The discovery of pulsars - a graduate students tale CfA Colloquium 2020-02-13 | In this talk I will describe how pulsars were accidentally discovered, and reflect on several instances where they were 'nearly' discovered. I will highlight the implications for new telescopes with high data rates.
Matthew Payne (Minor Planet Center) Planetary Defense and the Minor Planet Center CfA Colloquium 2024-04-17 | Matthew Payne (Minor Planet Center)"Planetary Defense and the Minor Planet Center "NASA's Planetary Defense Coordination Office (PDCO) coordinates and funds a variety of activities related to Near Earth Objects, including detection, characterization, impact assessment, mitigation strategies and international collaboration. I'll highlight a number of these activities, including the lessons learned from the Double Asteroid Redirection Test (DART) mission, as well as the results expected from the forthcoming NEO Surveyor mission. I'll then discuss the Minor Planet Center's (MPC) role in these planetary defense efforts, in which the MPC acts as the central hub for the collection, verification, and dissemination of data on minor planets, comets, and outer irregular natural satellites, and functions as the backbone of the NEO network monitoring, ensuring that observations are accurately processed and shared, and enabling timely decisions in the event of a potential impact threat.
New Results from the Event Horizon Telescope CfA Colloquium 2024-03-28 | The EHT will be releasing a major new result this Wednesday (3/27). This is a project led by Sara Issaoun and Angelo Ricarte here at the CfA, and they have organized a special colloquium with a series of talks from key contributors to present the results (including Freek Roelofs at the CfA, and two visiting scientists, Avery Broderick and Michi Baubock).
Ashley Villar (Center for Astrophysics) Data-driven Methods to Understand the Dynamic Universe CfA Colloquium 2024-03-27 | Ashley Villar (Center for Astrophysics)"Data-driven Methods to Understand the Dynamic Universe"Wide-field astronomical surveys have enabled the discovery of thousands of transient phenomena annually. In parallel, the development of machine learning techniques has revolutionized how we analyze astronomical data. Variable phenomena–supernovae, active galactic nuclei, pulsating stars, etc.–are uniquely challenging due to their intrinsic and observed diversity. In this colloquium, I will explore the development of physics-informed machine learning, particularly in the context of time-domain astrophysics. I will highlight advances in realtime modeling of time-series and its application to the upcoming Vera C. Rubin Observatory Legacy Survey of Space and Time. I will end with reflections on recent developments in generative artificial intelligence and how these may impact future survey science.
Dillon Brout Cosmological Results from the Full 5 Years of the Dark Energy Survey Supernova Program CfA Colloquium 2024-03-20 | Dillon Brout (Boston University)"Cosmological Results from the Full 5 Years of the Dark Energy Survey Supernova Program"Type Ia Supernovae (SN) remain a central pillar of the Standard Model of Cosmology - Lambda Cold Dark Matter (LCDM). I will detail leaps of progress in the last 5 years of supernova cosmology culminating in the recent Dark Energy Survey (DES) analysis of ~1700 photometrically classified supernovae collected over 5 years that rivals the spectroscopically confirmed Pantheon+ compilation of more than 30 years of supernovae. The DES dataset is the largest dataset ever compiled from a single telescope and places strong constraints on dark matter and dark energy. I will detail the robust systematic uncertainties that have built confidence in these measurements and that paves the path forward for future surveys.
Jason Wang New Frontiers in Exoplanet Imaging and Pathways to Habitable Worlds CfA Colloquium 2024-03-11 | Jason Wang (Northwestern University)"New Frontiers in Exoplanet Imaging and Pathways to Habitable Worlds"By spatially resolving faint planets from their bright host stars, we can directly characterize them as individual worlds. Exoplanet imaging is a technology-driven field, and I will discuss three new novel instruments that allow us to detect and characterize imaged Jovian exoplanets and whose technology, if placed on the next generation of observatories, has the potential for us to study habitable worlds. The Keck Planet Imager and Characterizer (KPIC) combines high-contrast imaging and high-resolution spectroscopy techniques together for the first time and allows us to spectrally resolve molecular absorption lines in the atmospheres of directly imaged planets. Long-baseline interferometry with VLTI/GRAVITY gives us the spatial resolution of a 140-meter telescope, enabling the positions of exoplanets to be measured to within 50 microarcseconds, the circumplanetary environment to be resolved to sub-au scales, and the first direct detection of a radial-velocity discovered exoplanet. Both techniques also unlock the ability to study giant planets at 1-10 au like our own Jupiter. Moving to space, the Roman Space Telescope Coronagraph Instrument will demonstrate high-order wavefront control and potentially image a planet in reflected light for the first time, allowing us to measure planetary albedo. I will highlight recent science results from these instruments, summarize lessons learned in pioneering these techniques, and discuss their prospects for imaging habitable worlds on the next generation of observatories.
