MIT Astrophysics Colloquia - Fall 2017

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.

Tuesday 12 September :
Imaging All the Sky All the Time in Search of Radio Exoplanets
Gregg Halinan
Host: Jacqueline Hewitt

Abstract: All the magnetized planets in our solar system, including Earth, produce bright emission at low radio frequencies, predominantly originating in high magnetic latitudes and powered by auroral processes. It has long been speculated that similar radio emission may be detectable from exoplanets orbiting nearby stars, which would provide the first direct confirmation of the presence, strength and extent of exoplanetary magnetospheres, as well as informing on their role in shielding the atmospheres of potentially habitable exoplanets. Despite 4 decades of observations, no detection has been achieved. Surprisingly, however, brown dwarfs have been found to produce both radio and optical emissions that are strikingly similar to the auroral emissions from solar system planets, albeit 10,000 times more luminous, bolstering the continued search for similar emission from exoplanets. I will discuss the auroral radio emission from exoplanets and brown dwarfs and introduce a new radio telescope, consisting of 352 antennas spaced across 2.5 km, that images the entire viewable sky every ten seconds at low radio frequencies, thereby monitoring thousands of stellar systems simultaneously in the search for radio emission from exoplanets

Tuesday 19 September :
Discovery potential of pulsar timing arrays
Xavier Siemens
University of Wisconsin-Milwaukee
Host: Matthew Evans

Abstract: For over a decade the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) has been using the Green Bank and Arecibo radio telescopes to monitor millisecond pulsars. The goal of the NANOGrav is to directly detect low-frequency (nano-Hertz) gravitational waves which cause small correlated changes to the times of arrival of radio pulses from millisecond pulsars. The most promising sources of gravitational waves at these frequencies are supermassive binary black holes that coalesce following the mergers of galaxies. In this talk I will give a status update and discuss the work of NANOGrav, as well as our sensitivity to gravitational waves from astrophysical sources.

Tuesday 26 September :
Tracking Planet Footprints in Dusty Disks
Catherine C. Espaillat
Boston University
Host: Nevin Weinberg

Abstract: We know that most stars were once surrounded by protoplanetary disks. How these young disks evolve into planetary systems is a fundamental question in astronomy. Observations of T Tauri stars (TTS) may provide insights, particularly a subset of TTS with "transitional disks" that contain holes or gaps in their dust disk. Many researchers have posited that these holes and gaps are the "footprints" of planets given that theoretical simulations predict that a young, forming planet will clear the material around itself, leaving behind a cavity in the disk. In this talk, I will review the key observational constraints on the dust and gas properties of transitional disks and examine these in the context of theoretical planet-induced disk clearing models. I will also discuss possibilities for future work in this field in the era of ALMA and JWST observations.

Tuesday 03 October :
Puzzles in Galaxy Scaling Relations
Stephane Courteau
Queen's University
Host: Michael McDonald

Abstract: Galaxies like our Milky Way can be described in terms of their structure, dynamics, and stellar populations. Some very robust correlations between galaxy structural properties, such as total luminosity, maximum circular velocity, and size display rather small scatter, hinting at well-regulated galaxy formation processes. A major challenge to understanding these scaling relations, their tight scatter, and ultimately galaxy formation and evolution, is the elusive interplay between visible and dark matter. I will present the latest results on galaxy scaling relations in order to constrain modern structure formation models and the distribution of dark matter in galaxies.

Tuesday 10 October :
How Rare are Earths and how Common are Jupiters?
David Hogg
New York Univeristy
Host: Deepto Chakrabarty

Abstract: The NASA Kepler and K2 missions have increased vastly the number of planets known around other stars. I will describe how we have found some of the hardest-to-find planets (those with small radii, like Earth, or those with long periods, like Jupiter) in these data sets. It involves noise modeling and linear algebra. I will also discuss how it is possible to make inferences about the full population of planets in the Galaxy, given the noisy measurement and uncertain detection of every important known system. It involves building statistical models with hierarchical structure. The questions in the title are not conclusively answered yet: Earths and Jupiters both appear common, but there certainly are some respects in which our Solar System appears unusual. I discuss the prospects and possible show-stoppers in the next generations of data, and concentrate on methodologies that will be critical in the era of TESS.

