MIT Astrophysics Colloquia - Spring 2021

(To be revised for virtual format.)
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.
MKI logo

Tuesday 02 March :
Understanding Planetary System Formation Through Astrochemistry
Ilse Cleeves
University of Virginia
Host: Michelle Kunimoto

Abstract: Historically, our understanding of planet formation and the origins of planets' compositions has been largely informed by our Solar System. However, we are just one system, and now with facilities like NASA's Kepler and TESS telescopes, we are discovering a wide variety of planet types and architectures, many of which are unlike our own. In the last five years, the Atacama Large Millimeter/Submillimeter Array has simultaneously revolutionized our understanding of planet formation by imaging disks around young stars at high resolution and with high sensitivity. In this presentation, I will discuss how observations of molecular spectral line emission in protoplanetary disks can shed light on 1) the compositions of future planets; and 2) the key physics governing disk evolution during the first few million years of evolution, to help us move toward a more general picture of planet formation, at home and abroad.

Tuesday 09 March :
Galactic Archaeology: Piecing Together the History of Our Galaxy
Keith Hawkins
University of Texas, Austin
Host: Lina Necib

Abstract: One of the key objectives of modern astrophysics is to understand the formation and evolution of galaxies. In this regard, the Milky Way is a critical testing ground for our theories of galaxy formation. However, dissecting the assembly history of the galaxy requires a detailed mapping of the structural, dynamical chemical, and age distributions of its stellar populations. Recently, we have entered an era of large spectroscopic and astrometric surveys which has begun to pave the way for the exciting advancements in this field. Combining data from the many multi-object spectroscopic surveys already underway and the rich dataset from Gaia will undoubtedly be the way forward in order to disentangle the full chemo-dynamical history of our Galaxy. In this talk, I will discuss my current work in Galactic archaeology and how large spectroscopic surveys can and have been used to dissect the structure of our Galaxy. I will also explore the future of Galactic archaeology through chemical cartography.

Tuesday 16 March :
Signals Of Light Relics In Cosmic Surveys
Daniel Green
UC San Diego
Host: Kiyoshi Masui

Abstract: I will discuss recent progress understanding how light particles thermalized in the early universe can be detected in the CMB and large scale structure surveys. We will focus in particular on the phase shift of the acoustic peaks caused by free streaming of these relics prior to recombination. We will discuss the origin of this signal in detail including the robustness to nonlinear evolution in the late universe. Finally, we will discuss the recent measurement of the cosmic neutrino background in BOSS DR12 from this phase shift and future applications of these types of analyses.

Tuesday 30 March :
Stellar-sized, mass-gap and intermediate-mass black holes across different environments
Michela Mapelli
University of Padova
Host: Salvatore Vitale


Abstract: The latest results from the LIGO-Virgo-KAGRA collaboration draw a spectacular fresco of binary black hole mergers, ranging from a few to more than hundred solar masses. In this talk, I will discuss the main astrophysical formation channels of binary black holes, highlighting their issues and open questions. On the one hand, models of stellar evolution and pair instability suggest the existence of a gap in the mass spectrum of black holes between ~60 and ~120 solar masses. The boundaries of this gap drastically depend on stellar rotation and on the efficiency of envelope removal. On the other hand, extreme dynamical processes in dense star clusters can fill the mass gap, via multiple stellar collisions, dynamical exchanges and hierarchical mergers. These processes might build up intermediate-mass black holes with mass up to several thousand solar masses, especially in the most metal-poor and massive clusters. The evolution of such dynamical mergers across cosmic time will be one of the most exciting targets of next-generation ground-based detectors.

Tuesday 06 April :
New Discoveries of Extreme Transients
Iair Arcavi
Tel Aviv University
Host: Erin Kara


Abstract: As transient surveys become wider and faster, and followup facilities become more automated and global, we are able to discover and characterize new phase spaces of transients. Among these I will review new classes of supernovae which evolve either too rapidly or too slowly, or emit too much energy to be explained by conventional models. Even as more events are found in each of these classes, their progenitors and the physical mechanisms driving them, remain a mystery. I will also mention new and diverse classes of transients which we are discovering in galaxy centers and believe are tied to accretion onto supermassive black holes. Hopefully, new observations and theoretical models will continue to help uncover the mysteries of extreme transients, and what physics and astrophysics they can teach us.