Dan Milisavljevic (Purdue University) Supernovae: Laboratories of Extreme Astrophysics CfA Colloquium 2024-03-06 | Dan Milisavljevic (Purdue University)"Supernovae: Laboratories of Extreme Astrophysics"Core-collapse supernovae are among the most powerful explosions in the universe. They shape and enrich their host galaxies; produce a variety of exotic objects including neutron stars, black holes, and some gamma-ray bursts; are a major site of nucleosynthesis; and serve as ideal laboratories of multimessenger astrophysics. I will review how radio-through-X-ray investigations of transients are revolutionizing our understanding of stellar death and its myriad impacts. Particular attention will be placed on how observations are characterizing the terminal and often dramatic evolutionary phases massive stars pass though when approaching core collapse, and how 3D reconstructions of supernova remnants can directly address uncertainties in supernova progenitor systems and explosion mechanisms. The special role of observations made possible by the James Webb Space Telescope will be highlighted. Finally, I will conclude by describing how new synergy between electromagnetic, neutrino, and gravitational wave facilities can enable transformative progress towards finally solving the enigma of core collapse by accurately interpreting the multi-messenger signals from the next Galactic supernova -- a once-in-a-generation event that will be detected by a network of neutrino detectors called the Supernova Early Warning System.
Aki Roberge (NASA Goddard Space Flight Center) Towards the Habitable Worlds Observatory CfA Colloquium 2024-02-28 | Aki Roberge (NASA Goddard Space Flight Center)"Towards the Habitable Worlds Observatory"Humans have long wondered if there were worlds around other stars – and life on those worlds. Over the last two decades, astronomers have found that the answer to the first question is a resounding YES. Those accomplishments have put us in a position to possibly answer the second question. The 2020 Astrophysics Decadal Survey (Astro2020) has laid out a path to the first telescope that can find out if habitable exoplanets are common or rare and search them for signs of global biospheres, while performing a wide range of transformative astrophysics studies.NASA has dubbed that telescope the Habitable Worlds Observatory (HWO). In this talk, I’ll cover Astro2020’s recommendations and expectations for HWO, as well as the previous mission concept studies that fed into it (LUVOIR and HabEx). I’ll then move on to NASA’s current plans for maturation of the HWO concept, highlighting key challenges and activities. This multi-faceted journey will require enthusiastic collaboration between a wide range of disciplines, interests, and stakeholders. Continuing out into future decades, the path may lead to a new generation of Great Observatories in space and new perspectives on the universe around us.
Jason Wang New Frontiers in Exoplanet Imaging and Pathways to Habitable Worlds CfA Colloquium 2024-02-21 | Jason Wang (Northwestern University)"New Frontiers in Exoplanet Imaging and Pathways to Habitable Worlds"By spatially resolving faint planets from their bright host stars, we can directly characterize them as individual worlds. Exoplanet imaging is a technology-driven field, and I will discuss three new novel instruments that allow us to detect and characterize imaged Jovian exoplanets and whose technology, if placed on the next generation of observatories, has the potential for us to study habitable worlds. The Keck Planet Imager and Characterizer (KPIC) combines high-contrast imaging and high-resolution spectroscopy techniques together for the first time and allows us to spectrally resolve molecular absorption lines in the atmospheres of directly imaged planets. Long-baseline interferometry with VLTI/GRAVITY gives us the spatial resolution of a 140-meter telescope, enabling the positions of exoplanets to be measured to within 50 microarcseconds, the circumplanetary environment to be resolved to sub-au scales, and the first direct detection of a radial-velocity discovered exoplanet. Both techniques also unlock the ability to study giant planets at 1-10 au like our own Jupiter. Moving to space, the Roman Space Telescope Coronagraph Instrument will demonstrate high-order wavefront control and potentially image a planet in reflected light for the first time, allowing us to measure planetary albedo. I will highlight recent science results from these instruments, summarize lessons learned in pioneering these techniques, and discuss their prospects for imaging habitable worlds on the next generation of observatories.
Prof. Jun Ye (JILA & NIST) Quantum degenerate gas of polar molecules CfA Colloquium 2024-02-07 | Prof. Jun Ye (JILA & NIST)"Quantum degenerate gas of polar molecules"In his 2001 seminal paper on cold chemistry, Prof. Dalgarno predicted a considerable rate for chemical reaction near zero temperature owing to “tunneling through the repulsive barrier”. His vision inspired a vibrant development of the field of ultracold molecules and was fully validated when a molecular gas was brought into the quantum regime, where ultracold collisions and chemical reactions were shown to be governed by quantum statistics and long-range interactions.Quantum degerate gas of molecules sets the stage to explore novel dynamics. External electric fields are used to tune elastic interactions, suppress reactive losses, produce sharp collision resonances, and control evaporative cooling. Confining molecules in optical traps with various spatial geometries allows realization of stereo chemistry and construction of tunable many-body spins for quantum magnetism. Hamiltonian engineering is paving the way for production of entangled molecules that will benefit precision measurement.
