Population-Level Planetary Physics
Comparative planetary science, as typically practiced over the past few decades, has looked for connections in physical processes between very small numbers of planets. However, due to the exoplanet revolution, and in particular for transiting planets, we can use the astronomical perspective to look for trends in planetary structure and physical processes that can only be seen with a large sample size. This work complements the pursuit of more detailed science questions that can be asked in the solar system. I will discuss modeling work that we have done to address several exoplanet topics, including the "evaporation valley" for sub-Neptunes and super Earths, the giant planet mass-metallicity relation, connections with atmospheric characterization for these planets, and the long-standing issue of the radius anomaly of hot Jupiters. Event Status:
Proving Einstein Right: What's In It & Why I Co-authored It
Abstract: The book "Proving Einstein Right" was created with the goal of telling a "scientific band of brothers story" about the lives of astronomers who strove from 1911 to 1922 to observe the deflection of starlight by the sun. A perspective on the importance of their stories for students and non-scientists today is presented.
Molecular characterization of preserved tissues in a Cretaceous ankylosaur
We used geochemical techniques to study the chemistry of soft tissues of an extraordinarily well-preserved Cretaceous dinosaur from Alberta, Canada. Analysis of the skin and armor plates revealed aspects of the pigmentation and the steroids present in the gut contents showed evidence of the prior presence of a microbiome. This talk will address the evidence that allowed us to make these assessment and other instances where steroids present in hominin guts can provide knowledge of value to archaeologists and health scientists.
The discovery of pulsars - a graduate student's tale
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.
Bok Prize Lecture: Measuring Spins of Stellar-Mass Black Holes with X-ray Spectroscopy
February 6, 2020
Center for Astrophysics | Harvard & Smithsonian
Abstract: One of the most remarkable properties of an astrophysical black hole is that it can be completely described by just its mass and spin. Knowledge of spin is fundamental for testing how black holes form, for testing the production of relativistic jets, for producing GRBs, and more. X-ray techniques measure black-hole spin by measuring the inner radius of the accretion disk, which corresponds to the innermost stable circular orbit (ISCO). I will describe the foundation of these measurements and summarize our progress, highlighting where we stand 15-years into the field, having achieved a census comprised of several dozen black holes in our Galaxy and Local Group. This talk is dedicated to the memory of Jeff McClintock, whose vision and indomitable curiosity drove him to discover all that could be known about a black hole.
Justin Casper - First Discoveries by Parker Solar Probe and the SWEAP Investigation
The chemical structure of planet forming disks: the story of Nitrogen
What sets the composition of nascent planets is a fundamental question in astronomy, and one that is extremely timely considering the large number of exoplanets with very different characteristics that have been discovered in the past years. Whether these planets can host life depends directly on the composition and distribution of the gas where they form, i.e. protoplanetary disks. Thanks to ALMA we can now image the emission of key organic species at scales of ~15 au. In this talk I will show recent ALMA observations of HCN, one of the simplest but bright N-bearing species in disks, and of its two isotopologues. HCN is of particular interest as it is thought to be the starting point for the formation of the precursors of RNA and proteins. Moreover, the N isotopic ratio 14N/15N is often used to determine the origin of the material in our solar system. However, the 14N/15N ratios varies dramatically between different solar system bodies, and the cause of this variation remains a mystery. I will then comment on the most complex N-bearing species detected in disks so far, and conclude by discussing our current efforts to constrain the chemical structure of disks in the planet and comet forming zone.
