to probe the temperature structure of AGN accretion disks by measuring time-delays between broad-band continuum light curves. I will discuss the detection of continuum lags in NGC 5548 (from the AGN STORM project) and implications for the accretion disk. I will also present evidence for continuum lags in two other AGN for which we recently measured black hole masses from continuum-Hbeta reverberations. The mass measurements allow us to compare the continuum lags to predictions from standard thin disk theory, and our results indicate that the accretion disks are larger than the simplest expectations.



Monday January 23

Effect of surface-mantle water exchange parameterizations
on the prevalence of waterworlds

Thaddeus Komacek

University of Arizona

Terrestrial exoplanets in the canonical habitable zone may have a variety of initial water fractions due to random volatile delivery by planetesimals. If the total planetary water complement is high, the entire surface may be covered in water, forming a “waterworld.” On a planet with active tectonics, competing mechanisms act to regulate the abundance of water on the surface by determining the partitioning of water between interior and surface. We have explored how the incorporation of different mechanisms for the degassing and regassing of water changes the volatile evolution of a planet. For all of the models considered, volatile cycling reaches an approximate steady-state after ~2 Gyr. Using these steady-states, we find that if volatile cycling is either solely dependent on temperature or seafloor pressure, exoplanets require a high abundance (more than 0.3% of the total mass) of water to have fully inundated surfaces. However, if degassing is more dependent on seafloor pressure and regassing mainly dependent on mantle temperature, the degassing rate is relatively large at late times and a steady-state between degassing and regassing is reached with a substantial surface water fraction. If this hybrid model is physical, super-Earths with a total water fraction similar to that of the Earth can become waterworlds. As a result, further understanding of the processes that drive volatile cycling on terrestrial planets is needed to determine the water fraction at which they are likely to become waterworlds.

Host: Taweewat Somboonpanyakul

Monday January 30

The Evolution and Explosion of Massive Stars

Tuguldur Sukhbold

University of Ohio

How a massive star ends its life depends upon how that life has been lived - the rotation, mass and composition it was born with, mass loss and exchange, and the complex convective and nuclear burning episodes it experienced along the way. In the end, the presupernova stellar core has a density structure that can be characterized by its “compactness” - essentially how fast the density declines outside the iron core. The likelihood that a massive star explodes, by any means, is sensitive to this compactness. It turns out, perhaps surprisingly, that the compactness is not a monotonic function of the star’s birth mass, and, in some mass regions, whether the star explodes or not is almost random. In this talk, I will review the underlying stellar physics for the development of presupernova core compactness, and will present explosion results from a fine grid of masses by assuming neutrino-powered mechanism. Unlike all of the prior explorations, in this survey we give up the “luxury” of exploding a star in any way we want, instead, the explosion energies, nucleosynthesis yields, light curves and remnant masses are all uniquely tied to the progenitor core structure. I’ll end the talk by sharing new results on the role of rotation in stellar explosions.

Host: Anna Frebel

Monday March 6

Fossil System Progenitors in the CASSOWARY Catalog:
Properties of Progenitors and Fossil Formation Implications

Lucas Johnson

University of Alabama

Galaxy groups and clusters are among the oldest and most massive structures in the universe, housing enormous elliptical galaxies in their centers. A subset of these galaxy systems, known as fossil systems, take this to the extreme by hosting overly-luminous central galaxies compared to the systemʼs size. The origin of fossil systems has been debated for many years. Are they formed differently than other groups, or is a fossil merely a transitory phase in the normal evolution of a galaxy system? Are fossil systems born or made? The discovery of the Cheshire Cat fossil group progenitor (Irwin et al. 2015) shows that fossil systems can still be formed, demonstrating that we can find the progenitors to fossil systems today. Finding more fossil progenitors would allow us to determine which formation mechanisms exist for fossil systems. By studying the CASSOWARY catalog of strong gravitational lensing events in the Sloan Digital Sky Survey, we identified 18 additional fossil progenitors along with some of their properties. We present our findings of these progenitors, possible implications for fossil system formation, and preliminary X-ray data for some of the most promising systems.