Joanna Drazkowska The Emerging Theory of Planet Formation CfA Colloquium 2024-01-31 | Joanna Drazkowska (Max Planck Institute for Solar System Research)"The Emerging Theory of Planet Formation"The classical theory of planet formation originated when our knowledge about planets was limited to the Solar System alone. The numerous discoveries of exoplanet systems have compelled a revision of this theory, aided by cutting-edge observations of circumstellar disks and precise laboratory studies of Solar System materials. Nonetheless, the formation of planets remains one of the major unsolved problems in modern astrophysics. In this talk, I will outline the emerging paradigm in which centimeter-sized dust aggregates, colloquially known as pebbles, take center stage. I will explore the challenges in the growth process from tiny dust grains to pebbles, identifying key barriers that hinder this progression. Due to these barriers, the formation of planetesimals, the first gravitationally-bound building blocks preceding today's asteroids and comets, remains particularly enigmatic. Notably, the emergence of planetesimals is now believed to occur only in specific locations within the circumstellar disk, such as the water snow line. Therefore, these early stages of planet formation may significantly influence the final architectures of planetary systems.
Jessica Dempsey(ASTRON) Harnessing the Opportunities in the Approaching Radio Astronomy Renaissance CfA Colloquium 2024-01-17 | Jessica Dempsey (ASTRON)"Harnessing the Opportunities in the Approaching Radio Astronomy Renaissance"Radio astronomy is about to step into a new era of technical and scientific capability. ASTRON leads the expansion of LOFAR, while SKA enters construction of the largest radio observatory ever built. On the horizon, to complement and compete, are the ngVLA and DSA-2000. ASTRON, as the Netherlands centre for radio astronomy, looks now to its role in this renaissance, and in its opportunities with our LOFAR and SKA partners. ASTRON is developing a science data centre to lead the world in providing the broadest accessibility to science-ready data products in the radio regime, committed to lowering the barrier of access to the radio universe for the widest community of astronomers possible, in advance of hosting the upcoming SKA regional centre. The coming renaissance is not without challenge – particularly in facing and overcoming the daunting data collection, processing and storage bottlenecks and to develop green solutions for our expanding environmental impacts. ASTRON is developing its long-look strategic plan around these goals with our national and international community, focusing on ensuring in this decade of world-spanning facilities, we maintain agency and opportunity for our researchers to lead impactful science and technology developments.
Nadja Drabon (Harvard University) CfA Colloquium 2023-11-15 | Nadja Drabon (Harvard University)"The Effect of the Giant 3.26 Gyr Meteorite impact on the Earth’s Early Surface Environment and Life"Large meteorite impacts strongly affected the habitability of the early Earth. The rocks of the Archean Eon record at least 16 major impact events with bolides larger than 10 km in diameter. These impacts likely had severe consequences for the surface conditions of early Earth. I will present on the analysis of sedimentary rocks across the 3.26 Ga S2 impact event (37-58 km bolide) to evaluate its environmental effects and consequences for early life. The results reveal that the impact caused a giant tsunami, evaporation of the uppermost layer of the ocean, and an increase in iron by mixing Fe 2+ -rich deep with Fe 2+ -poor shallow waters. Although meteorite impacts are usually seen as destructive and the S2 impact certainly had disastrous short-term consequences, it may have had some medium-term benefits for the early biosphere. The mixing of Fe 2+ to the upper water column potentially provided electron donors to the photic zone. In addition, the post-impact hothouse environment may have led to increased erosion and weathering, injecting nutrients like phosphorus into the oceans. Overall, meteorite impacts may have had transient benefits for early life on Earth.
CfA Colloquium 1:30pm Brian Metzger CfA Colloquium 2023-10-25 | Brian Metzger (Columbia University)"Strange Occurrences in Galactic Nuclei"Galactic nuclei can host a number of transient phenomena involving the violent interactions of stars with the central supermassive black hole (SMBH) or its accretion disk. Stars placed on parabolic orbits which pass too close to the SMBH give rise to tidal disruption events (TDE). I will highlight several puzzles related to the appearance of TDEs flares, particularly their unexpectedly modest effective temperatures (larger than expected photosphere radii) and the recent discovery of late-time radio flares delayed many months or years after the optical light curve peak. Motivated by these issues, I will describe a new model in which the disrupted stellar debris forms a quasi-spherical pressure-supported envelope around the SMBH, which undergoes cooling-driven (Kelvin-Helmholtz) contraction, only gradually forming the inner accretion disk responsible for X-ray and radio jetted emission. In contrast to TDEs, stars which migrate into the galactic center through gravitational wave emission on low-eccentricity orbits ("extreme mass-ratio inspirals"; EMRIs) can survive in the immediate vicinity of SMBH for considerably longer, at least until a gaseous disk is produced (e.g., via a TDE). I will show that the recently discovered sources of X-ray "quasi-periodic eruptions" (QPEs) arise naturally from repeated collisions of an orbiting stellar EMRI with a SMBH disk. If this explanation is correct, QPEs offer a new probe of the EMRI population in galactic nuclei; the structure of radiation-dominated accretion disks; and strong-gravity effects such as apsidal/nodal orbital precession.