Tuesday 17 October :
Interpreting Dwarf Galaxies
Alyson Brooks
Rutgers University
Host: Paul Schechter

Abstract: Dwarf galaxies have long provided a challenge to galaxy formation theory within the favored Lambda Cold Dark Matter cosmology. In recent years, advances in cosmological simulations of galaxies have yielded new insights into dwarf formation, pointing to the fact that baryonic physics can reconcile tensions between theory and observations. Despite these advances, new challenges related to dwarf galaxies continue to arise, particularly in matching the predicted abundance of dwarf galaxies with observations. I will discuss whether baryonic physics can continue to reconcile these new challenges. I will also present first results from a suite of simulations that is providing insight and predictions for the upcoming era of dwarf galaxy discovery.

Tuesday 24 October :
Astrophysical Signatures of Black Hole Mergers
Manuela Campanelli
Host: Salvatore Vitale

Abstract: Numerical relativity simulations of binary black hole inspirals and mergers played a crucial role in the calculations of the gravitational wave signals that were just observed by the advanced LIGO and Virgo detectors. For supermassive binary black holes, these signals might also be accompanied by observable electromagnetic signals from surrounding accreting gas. I will briefly review the history of simulation efforts to model these systems in their astrophysical environments. I will also present some exciting new results in the context of magnetohydrodynamical simulation, indicating that supermassive binary black hole sources might indeed be also detectable in the electromagnetic spectrum in the not too distant future.

Tuesday 31 October :
A cosmological simulations view on galaxy size and angular momentum evolution
Shy Genel
Center for Computational Astrophysics, Flatiron Institute
Host: Mark Vogelsberger

Abstract: Scaling relations involving galaxy size and angular momentum content are some of the most telling for galaxy formation scenarios. While cosmological simulations are still struggling to reproduce the observed relations, rapid progress is being made. This allows us to draw conclusions on the processes at play from such simulations with increasing confidence. I will discuss several recent results that make connections between galaxy size, angular momentum, feedback, and formation history, which touch on one of the oldest puzzles about galaxies: why do they come in two distinct types?

Tuesday 07 November :
Matter in compact binary mergers
Jocelyn Read
California State University
Host: Nevin Weinberg

Abstract: Mergers of binary neutron stars or neutron-star/black-hole systems are promising targets for gravitational-wave detection. The dynamics of merging compact objects, and thus their gravitational-wave signatures, are primarily determined by the mass and spin of the components. However, the presence of matter can make an imprint on the final orbits and merger of a binary system. I will outline efforts to model and understand the impact of neutron-star matter on gravitational waves, using both theoretical and computational input, so that gravitational-wave observations can be used to learn about neutron-star matter.

Tuesday 14 November :
Rattle and Shine: Joint Detection of Gravitational Waves and Light from the Binary Neutron Star Merger GW170817
Edo Berger
Harvard CfA
Host: Erik Katsavounidis

Abstract: The much-anticipated joint detection of gravitational waves and electromagnetic radiation was achieved for the first time on August 17, 2017, for the binary neutron star merger GW170817. This event was detected by Advanced LIGO/Virgo, gamma-ray satellites, and dozens of telescopes on the ground and in space spanning from radio to X-rays. In this talk I will describe the exciting discovery of the optical counterpart, which in turn led to several detailed studies across the electromagnetic spectrum. The results of the observations carried out by our team include the first detailed study of a "kilonova", an optical/infrared counterpart powered by the radioactive decay of r-process nuclei synthesized in the merger, as well as the detection of an off-axis jet powering radio and X-ray emission. These results provide the first direct evidence that neutron star mergers are the dominant site for the r-process and are the progenitors of short GRBs. I will also describe how studies of the host galaxy shed light on the merger timescale, and describe initial constraints on the Hubble Constant from the combined GW and EM detection.

Tuesday 21 November :
The Chemical Composition of Planet Forming Disks
Karin Oberg
Harvard University
Host: Ian Crossfield

Abstract: In the past few years a number of rocky, temperate exoplanets have been identified. How often can we expect such planets to be hospitable to life from a chemical point of view? In other words how often do such planets acquire a rich organic inventory? In this talk I will argue that characterizing the chemistry of protoplanetary disks, the formation sites of planets, is key to address the likelihood of finding life on planets, and to identify life when present. The most direct path to constrain the chemistry in disks is to directly observe it. The arrival of ALMA has provided us with the perfect tool to do exactly that. Recent ALMA highlights include constraints on snowline locations, maps of the distributions of small organic molecules, and first detections of more complex organics. Observations can only provide chemical snapshots, however, and even ALMA is blind to the complete chemical composition of disks. To interpret disk chemistry observations and address the full chemical complexity in disks, we require models informed by laboratory experiments, as well as constraints from protostellar and cometary chemical inventories. I will present key constraints obtained from such models, laboratory experiments and observations, and discuss how they inform our interpretations of ALMA observations, and further, predictions of the chemical compositions of nascent planets.

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