Host: Eric Miller




Monday April 3

Gaps in Globular Cluster Streams: Pal 5 and the Galactic Bar

Sarah Pearson



Stellar tidal streams in the Milky Way form as globular clusters and dwarf galaxies tidally disrupt in the Milky Way's Galactic potential. Their dynamical evolution can be used to constrain Galactic parameters such as the distribution of dark matter. Additionally, it has been hypothesized that thin stellar streams can serve indirect detection sights for dark matter subhalos as gaps form when they interact with the streams. In principle, this enables an investigation of the predicted power spectrum of dark matter subhalos in our Galaxy and therefore a direct test of LCDM. In this talk I focus on the globular cluster stream Palomar 5 (Pal 5), which has received much attention since its discovery due to its constraining power on the Galactic potential and due to apparent gaps along its stream. Recent Pan-STARRS data show that the leading arm from Pal 5 appears shorter than the trailing arm, while simulations of Pal 5 predict similar angular extents. I demonstrate that including the spinning Galactic bar with appropriate pattern speeds in the dynamical modeling of Pal 5 can reproduce the Pan-STARRS data. As the bar sweeps by, some stream stars experience a difference in net torques near pericenter. This leads to the formation of apparent gaps along Pal 5's tidal streams and these gaps grow due to an energy offset from the rest of the stream members. I conclude that only streams orbiting far from the Galactic center or streams on retrograde orbits (with respect to the bar) can be used to unambiguously constrain dark matter subhalo interactions. Additionally,  the Pal 5 leading arm debris should re-appear south of the Pan-STARRS density truncation, which, if detected, can provide a constraint on the bar pattern speed.


Monday April 10 -- Note 2:30 start time

Discovery of an Extremely Rare Galaxy: PGC 1000714

Burcin Mutlu

University of Minnesota

Non-barred ringed galaxies are among the ideal galaxies to study the role of both the internal dynamics of galaxies and the physics of accretion/interaction mechanisms. An especially interesting ring case is Hoag's Object with its peculiar morphology: an elliptical-like core with a nearly perfect outer ring, and no signs of bar and stellar disc. Hoag-type galaxies, which bear strong resemblance to Hoag's Object, are extremely rare and their origin is still debated. I will present our recent work that has revealed a unique case for Hoag-type galaxies: PGC 1000714 presents strong resemblance to Hoag's Object, with an additional surprise — a second inner ring, which is older, more diffuse, and redder than the outer ring. This is a first description of a double ringed elliptical galaxy. Such peculiar systems help our understanding of galaxy formation in general, since they present extreme cases, providing clues on formation mechanisms.


Monday April 24

Simulating galaxies in the reionization era with realistic stellar feedback

Xiangcheng Ma


Galaxies at z>5 are the dominant sources for cosmic reionization, but current knowledge on these galaxies are restricted by the limited data available. JWST and next generation of ground-based telescopes allow us to probe z>5 galaxies in the rest-frame optical at higher resolution via deeper observations. I will present a new suite of high-resolution cosmological zoom-in simulations of z>5 galaxies, using realistic models of the multi-phase ISM, star formation, and stellar feedback from the FIRE project. These simulations form a more realistic sample of model galaxies and can provide useful predictions for future observations. I will show how these galaxies would appear in JWST, and present the prediction of the stellar mass function, and luminosity functions from z=5 to 12. I will also briefly mention the possible follow-up projects using these simulations, including modeling the dust attenuation at these redshifts, possible present-day globular cluster formation channel, Lyman-alpha emission and Lyman continuum escape fractions, so on and so forth.

Host: Paul Torrey


Monday May 15

The KELT Transit Survey:  Hot Planets around Hot, Bright Stars

Scott Gaudi
Ohio State University


The KELT Transit Survey consists of a pair of small-aperture, wide-angle automated telescope located at Winer Observatory in Sonoita, Arizona and the South African Astronomical Observatory (SAAO) in Sutherland, South Africa. Together, they are surveying roughly 60% of the sky for transiting planets. By virtue of their small apertures (42 mm) and large fields-of-view (26 degrees x 26 degrees), KELT is most sensitive to hot Jupiters transiting relatively bright (V~8-11), and thus relatively hot stars. Roughly half of the dwarf stars targeted by KELT are hotter than 6250K; such stars pose novel challenges, but also provide unique opportunities. I will summarize the properties of the ~20 low-mass transiting companions discovered to date by KELT, focusing in detail on a few particularly interesting systems. 


Thursday May 25 @ 12:05pm

Stellar Velocity Dispersions of Quiescent Galaxies 

Jabran Zahid


Abstract: Galaxies form as gas cools and condenses onto halos of dark matter. Baryons composing the observable parts of galaxies are influenced by dark matter through gravitational interactions. Thus, the motions of stars in galaxies are governed by the baryonic and dark matter content. Stellar velocity dispersion is an observable property directly connecting the baryonic content of galaxies to the unobserved dark matter halo. I will discuss our program to use velocity dispersions as a tool for understanding the dynamical properties of quiescent galaxies and for constraining their evolution. Our results suggest that quiescent galaxies are in approximate virial equilibrium and that the stellar velocity dispersion may be the best observable to connect galaxies to their dark matter halo. I will discuss how these observations may be used to constrain simulations.