CfA Colloquium 1:30pm Iouli Gordon CfA Colloquium 2023-10-18 | Iouli Gordon (CfA)"The HITRAN and HITEMP Molecular Spectroscopic Databases: Empowering Remote Sensing Missions"The HITRAN molecular spectroscopic database, traditionally used as an international reference standard for terrestrial atmosphere research, has evolved significantly over its five-decade history. Recent advances in the quality of spectra obtained from on-ground and space-based spectrometers have established the incredible diversity of planetary and exoplanetary atmospheres, not just in thermodynamic conditions but also in composition. To meet the demands of this evolving field, HITRAN has expanded its scope through 1) the inclusion of molecules relevant to non-terrestrial atmospheres; 2) updating and expanding the high-temperature counterpart of the database, HITEMP; 3) adding collision-induced absorption data; and 4) adding pressure-broadening parameters due to ambient gases dominating planetary atmospheres (e.g., CO2, H2 and He). The philosophy behind database development can be summarized as a PACT: Practicality, Accuracy, Completeness, Traceability. Examples of applying this philosophy will be given, highlighting, in particular, a robust process for curation and validation of the parameters, for instance, against high-quality laboratory spectra or atmospheric measurements.This presentation will explore the current state of these databases and associated tools and provide insight into their future developments.
Julien de Wit Robustly Characterizing Exoplanets in a New Era of High-Precision Transit Measurements CfA Colloquium 2023-10-11 | Robustly Characterizing Exoplanets in a New Era of High-Precision Transit MeasurementsOctober 10, 20231:30 PMAbstract: With a new-generation of great observatories coming online this decade, unprecedented insights into exoplanets are becoming within reach. JWST for example can enable the study of atmospheres around terrestrial exoplanets and reveal tri-dimensional structures in the atmospheres of their larger counterparts. Yet, robustly leveraging new data to reach such achievements will require extra care as the models currently used may not be up to par.During this presentation, I will introduce work done by MIT’s Disruptive Planets group and collaborators towards supporting the robust in-depth characterization of exoplanets. I will specifically discuss how not accounting for the true shape of a planet can lead to a misinterpretation of its interior properties as well as atmospheric structure; how the current state of our understanding of light-matter interactions can similarly affect our interpretation of planetary spectra and thus inferences regarding their atmospheric properties; and how the current state of emission spectrum models for stars may even prevent from disentangling between the contribution of a planet and its host star, to start with. I will then present possible ways to address these challenges and end with a step-by-step roadmap to the robust characterization of temperate terrestrial planets with JWST, which includes habitability assessment.
Blake Sherwin: Do we understand cosmic structure growth? CfA Colloquium 2023-10-04 | Blake Sherwin (University of Cambridge)"Do we understand cosmic structure growth? Insights from new CMB lensing measurements with the Atacama Cosmology Telescope"
Morgan MacLeod pH Lecture: Breaking Waves on a Stellar Shore CfA Colloquium 2023-09-27 | Morgan MacLeod pH Lecture: Breaking Waves on a Stellar Shore
Jenna Samra High-Altitude Instrumentation for Infrared Observations of the Solar Corona CfA Colloquium 2023-09-20 | Jenna Samra High-Altitude Instrumentation for Infrared Observations of the Solar Corona
Sandro Tacchella First Insights from JWST: Star Formation in Cosmic-Dawn Galaxies CfA Colloquium 2023-09-13 | First Insights from JWST: Star Formation in Cosmic-Dawn GalaxiesSeptember 12, 20231:30 PM - 2:30 PMAbstract: After successful deployment and commissioning, JWST has now been delivering data for over a year. I will give an overview of the first-year surprises and discoveries, zooming in on some of the earliest galaxies and presenting new results on the spatially resolved growth histories of high-redshift galaxies. Specifically, I will discuss the structural and stellar population properties of the galaxies at the redshift frontier, with redshifts z 10. I will then connect the mode of star formation with the early enrichment of galaxies: for galaxies where we have constraints on the gas-phase metallicity (at redshift of z=8), I will show how star-bursting galaxies can have different gas-phase metallicities due to internal and external mechanisms that drive the starburst. Finally, I will focus on the structural evolution of galaxies, presenting how dense stellar cores form within galaxies at z 7. I will wrap up by highlighting what JWST will be able to deliver regarding high-redshift galaxy evolution in the upcoming years.Speaker InformationSandro TacchellaUniversity of Cambridge
Searching for the Black Holes from the nearby to faraway universe 4 11 23 0 CfA Colloquium 2023-05-25 | ...
How Supermassive Black Holes Ignite the Intergalactic Medium April 4, 2023 CfA Colloquium 2023-05-24 | ...
A Sky Full of Satellites: The changing orbital population and the impact of megaconstellations CfA Colloquium 2023-05-24 | aired 1/31/23
Exoplanets with JWST: How one telescope revolutionized a field in about 100 Days CfA Colloquium 2023-05-24 | ...