Decoding the Milky Way galaxy
Stars orbiting in the Milky Way halo comprise only 1% of our Galaxy's stellar mass. However, these stars are among the oldest in the Galaxy, and thanks to their long relaxation times, they preserve a historical record of the Milky Way forming. To access this historical memory of the halo, we need to measure the precise 3D positions, 3D velocities, and chemical abundances of large numbers of stars. Thanks to the Gaia mission, 5D positions are now available for almost 2 billion stars. In this talk, I will present first results from spectroscopic surveys that use Gaia data to preferentially target halo stars, and have so far mapped the full 6D phase-space for 70,000 stars. I will first discuss how a blind halo survey can be used to discover even the oldest, completely dissolved, progenitors of the Milky Way because they remain distinct in the space of conserved orbital quantities. The abundance and chemical properties of the Milky Way progenitors will provide a unique window into the early universe. Then, I will show how similar observations targeted at a known, cold stellar stream, suggest it recently had a close encounter with a massive and dense perturber, and also constrain the perturber's orbit and present-day location. Known baryonic objects are unlikely perturbers based on their orbital properties, but observations permit a low-mass dark-matter subhalo as a plausible candidate. This observation opens up the possibility that detailed studies of stellar streams could measure the mass spectrum of dark-matter substructures and illuminate the nature of dark matter.
The Galactic Center Gamma-Ray Excess: A Puzzle at the Heart of the Milky Way
The region around the Galactic Center contains a well-characterized excess of gamma rays, which has garnered great interest as a possible signal of either dark matter particles colliding and annihilating, or a previously undiscovered population of pulsars in the stellar bulge. Analyses of the photon statistics of this excess have been used to argue that the pulsar interpretation is strongly favored -- however, I will present recent work arguing that it may be premature to exclude a dark matter origin for the excess on these grounds. I will outline the history of our understanding of the excess and the arguments for various interpretations, describe the current status of the controversy, and discuss future paths forward
Planets in a bottle: Exploring planetary atmospheres in the lab
From exoplanets, with their surprising lack of spectral features, to Titan and its characteristic haze layer, numerous planetary atmospheres may possess photochemically produced particles of "haze". With few exceptions, we lack strong observational constraints (in situ or remote sensing) on the size, shape, density, and composition of these particles. Photochemical models, which can generally explain the observed abundances of smaller, gas phase molecules, are not well suited for investigations of much larger, solid phase particles. Laboratory investigations of haze formation in planetary atmospheres therefore play a key role in improving our understanding of the formation and composition of haze particles. I will discuss a series of experiments aimed at improving our understanding of the physical and chemical properties of planetary atmospheric hazes on Titan, Pluto, super-Earths, and mini-Neptunes.
Hubble’s Panchromatic Comparative View of Exoplanet Atmospheres
1080P - 60
Host: Mercedes Lopez-Morales Speaker: David Sing (STScI) Abstract: To date, Hubble has played the definitive role in the characterization of exoplanet atmospheres. From the first planets available, we have learned that their atmospheres are incredibly diverse. With HST, JWST, and TESS a new era of atmospheric studies is opening up, where wide scale comparative planetology is now possible. Such studies can provide insight into the underlying physical process through comparative studies. Hubble’s full spectroscopic capabilities are now being used to produce the first large-scale, simultaneous UVOIR comparative study of 20 exoplanets ranging from super-Earth to Neptune and Jupiter sizes. With full UV to infrared wavelength coverage, an entire planet’s atmosphere can be probed simultaneously and with sufficient numbers of planets, it will be possible to statistically compare their features with physical parameters. The panchromatic treasury program aims at build a lasting HST legacy, providing the UV and blue-optical exoplanet spectra which will be unavailable to JWST, providing key insights into clouds and mass loss. I will review the highlights of the program to date, which include atmospheric water resolved in emission and new absorption features seen in transmission such as escaping ionized metals. I will also present the latest findings from the ongoing Hubble Treasury program and discuss synergies with JWST.
