MIT Astrophysics Colloquia

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 05 February :
New Measurements of the EDGES 21cm Profile
Judd Bowman
Host: Jackie Hewitt

Abstract: After stars form in the early Universe, their ultraviolet light alters the excitation state of the 21cm hyperfine line of neutral hydrogen gas in the intergalactic medium. This initially causes the gas to absorb photons from the cosmic microwave background. Later, energy deposited into the gas by the ultraviolet and X-ray emission from these early stars and their remnants heats the gas and eventually ionizes it. In early 2018, we reported the first evidence for detection of this redshifted 21cm signal in all-sky radio observations acquired by our Experiment to Detect the Global EoR Signature (EDGES). We found a flattened absorption profile in the sky-averaged radio spectrum centered at a frequency of 78 MHz with full width at half maximum of 19 MHz and an amplitude of 0.5 K. The frequency (redshift) of the profile is roughly consistent with astrophysical models of early star formation. However, the amplitude of the observed profile is more than a factor of two greater than the largest standard predictions and suggests that the gas was either significantly colder than expected or the background radiation temperature was hotter than expected. A number of possible explanations have been proposed, including interactions between baryons and various candidate dark matter particles, as well as possible astronomical sources capable of producing an intense radio synchrotron background in the early Universe. At the same time, although we reported a number of verification tests with the detection, concerns have been expressed that the instrument or parameter estimation procedure we used could be erroneously creating the signal. New measurements with EDGES using a scaled antenna that shifted frequency-dependent beam properties reproduced the reported profile, reducing the likelihood that instrumental artifacts are responsible for the feature. I will review the original detection, present the new measurements and additional analysis, and describe plans for a third-generation EDGES instrument that could yield improvements in the calibration accuracy and reduce frequency-dependent structure in the antenna beam pattern.

Tuesday 12 February :
Microlensing Perspectives on Cool Planet Populations
Jennifer Yee
Host: Ian Crossfield

Abstract: Microlensing is uniquely capable of studying planets across a wide range of masses at a few AU from their host stars. At the lower masses, these planets are difficult to impossible to find with other techniques. I will discuss recent results in microlensing suggesting a turnover in the planet mass ratio function around 10^-4, i.e. that planets with mass ratios similar to Neptune are the most abundant at separations probed by microlensing. I will also discuss how current and future microlensing surveys, including WFIRST, will advance our understanding of cool planet populations.

Tuesday 19 February :
The Lifetimes of Planetary Systems around Small Stars
Sarah Ballard
Host: Ian Crossfield

Abstract: The Solar System furnishes our most familiar planetary architecture: many planets, orbiting nearly coplanar to one another. However, a typical system of planets in the Milky Way orbits a much smaller M dwarf star. Small stars present a very different blueprint in key ways, compared to the conditions that nourished evolution of life on Earth. Using ensemble studies of hundreds-to-thousands of exoplanets orbiting small stars, I aim to understand the links between planet formation from disks, orbital dynamics of planets, and the content and observability of planetary atmospheres. These processes bear upon the potential for M dwarf systems to evolve and sustain life. In particular, I will address the conflicting hypotheses about the longevity of M dwarf planetary systems over Gyr timescales. In addition to summarizing current knowledge of the large-scale patterns of planets orbiting small stars, I will describe an observational mechanism to test the hypothetical disruption timescale of these planetary systems. Using the forthcoming TESS yield of planets orbiting M dwarfs, we will be able to investigate whether self-disruption commonly occurs in these planetary systems, or whether they exhibit the long-term stable conditions necessary for life to evolve

Tuesday 26 February :
Connecting Galaxies and the Intergalactic Medium Near Reionization
George Becker
UC Riverside
Host: Rob Simcoe

Abstract: The reionization of hydrogen was a landmark event in cosmic history. Within one billion years of the Big Bang the first galaxies emitted enough ultraviolet photons to ionize the gas in deep space, permanently transforming the Universe. Determining exactly when and how reionization occurred is therefore central to our efforts to understand these early sources, as well as the physics that governs the interaction between galaxies and their environments. I will describe what we know about reionization from the study of quasar absorption lines and other probes of the high-redshift Universe, focusing especially on what we’re learning about the intergalactic medium (IGM) shorty after reionization is believed to end. By combining observations of high-redshift quasars with wide-field galaxy surveys we are beginning to better appreciate the complexity of the IGM at this epoch, and recognize how it may help us to construct a more complete model of reionization.