Searching for Black Holes From the Nearby to the Faraway Universe CfA Colloquium 2023-04-12 | I will provide an overview of our quest to understand, with theory and observations, undetected populations of black holes, proceeding from the nearby to the faraway Universe. First, I will unveil a new extensive, multiwavelength observational campaign likely leading to the discovery of the second closest supermassive black hole, located in the Milky Way satellite Leo I. I will then describe some orbital and radiative properties of intermediate-mass black holes potentially wandering in local galaxies, including the Milky Way. Moving higher in the redshift ladder, I will detail the detection of the farthest lensed quasar to date and theoretical arguments as to why we are missing a significant fraction of this population. Still in the reionization epoch, I will delineate the discovery of two candidate galaxies at redshift z greater than 10 and the possibility that they host supermassive black holes. I will then conclude with the observational properties of the same sources that have started the reionization at z ~ 30: the first population of stars and black holes, and how we may soon be able to detect them directly.
How Supermassive Black Holes Ignite the Intergalactic Medium CfA Colloquium 2023-04-05 | The Lyman-α forest is a key diagnostic for the state of diffuse baryons in the intergalactic medium (IGM) and for fundamental cosmological parameters. At high redshift (i.e., z ≥ 2) the Lyman-α forest is observed in optical wavelengths from ground-based observatories and has been used to constrain small-scale cosmic structure, the dark matter distribution, the IGM gas temperature, and the evolution of the ultraviolet ionizing background (UVB) radiation. However, the low redshift (z~0.1) Lyman-α forest, observed from space-based UV instruments such as HST COS, has challenged the most advanced cosmological simulations. In this talk, I will show how active black hole (AGN) feedback can be at least as important (or more!) as the UVB for setting the amount and distribution of neutral gas in the low redshift IGM. The AGN feedback signatures appear in the neutral gas statistics beginning around z~1, thus possibly affecting 21-cm intensity mapping programs that target this redshift range (e.g., HIRAX). These findings herald a significant paradigm shift for cosmological simulations and observations that target the low redshift IGM and suggest that cosmological simulations should develop AGN feedback models with both galaxies and the IGM in mind.
The Emergence of Galactic Structure in the JWST Era CfA Colloquium 2023-03-29 | The launch and commissioning of the James Webb Space Telescope is ushering in a new era in our understanding of our cosmic origins. Galaxies are a fundamental building block of the universe, yet how they formed has remained enigmatic owing to our inability to observe them at early cosmic times. In just the first eight months of data JWST is already revolutionizing our understanding of the earlyuniverse by allowing us to detect and resolve early galaxies at infrared wavelengths. In this talk, I will discuss some of the results that have come out of our work with JWST and their impact on our understanding of the formation and evolution of galaxies. This includes the discovery of candidate galaxies so old and massive they should not exist, the surprising shapes of HST-dark galaxies, and a new method for measuring kinematics that has revealed a monstrous spinning disk 1 billion years after the big bang. The first spatially-resolved infrared look at distant galaxies has revealed that our previous understanding of the emergence of galactic structure was faulty and requires a next-generation set of tools, which we are building. I’ll conclude with a discussion of where the field is moving and the rich discovery space in this new era of extragalactic astrophysics.
How to Form a Habitable Planet CfA Colloquium 2023-03-22 | Abstract: Planets form from disks of dust and gas surrounding young stars. As they grow, these new planets inherit their chemical composition from the surrounding material and then sculpt it through gravitational interactions to form gaps and other asymmetric structures. In the last decade, the Atacama Large Millimeter/submillimeter Array (ALMA) has revolutionized our ability to study planet formation, allowing us to examine this process in high resolution. I will present highlights from ongoing work using ALMA and other facilities that explores how planetary systems form and evolve by (1) connecting disk structure to sculpting planets and (2) understanding the impact of stellar flares on planetary habitability. Together these results provide an exciting foundation to investigate the evolution of planetary systems as a whole through multi-wavelength observations. In the future, new facilities, specifically in the far-infrared, will help complete our understanding by tracing the chemistry of water and other volatiles critical for life.
Imagining Other Worlds: How Planet-Forming Disks Can Shed Light on What is Possible CfA Colloquium 2023-03-08 | The last decade of ALMA has transformed our view of planet-forming environments in all respects. High resolution images have revealed a diverse array of structured belts of millimeter-sized dust and a variety of distinct molecular compositions both within disks and between different disk systems. How does this diversity translate into the initial conditions for the formation of planets and the compositions (gas and solid) that they receive? Are planets likely to receive water and organic material at formation, or at some later phase from a belt of volatile-rich icy comets? I will present an overview of how our picture of the chemical and physical environment of planet formation has shifted in recent years, how this has pushed us to revise models, and how multiwavelength observational campaigns, including upcoming JWST programs, can help us find patterns in the apparent variety of protoplanetary environments.