Blast from the Past: Ultraviolet Investigations of Exoplanet Systems and their Habitability
Roughly seventy-five billion low-mass stars (a.k.a. M dwarfs) in our galaxy host one or more small planets in the habitable zone (HZ). The stellar ultraviolet (UV) radiation from M dwarfs is strong and highly variable, and impacts planetary atmospheric loss, composition and habitability. In fact, superflares occur daily in their first ~100 Myr, and these effects are amplified by the extreme proximity of their HZs. Understanding the UV environments of M dwarf planets is crucial to understanding atmospheric composition and evolution, and providing context for measured exoplanet spectra. For HZ terrestrial planets, characterization of the UV provides a key parameter in a planet’s potential for habitability as well as for discriminating between biological and abiotic sources of observed biosignatures. Our efforts to study the stellar UV span past, present and future space telescopes: the Galaxy Evolution Explorer (GALEX), the Hubble Space Telescope (HST), and the upcoming NASA-funded Star-Planet Activity Research CubeSat (SPARCS), due for launch at the end of 2021. SPARCS will be a 6U CubeSat completely devoted to continuous photometric monitoring of M stars, measuring their variability, flare rates and evolution, while also being a pathfinder for much-needed future UV missions.
Planck results, curiosities and tensions in the LCDM model
Planck is an ESA satellite aimed at the observation of the Cosmic
Microwave Background. The Planck collaboration has recently published
its last legacy release. In this talk I will shortly review the main
Planck results and their robustness, highlight some of the curious
features present in the data and the Planck point of view on tensions
with a few other astrophysical probes, notably with the Hubble constant
measurements from local distance measurements.
So What's All This Fuss About Decadal Surveys?
Astronomers have been doing decadal surveys for more than half a century. There must be good reasons that our community keeps spending so much time and energy on these reports every ten years. And yet, some in our community have suggested that the whole enterprise has outlived its usefulness and should be rethought. This talk will briefly review the history of the surveys, examine how the reports are used (or not) by policymakers and lobbyists in Washington, and look forward to what we might expect when Astro2020 hits the streets.
The comets that orbit other stars
I will talk about recent work on the small bodies that orbit other stars. We see the dust produced in collisions between these bodies as debris disks, and the large bodies when their comae transit their host stars. A few comet transits have now been detected in broadband space-based photometry, and I will outline the first efforts towards these discoveries. These new detections are complimented by prior transient calcium absorption features, and I will show how the acceleration of these lines provides new constraints on the orbits of the transiting bodies around beta Pictoris. Finally, I will show some new detections of warm dust towards Sun-like stars, and discuss why these detections suggest comets as a probable dust source.
Core-Collapse Supernova Explosions in 3D
October 10, 2019
Abstract: Using our state-of-the-art code Fornax we have simulated the collapse and explosion of the cores of many massive-star models in three spatial dimensions. This is the most comprehensive set of realistic 3D core-collapse supernova simulations yet performed and has provided very important insights into the mechanism and character of this 50-year-old astrophysical puzzle. I will present detailed results from this suite of runs and the novel conclusions derived from our new capacity to simulate many 3D, as opposed to 2D and 1D, full physics models every year. This new capability, enabled by this new algorithm and modern HPC assets, is poised to transform our understanding of this central astrophysical phenomenon.
Chandra's Sharp View of the X-ray Sky: 20 years and counting.
NASA's Chandra X-ray Observatory is celebrating 20 years of operation in 2019. Chandra's uniquely sharp X-ray vision has resulted in major strides in our understanding of all kinds of celestial sources, and it continues to be an indispensable tool for expanding the frontiers of our knowledge. From the discovery of an X-ray jet in its first targeted source and finding the compact stellar remnant of a supernova in the second, the excitement and surprises continue. Chandra uniquely pinpoints the youngest stars buried amongst the gas and dust of star-forming regions, observes the explosions as massive stars run out of fuel and tracks the evolution of the resulting supernova remnants, measures the complex structure of the hot gas which dominates the baryonic matter in clusters of galaxies, tracing their turbulent past and present, explores the properties of dark matter, and observes the matter being captured by black holes of all sizes. I will review highlights of Chandra and its scientific discoveries, from launch on the shuttle Columbia commanded by Eileen Collins, the first female commander, to recent work such as Chandra's first X-ray detection and continued monitoring of GW170817, the merging neutron stars detected in gravitational waves by LIGO.