Tuesday 05 March :
The Changing Sun
Sarbani Basu

Abstract: The solar cycle causes the Sun to change on timescales that can affect life. The maxima of cycles are marked by an increased occurrence of sunspots that cause solar flares and coronal mass ejections that can cause potentially harmful geomagnetic storms. Despite the fact that the existence of the solar cycle has been known for hundreds of years, we do not yet have an explanation as to why and how the solar cycle occurs, and we do not know enough to be able to predict the nature of future solar cycles. Until quite recently, the only way to study the solar cycle was to look at surface indicators like the photospheric magnetic fields and sunspot numbers, however, with the availability of helioseismic data, we can now look inside the Sun to understand changes within. In this talk I shall describe some of the results that have been obtained with helioseismic data and show that solar cycle 24 has been very different from solar cycle 23, and discuss some of the implications of the results.

Tuesday 12 March :
The astrophysical r-process: what we are learning from gravitational waves, dwarf galaxies, and stellar archaeology
Ian Roederer
U. Michigan

Abstract: Understanding the origin of the elements is one of the major challenges of modern astrophysics. The rapid neutron-capture process, or r-process, is one of the fundamental ways that stars produce the elements listed along the bottom two-thirds of the periodic table, but key aspects of the r-process are still poorly understood. I will describe four major advances in the last few years that have succeeded in confirming neutron star mergers as an important site of the r-process. These include the detection of freshly produced r-process material powering the kilonova associated with the merger of neutron stars detected via gravitational waves (GW170817), the identification of a dwarf galaxy where most of the stars are highly enhanced in r-process elements (Reticulum II), new connections between the r-process and its Galactic environment (thanks to data from the Gaia satellite), and advances in deriving abundances of previously-undetected r-process elements (such as Se, Te, Pt) in ultraviolet and optical spectra of metal-poor stars. I will highlight opportunities to connect these research directions with future facilities (like FRIB, CETUS, and HabEx) to associate specific physics with specific sites of the r-process

Tuesday 19 March :
Constraining axion physics with small-scale CMB measurements
Renee Hlozek
U. Toronto

Abstract: The CMB presents a unique probe of dark matter physics. Ultra-light axions of mass around 1e-22 eV are a promising dark matter candidate well motivated by high energy physics. The Simons Observatory (SO) is a new cosmic microwave background (CMB) experiment being built on Cerro Toco in Chile, due to begin observations in the early 2020s. SO will measure the temperature and polarization anisotropy of the cosmic microwave background in six frequency bands: 27, 39, 93, 145, 225 and 280 GHz. The initial configuration of SO will have three small-aperture 0.5-m telescopes (SATs) and one large-aperture 6-m telescope (LAT), with a total of 60,000 cryogenic bolometers. I will highlight some of the cosmology forecasts for SO science, and focus on the constraints for ultralight axions

Tuesday 02 April :
Nucleosynthesis In Degenerate Objects
Evan Kirby
Host: Anna Frebel

Abstract: Although thermonuclear (Type Ia) supernovae and neutron star mergers are some of the most important astrophysical events, our understanding of these explosions is vague. I will present abundance measurements of elements across the periodic table (Mg, Fe, Ni, Ba, and others) that address the nature of both types of explosions. The measurements are based on Keck/DEIMOS spectroscopy of red giants in dwarf galaxies, which experienced a large number of Type Ia supernovae. The iron-peak elemental abundances strongly suggest that the majority of Type Ia supernovae in dwarf galaxies exploded below the Chandrasekhar mass, i.e., the double-degenerate model or the single-degenerate, double-detonation model. The DEIMOS spectra also reveal that barium comes from the r-process and appears in the dwarf galaxies on a timescale similar to iron (at least 100 Myr). Therefore, the mostly likely origin is not supernovae but neutron star mergers. The evolution of the [Ba/Fe] ratio indicates a neutron star merger rate consistent with results from LIGO.

Tuesday 09 April :
Departments that Excel in Diversity, Equity, and Inclusion: Who, What, Why, and How
Ed Bertschinger
Host: Ian Crossfield


Tuesday 16 April :
The Importance of Brown Dwarfs
Jackie Faherty
Host: Ian Crossfied

Abstract: Brown Dwarfs are objects with masses that straddle “planet” and star classifications. They are defined by an inability to sustain stable Hydrogen burning. Their spectral energy distributions morph with time as they cool from objects as hot as stars to those as cool as Jupiter. The atmospheres of brown dwarfs contain a potpourri of molecules and exotic condensate cloud materials that are extremely relevant to planetary science investigations (both solar system and objects discovered beyond the Sun). In this talk I will discuss the importance of brown dwarfs in the context of how and where they overlap with exoplanet studies. I will particularly focus on brown dwarfs that challenge our understanding of the line between a companion planet and an isolated object. Specifically on the collection of brown dwarfs dubbed “Super Jupiters” that have masses, temperatures, and atmosphere features nearly identical to what is seen or what is expected in exoplanet studies. I will show how Gaia parallaxes paired with long term ground based parallax surveys reveal a diversity in the fundamental properties of brown dwarfs that must be taken into account when considering hot Jupiter and directly imaged exoplanet studies in the JWST era.