Hot Accretion Flows Around Black Holes CfA Colloquium 2023-03-01 | Luminous quasars and near-Eddington X-ray binaries are explained wellby a standard model of black hole accretion called the thin accretiondisk. At lower luminosities, a different model in which the accretinggas is much hotter provides a better explanation of observations. Somefeatures of this latter model are rather extreme, notably theassumption of a two-temperature plasma with proton temperatureapproaching 10^12 K near the black hole. Event Horizon Telescopeobservations of two low-luminosity black holes, M87* and SagittariusA*, have validated the most problematic features of the hot accretionflow model, increasing our confidence in the model. Manylow-luminosity black holes have relativistic jets. Computersimulations reveal that such jets form easily in hot accretion flows,provided the plasma is threaded by a strong magnetic field and theblack hole spins rapidly. The talk will review these and relatedfeatures of hot accretion flows.
IXPE: Our First Look Around the X-ray Polarized Sky CfA Colloquium 2023-02-22 | For 50 years, the only astronomical source with measured soft X-ray polarizationwas the Crab nebula, initially detected with sounding rockets. Nowwe finally have the Imaging X-ray Polarimetry Explorer mission on orbit, whichhas already measured complex X-ray polarization signals from dozens of neutronstar and black hole sources, with over 100x the sensitivity of the initialexperiments. I summarize the motivation for such polarization studies,the technology advances that made IXPE possible, and some exciting resultsfrom our first year's program. As the wider community gets involved,additional novel probes of high energy astrophysical sources are expected soon.
Tidal Disruption Events: A New Black Hole Census CfA Colloquium 2023-02-08 | Tidal disruption events, or TDEs, occur when stars pass too close to massive black holes (MBHs) and are spaghettified by tidal forces. The ensuing fallback of stellar debris powers a luminous flare brighter than most supernovae, with most of the luminosity emerging as quasi-thermal optical, UV, and soft X-rays. These transients have great potential to measure the demographics of MBHs in the Universe, especially at the uncertain bottom end of the MBH mass function: they preferentially occur in the smallest nucleated galaxies, they are visible to cosmological distances, and their light curves are governed by just a handful of free parameters. Furthermore, upcoming advances in time domain astronomy will turn TDE discovery from an artisanal into an industrial process: while less than 100 TDEs are known today, the upcoming LSST optical survey and ULTRASAT UV survey will each find thousands per year. In this talk, I will discuss the potential of TDEs to resolve long-standing questions on MBH demographics, origins, and evolution, focusing both on what can already be done today with techniques such as X-ray continuum fitting, and on the theoretical challenges that must be resolved before making further progress in optical/UV modeling. With an emphasis on recent progress in ab initio simulations, I will review the key theoretical questions concerning TDE flares: how does the debris circularize into an accretion flow? And what is the power source and geometry of the optical/UV emission? I will argue that the answer to these long-standing questions are finally emerging, and that observed optical/UV light curves can now be modeled from first principles.
Copy of Tidal Disruption Events: A New Black Hole Census CfA Colloquium 2023-02-07 | Tidal disruption events, or TDEs, occur when stars pass too close to massive black holes (MBHs) and are spaghettified by tidal forces. The ensuing fallback of stellar debris powers a luminous flare brighter than most supernovae, with most of the luminosity emerging as quasi-thermal optical, UV, and soft X-rays. These transients have great potential to measure the demographics of MBHs in the Universe, especially at the uncertain bottom end of the MBH mass function: they preferentially occur in the smallest nucleated galaxies, they are visible to cosmological distances, and their light curves are governed by just a handful of free parameters. Furthermore, upcoming advances in time domain astronomy will turn TDE discovery from an artisanal into an industrial process: while less than 100 TDEs are known today, the upcoming LSST optical survey and ULTRASAT UV survey will each find thousands per year. In this talk, I will discuss the potential of TDEs to resolve long-standing questions on MBH demographics, origins, and evolution, focusing both on what can already be done today with techniques such as X-ray continuum fitting, and on the theoretical challenges that must be resolved before making further progress in optical/UV modeling. With an emphasis on recent progress in ab initio simulations, I will review the key theoretical questions concerning TDE flares: how does the debris circularize into an accretion flow? And what is the power source and geometry of the optical/UV emission? I will argue that the answer to these long-standing questions are finally emerging, and that observed optical/UV light curves can now be modeled from first principles.
A Sky Full of Satellites CfA Colloquium 2023-02-01 | The number of active satellites in Earth orbit has increased fivefold in the past decade and is expected to increase by at least one further order of magnitude in the next decade. The advent of large constellations of big satellites in low orbit will transform the night sky and present a challenge to wide-field ground-based astronomical imaging.I discuss the rapidly changing demographics of the current low Earth orbit artificial satellite population and show that the proposed "megaconstellation" of circa 30,000 Starlink internet satellites dominates the lower part of Earth orbit, below 600 km. Such large, low altitude satellites appear visually bright to ground observers, and Starlinks are naked eye objects. Higher orbit satellites such as the OneWeb constellation are fainter but have a larger impact on optical ground-based imaging. I model the expected number of illuminated satellites as a function of latitude, time of year, and time of night and summarize the range of possible consequences for ground-based astronomy. In winter at lower latitudes typical of major observatories, the satellites will not be illuminated for six hours in the middle of the night. However, at low elevations near twilight at intermediate latitudes (45-55 deg, e.g. much of Europe) hundreds of naked-eye satellites may be visible at once to observers and wide field deep images may have of order 1 satellite trail per exposure, or more, throughout the summer night. Astronomers are engaging with regulators both in the US and internationally to argue that the night sky should be considered an environmentally protected resource and large constellations should be regulated with this in mind.