Tuesday 13 April :
Event Horizon Scale Magnetic Fields From Polarized Black Hole Images
Jason Dexter
University of Colorado, Boulder
Host: TBA

Abstract: Magnetic fields have long been thought to play key roles in driving accretion onto black holes and launching their relativistic jets. I will present spatially resolved long-baseline interferometry observations of linearly polarized synchrotron radiation from near the event horizons of two weakly accreting supermassive black holes, Sgr A* and M87. I will show how the data can be used to infer properties of the radiating plasma, and map the magnetic field structure. The data imply the presence of dynamically important magnetic fields in the emission region in both cases, which can alter the structure of the accretion flow and launch powerful jets.

Tuesday 20 April :
HectoMAP: Mapping the Middle-Aged Universe
Margaret Geller
Harvard Center for Astrophysics
Host: TBA


Abstract: HectoMAP combines data from two of the world's largest telescopes to map the universe at ages 7.5 Gyr. Hectospec on the MMT provides redshifts and other spectroscopic measurements. Hyper Suprime-Cam on Subaru provides spectacular imaging over the entire 53 square degree strip. HectoMAP includes more than 2000 galaxy redshifts per square degree. The survey focuses on quiescent galaxies and the way they trace the cosmic web. The high survey density enables exploration of the most massive galaxies, brightest cluster galaxies (BCGs), and their environments. More than 500 BCGs and their home clusters test of theories of galaxy and structure formation.

Tuesday 27 April :
Shock Physics In The Merging Galaxy Cluster A2146
Helen Russell
University of Nottingham
Host: Michael McDonald


Abstract: The galaxy cluster A2146 is undergoing a major merger and hosts two huge, bright Mach 2 shock fronts, which provide a unique opportunity to measure the electron-ion equilibration timescale along with other key transport phenomena. Collisionless shocks occur over a wide range of scales from accretion shocks to supernova remnants and heliospheric shocks. However, only clusters, particularly A2146, allow us to map the large-scale equilibration with a single observation, which is unaffected by cross-calibration uncertainties. I will present preliminary results from a new 2.4Ms Chandra observation that provides a measure of this timescale, reveals detailed shock structure and traces the disruption of both cool cores, including heating and mixing of metal-rich, cool gas blobs in ram pressure stripped tails.

Tuesday 04 May :
Forging A Path To Excellence: Distributed Leadership For Elevating STEM Disciplines
Robbin Chapman
Host: Edmund Bertschinger

Abstract: STEM research and innovations have positively impacted many aspects of our society. Today, the quality of the STEM enterprise must be elevated if the quality of innovation is to meet an increasing complexity of societal needs; and that elevation is inextricably linked to diversifying STEM disciplines. However, an ongoing challenge to STEM diversification continues to reside at the intersection of racial bias and unfettered membership in an academy that reverberates deeply with the historical and ongoing realities of racial oppression of people of color in the United States. Academia was institutionalized to operate within this racialized framework and therefore contains systemic barriers to access, opportunity, and power; these barriers impact success outcomes for marginalized STEM students. When we consider the current racial climate within the United States, coupled with the urgent national need for STEM innovation and scholarship, ensuring marginalized scholar and student success becomes a clear priority. Progress will require looking behind the STEM "cultural curtain" and critical examination of areas that require change if we are to elevate our STEM disciplines. During this talk, we will examine the assets marginalized students and scholars bring that elevate the academy and describe some efforts that have shown success for promoting diversity in STEM. We will also consider how higher education institutions, STEM disciplines, and STEM faculty communities can take decisive action and provide leadership for increasing capacity for noticing and addressing to structural inequities in STEM.

Tuesday 11 May :
A NICER view of neutron stars
Anna Watts
University of Amsterdam
Host: Deepto Chakrabarty

Abstract: NICER, the Neutron Star Interior Composition Explorer, is an X-ray telescope that installed on the International Space Station. Its mission is to study the nature of the densest matter in the Universe, found in the cores of neutron stars. NICER uses Pulse Profile Modeling, where we use relativistic effects on X-rays emitted from the hot magnetic polar caps of millisecond pulsars to measure the mass and radius of the neutron star. The technique also lets us create a map of the hot emitting regions on the neutron star's surface. I will present NICER's latest results - including a measurement of the size of the most massive neutron star - and discuss the implications for our understanding of ultradense matter, pulsar emission, and stellar magnetic fields.

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