Tuesday 23 April :
Extreme Plasma Astrophysics: Successes, Opportunities, and Challenge
Dmitri Uzdensky
U. Colorado

Abstract: Our Universe is filled with plasma which often manifests itself via various spectacular phenomena. Our understanding of many of them has benefitted tremendously over the past decades from direct application of classic plasma physics results originally obtained in traditional plasma physics areas like space physics and magnetic fusion. Often, however, the classical plasma physics falls short, becomes inapplicable in many high-energy astrophysics situations. The physical conditions in plasmas surrounding relativistic compact objects like black holes and neutron stars are quite extreme, and the effects of special and general relativity, radiation, pair production, ultra-strong magnetic fields — effects viewed as exotic by a traditional plasma physicist — become critical. Understanding how various plasma processes (waves and instabilities, magnetic reconnection, turbulence, shocks) operate under such extreme conditions is thus an important new research frontier. I will present the current status of this emerging and exciting branch of plasma astrophysics and will review the rapid progress it has enjoyed in recent years. This progress is made possible by a combination of concerted theoretical efforts and recent computational breakthroughs, including the advent of plasma codes that now self-consistently incorporate new physics capabilities like synchrotron and inverse-Compton radiation and its back-reaction, QED pair-creation processes, and general relativity. I will give several examples of how recent first-principles numerical studies of plasma processes under extreme conditions have helped resolve some puzzles in high-energy astrophysics, and will also outline promising future directions of research.

Tuesday 30 April :
Overview and Status of the Giant Magellan Telescope Project
Breann Sitarski
Host: Ian Crossfield

Abstract: The Giant Magellan Telescope (GMT) will be a 25.4-meter optical/near-infrared telescope located at Las Campanas Observatory in the Chilean Atacama Desert. It is being built by an international consortium of universities and research institutions. The optical design of GMT consists of seven 8.4-meter borosilicate honeycomb mirror segments that reflect light to seven secondary mirror segments in a doubly-segmented Gregorian configuration. Commissioning will be performed using a fast-steering secondary mirror assembly which, during science operations, can be replaced with a state-of-the-art adaptive secondary assembly comprised of fully adaptive segments. The adaptive secondary mirrors will enable ground layer, natural guide star, and laser tomography adaptive optics to be used on GMT. I will give an overview of the GMT optical design and its unique capabilities among the other extremely large telescopes. I will further describe the first generation of GMT instruments and the diverse science cases the instrument suite allows. Lastly, I will give a status update on the final design work, mirror fabrication, prototyping, and site construction that is underway to bring GMT towards first light.

Wednesday 01 May :

Dimitrios Psaltis
Host: Scott Hughes

Abstract: The Event Horizon Telescope recently revealed the first picture of the black-hole shadow in the center of the M87 galaxy. I will discuss the technological and theoretical advances during the last decade that led to this result. I will then focus on how this picture allows us to accurate measure the mass of the black hole and test Einstein’s theory of General Relativity.

Tuesday 07 May :
Revealing Exoplanet Atmospheres in 3-D
Eliza Kempton
U. Maryland
Host: Ian Crossfield

Abstract: In the upcoming era of JWST and 30-meter class ground-based telescopes we will be forced to reckon with the full 3-D nature of extrasolar planets. Today, many current models treat exoplanet atmospheres in one dimension, as a vertically stratified global average -- similar to 1-D models of stars and many solar system planets. Yet we know that close-in transiting exoplanets in particular are expected to be tidally locked to their host stars with short radiative timescales in their atmospheres, and should therefore have strong longitudinal inhomogeneities in their temperature structure, composition, and atmospheric dynamics. I will present models and observations of exoplanet atmospheres that take into account their 3-D structure, focusing on how we can recover key physical, chemical, and dynamical properties. I will spend considerable time discussing the constraints on atmospheric circulation that can be obtained through high resolution spectroscopy (R~10^5), which allows for individual spectral lines to be resolved and for atmospheric motions to be directly probed through Doppler-shifted line profiles. This observational technique will reach its full potential with the planned complement of high-resolution spectrographs on upcoming 30-meter class ground-based telescopes.

Tuesday 14 May :
The Dynamics of the Local Group in the Era of Precision Astrometry
Gurtina Besla

Abstract: Our understanding of the dynamics of our Local Group of galaxies has changed dramatically over the past few years owing to significant advancements in astrometry and our theoretical understanding of galaxy structure. New surveys now enable us to map the 3D structure of our Milky Way and the dynamics of tracers of its dark matter distribution, like globular clusters, satellite galaxies and streams, with unprecedented precision. Some results have met with controversy, challenging preconceived notions of the orbital dynamics of key components of the Local Group. I will provide an overview of this evolving picture of our Local Group and how we can test the cold dark matter paradigm in the era of Gaia, LSST and JWST

This page is maintained by Ian Crossfield