Exoplanets with JWST: How one telescope revolutionized a field in about 100 Days CfA Colloquium 2023-01-25 | The discovery rate of new exoplanets has steadily increased since the 1990s, and the pace is expected to continue for at least another decade with the advent of new facilities, such as the Roman telescope. However, progress in our understanding of the actual physical and chemical properties of exoplanets, and how they form and evolve has been significantly slower because of limitations in instrumentation capable of characterizing their atmospheres.In this talk I will describe how the successful launch of JWST has already transformed the field of exoplanets and how we expect it will further transform it with just the first round of observations in 2023. I will summarize the findings of our Transiting Exoplanet Community Early Release Science Program so far and their implications for our understanding of exoplanet formation. I will also present a number of other early results, including our first observation of an earth-size exoplanet atmosphere, to conclude with a brief preview of other upcoming observations.
Copy of Exoplanets with JWST: How one telescope revolutionized a field in about 100 Days CfA Colloquium 2023-01-25 | The discovery rate of new exoplanets has steadily increased since the 1990s, and the pace is expected to continue for at least another decade with the advent of new facilities, such as the Roman telescope. However, progress in our understanding of the actual physical and chemical properties of exoplanets, and how they form and evolve has been significantly slower because of limitations in instrumentation capable of characterizing their atmospheres.In this talk I will describe how the successful launch of JWST has already transformed the field of exoplanets and how we expect it will further transform it with just the first round of observations in 2023. I will summarize the findings of our Transiting Exoplanet Community Early Release Science Program so far and their implications for our understanding of exoplanet formation. I will also present a number of other early results, including our first observation of an earth-size exoplanet atmosphere, to conclude with a brief preview of other upcoming observations.
Television as an Aid to Astronomy: The rise and fall of the image tube CfA Colloquium 2022-11-09 | As the electronic television industry grew during the early twentieth century, many in the astronomy community became interested in the possible application of television technology to telescopic observations. The potential advantage over unaided photography was acknowledged quickly by some, but the successful application to low-light-level astronomy remained a “vexing problem” for several decades. By the 1950s, a few groups had the interest, experience, and resources to begin investigating electronic imaging devices for astronomical research. While some experimented with commercially available image tubes already successfully employed in television cameras, others developed devices specifically designed to address astronomers’ needs.In this talk, I will examine the origins, subsequent development, and ultimate decline of image tubes in astronomy during the twentieth century. Physicists, who gained training in electronics during World War II, led the early push for astronomical image tubes. Vannevar Bush’s concern for scientific prestige led him to dedicate resources to development and postwar federal funding for the sciences helped sustain development efforts for over a decade. Even with an increase in financial and technical resources, images tubes failed to become a regular part of astronomical observations before being replaced entirely by the advent of solid-state detectors. By examining the development of a technology that ultimately failed to become a routine part of astronomical observations, we may better understand how astronomers acquire new tools and obstacles that prevent them from achieving their scientific goals.
What do we know about the first stars and galaxies ? CfA Colloquium 2022-11-02 | I will review the emerging theoretical framework for how stars, galaxies, and black holes transformed the early universe. Predictions for the enrichment of the intergalactic medium with heavy chemical elements, the rate of supernova explosions and gamma-ray bursts,as well as the number density and properties of the first galaxies, sensitively depend on theparticle-physics nature of dark matter. To constrain the elusive first generation of stars, we canbring to bear a powerful combination of probes at high redshifts and in our local neighborhood.The latter approach, known as “stellar archaeology” holds particular promise in light of ongoingand planned large surveys of metal-poor stars, both in the Milky Way and its dwarf satellites.I will discuss insights and constraints from the Early Release Science with the James Webb SpaceTelescope (JWST) .
The Sun’s Trajectory in the Last 10 Million years CfA Colloquium 2022-10-26 | The Anthropogenic climate crisis is a central challenge facing humanity today. Crises like this are not new, in fact Earth’s history has been marked by periods of rapid climatic change associated with dramatic consequences for the biosphere and its inhabitants. However, the drivers of these events are often unclear and are generally attributed to a few types of phenomena. The impacts of astronomical phenomena on climate and the evolution of biological systems have only been considered minimally. The Sun moves large distances (~19pc/Myr pc/Myr) through the quite variable Interstellar Medium. There is geological evidence from 60Fe and 244Pu isotopes that Earth received interstellar material about 2-3 Myr ago and 7 Myr ago. These isotopes were interpreted evidence for a nearby supernova, however that has been cast into doubt. In this talk I will discuss our new research indicating the encounter of Earth with massive cold cloud in Local Ribbon of Cold Clouds, 3 Myr ago and with the edge of the Local Bubble 7 Myr ago. Both encounters shrinked the Sun’s protective bubble—the heliosphere—to within Earth’s orbit exposing Earth to a cold dense interstellar medium. I will discuss the possible terrestrial consequences to climate and to biological evolution.