From Birth to Chirp - Astrophysics of Massive Stars as Gravitational Wave Progenitors
How did they form?’ is a question many asked when LIGO announced the first direct detection of gravitational waves originating from two surprisingly heavy stellar-mass black holes. With masses of about 30 solar masses each, they outweighed all of the known black holes known from X-ray binaries. Now, four years after the first detection, alerts of new triggers come in at a rate of almost one per week. The analysis of the first eleven events has been published and we learned that the first system was not exceptional: the majority of detected events involve heavy black holes. In parallel, classical telescopes have been revolutionizing our understanding of the properties of young massive stars. One of the most remarkable findings is that the majority of massive stars have one or more companions so close that the exchange of mass between them in inevitable during their lifetime. Yet, only a very tiny fraction of these stellar couples are still with their companion after both ended their lives to leave behind a neutron star or black hole. What does it take for a `stellar marriage’ to be such that not even death can part them? I will address some of the exciting new insights, but also the challenges and open questions that we are still facing. More generally I will argue that improving our understanding of massive stars is crucial for a variety of problems in astrophysics. This is because massive stars played a disproportionally large role in transforming the pristine Universe left after the Big Bang into the rich and diverse Universe in which we live today. The fact that we ourselves are largely made of the nuclear ashes of massive stars, makes the quest to understand their lives and deaths an integral part of our quest to understand our own cosmic origin.
The Puzzle of Multiple Populations in Globular Clusters
September 19, 2019
Liverpool John Moores University
Andrea Dupree and Ivan Cabrera
Abstract: Globular clusters (GCs) exhibit star-to-star variations in specific elements (e.g., He, C, N, O, Na, Al) that bear the hallmark of high-temperature H-burning. These abundance variations can be observed spectroscopically and also photometrically, with the appropriate choice of filters, due to the changing of spectral features within the band pass. This phenomenon is observed in nearly all of the ancient GCs, and has recently been found in many younger clusters as well. Many scenarios have been suggested to explain this phenomenon, with most invoking multiple epochs of star formation within the cluster; however, all have failed to reproduce various key observations. I will review the state of current observations and outline the successes and failures of some of the main proposed models. The traditional idea of using the stellar ejecta from a first generation of stars to form a second generation of stars, while conceptually straightforward, has failed to reproduce an increasing number of observational constraints. I conclude that the puzzle of multiple populations remains unsolved, hence alternative theories are needed, and will present new HST results that suggest that we may be finally closing in on origin of this enigmatic phenomenon.
And Then There was Light: What Determines the “Active” Zones of Jets?
September 12, 2019
Abstract: The energy release from black holes is a well-known problem of keen interest to many astrophysics sub-fields, from high-energy astroparticle physics to cosmology and galaxy evolution. For instance we know from the mismatch between ‘gastrophysics-free’ cosmological simulations and observations of the largest scales of structure, that black holes manage to communicate with regions well beyond their gravitational sphere of influence, and that outflows (particularly jets for the largest scales) must be the mechanism. We also know that black holes of all scales experience cycles of activity where the dominant form of energy output changes, and that jet-dominated phases are associated with the highest energy particle acceleration. The problem currently lies in tying all these phenomena together and being able to uniquely predict outflow properties as a function of accretion and environmental properties. In this talk I will discuss a problem I have been puzzling over for some time, which is the link between event-horizon scales and the particle acceleration that “lights up” the jets we observe, which relates to our ability to connect recent Event Horizon Telescope images to observations at other wavebands. In particular I will focus on some recent studies about an older problem well-known in ‘AGN circles’, but that is now being explored in real-time using transient jets from black hole X-ray binaries (BHXBs): what sets the so-called “dissipation zone” where the inner jets start to radiate? I will then discuss how these results can be used to inform our models of both EHT images, and help connect them to the multi-wavelength observations we perform each campaign, as well as increase our understanding of the source of ultra-high energy cosmic rays, TeV gamma-rays and neutrinos.