Day & Night in the Milky Way CfA Colloquium 2022-10-19 | 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. These stars present a very different dynamical blueprint in key ways when compared to the conditions that nourished evolution of life on Earth. Using ensemble studies of hundreds-to-thousands of exoplanets orbiting small stars, my research program investigates the links between planet formation from disks, orbital dynamics and stability of planetary systems, and the presence and observability of their atmospheres. These processes bear upon the potential for M dwarf systems to evolve and sustain living things. Planets orbiting small stars will comprise the majority of the targets for detailed space-based atmospheric follow-up investigation in the next decade. By leveraging the many thousands of planet discoveries in hand, and understanding the patterns among them, we can determine how best to use this precious and limited resource.
Next Generation Photoionization modelling for HII regions and Galaxies CfA Colloquium 2022-10-12 | I will describe our development of new 2D and 3D models for the ionized gas in HII regions, Star-forming and AGN galaxies. New developments include self-consistent treatment of elemental abundances and dust depletion, Stromlo Stellar Tracks, and 3D Monte Carlo radiative transfer to allow for arbitrary nebular gas geometries. These new models are now being used to help analyse the chemical abundances in HII regions, star forming galaxies, and AGN, nearby and at high redshift. I will present a sneak preview of this latest work.
Using quantum science to search for dark matter CfA Colloquium 2022-10-05 | The dark matter puzzle is one of the most important open problems in modern physics. I will describe some of the approaches that are used to search for non-gravitational interactions of ultralight dark matter, and the fundamental limitations on their sensitivity. The axion is a compelling dark matter candidate, since it resolves the strong-CP problem of quantum chromodynamics. I will focus on precision laboratory-scale experiments that search for axion-like dark matter, aiming to achieve, or circumvent, the quantum limits on their sensitivity. Specifically, the Cosmic Axion Spin Precession Experiment (CASPEr-e) uses nuclear magnetic resonance to search for the defining gluon coupling of the QCD axion. The prototype CASPEr-e experiment has achieved design sensitivity in the nano-electronvolt mass range. We are now developing the next-generation search, with the goal of achieving the sensitivity at the QCD axion level.
Galaxy evolution with machine learning: the perilous road from simulations to data CfA Colloquium 2022-09-28 | Obtaining accurate estimates of galaxy properties, from stellar mass to star formation history to chemical enrichment history, is crucial to solving open problems in galaxy formation and evolution. Traditionally, this has been done by using spectral energy distribution fitting methods on spectroscopic or photometric data; more recently, supervised machine learning methods have also been used, with promising results. One potential roadblock for the ML-based methodology is that identifying a training set for this problem is very hard, because the ground truth is not readily available - we have no way of knowing the true history of how galaxies have formed and evolved. As an alternative, we can train machine learning models on state-of-the-art cosmological simulations. In this talk, I will discuss three aspects of this approach: one, how we can estimate the performance of such models when they are applied to real data; two, whether an improved feature representation for spectroscopic and photometric galaxy data can help us increase generalizability and robustness; and three, how validating ML models on simulations can help us advance theoretical physical models.
Towards a 6D View of Planet Formation CfA Colloquium 2022-09-21 | We have observed a stunning diversity in the properties of exoplanetary systems which questions our understanding of planet formation. Does this diversity arise from differences in initial conditions? Are there multiple modes of planet formation which preferentially build different types of planets? Or does this diversity stem from the evolution of the newly born planetary system as the natal protoplanetary disk dissipates? Over the last several years we have gained an incredible insight into how planetary systems form and evolve thanks to facilities such as ALMA which allow us to resolve — spatially and spectrally — these planetary nurseries. In this talk I will discuss how our view of the protoplanetary disk has shifted from a smooth, flat 2D disk, to a highly structured and vertically extended disk which hosts complex flows and dynamical features driven by embedded planets. I will provide examples of how these observations, coupled with state-of-the-art numerical simulations, are shining new light on the planet formation process, and how new facilities, such as JWST, are going to allow us to tackle new questions related to our origins.
Betelgeuse - The Great Dimming and its Consequences CfA Colloquium 2022-09-14 | During December 2019, the supergiant star Betelgeuse, began to dim – morethan had occurred in over half a century of photoelectric measurements. Thisdimming continued, reaching a historic low in mid-February 2020 and thenbegan to recover. In addition to intense public interest, astronomers world-widefollowed this event with imaging and/or spectroscopy spanning X-rays, ultraviolet,optical, infrared and radio frequencies. These many measurements enable us to tracethe development of this phenomenon. We suggest that a major upheaval occurred from thestellar photosphere: a Surface Mass Ejection. Moreover, Betelgeuse still has not recoveredfrom the insult - its fundamental 400-day brightness variation has not returned, now morethan 2 years later. We discuss the evidence, the implications, and possible future behavior ofthis iconic star.
