Date: Wednesday 06-Sep-2023
Speaker: Dr. Sanjana Curtis (UC Berkeley)
Title: Heavy element nucleosynthesis and kilonovae from compact object mergers
From the gallium in our semiconductors to the gold in our jewelry, heavy elements occupy myriad roles in human life, including that of a mystery -- their astrophysical production via the r-process has been a topic of debate since the 1950s. Kilonovae are the only direct observational evidence of r-process nucleosynthesis in situ and thus hold great promise for uncovering how and where heavy elements are produced. While the landmark detection of the kilonova counterpart to GW170817 confirmed that neutron star mergers are a site of the r-process, several open questions remain when it comes to the details of nucleosynthesis in merger ejecta and resulting kilonova light curves. In this talk, I will discuss both binary neutron star mergers and black hole-neutron star mergers, with a focus on the r-process and kilonovae. I will present our latest predictions based on cutting-edge, general-relativistic magnetohydrodynamic simulations. Crucially, we include important neutrino physics that sets the electron fraction of the ejecta, and in turn, the heavy element abundances. I will show how detailed numerical modeling can allow us to link kilonovae to their progenitors, interpret past and future observations of these transients, and gain unprecedented insight into the origin of heavy elements.
Date: Wednesday 13-Sep-2023
Speaker: Dr. Marc Buie (SWRI Boulder)
Title: Small-body occultations as a tool to understand solar system formation
With the release of the Gaia DR2 star catalog, a new era has begun allowing the use of occultations by small solar system bodies. We can now deterministically obtain occultation sizes, shapes, and precise astrometry of nearly any object we want. We have successfully observed small near-Earth objects (eg., Phaethon, Didymos/Dimorphos, Apophis) all the way out to Jupiter Trojans and on into the Kuiper Belt. I will present results of work in support of New Horizons and Lucy that reveal a pathway to building a much more detail view of the formation of our solar system and show the current challenges that stand in the way as we stand on the threshold of another legacy data stream soon to come from VRO/LSST.
Date: Wednesday 20-Sep-2023
Speaker: Dr. Stephanie Olson (Purdue University)
Title: Habitability and Biosignatures of High-Obliquity and High-Eccentricity Exoplanets
There exists a large diversity of potentially habitable exoplanets, but we do not yet know which planetary scenarios are most likely to support remotely detectable life. Among terrestrial exoplanets, obliquity may be a particularly important consideration because it is a first-order control on climate. It has two major consequences: (1) shaping the balance of energy received at the equator vs. at the poles, and (2) driving seasonal variability. Eccentricity can also drive seasonality as the planet-star separation evolves. This talk will explore the consequences of obliquity and eccentricity for the habitability of surface environments of exoplanets. I will then discuss implications for life and resulting biosignature production. I will argue that although extreme seasonality may preclude some types of familiar life on high obliquity and high eccentricity worlds, these worlds may nonetheless be among our best targets for exoplanet life detection.
Date: Wednesday 27-Sep-2023
Speaker: Dr. Antonella Palmese (Carnegie Mellon)
Title: Probing the Universe’s expansion and the origin of compact object binaries with multi-messenger astronomy
The synergy between gravitational wave (GW) experiments, such as LIGO/Virgo, and optical sky surveys is prominent in the discovery of electromagnetic counterparts to GW events and the application of the standard siren method, which has already enabled several measurements of the Hubble Constant. Dark Energy Camera (DECam) follow-up observations of the first binary neutron star merger detected by LIGO/Virgo enabled the discovery of the first optical counterpart to a GW event and the first standard siren measurement, which we have updated using the latest afterglow constraints. We will continue searching for optical counterparts in the upcoming years with the Gravitational Wave Multi-Messenger Astronomy DECam Survey (GW-MMADS), which I will introduce. We have also extended the standard siren analysis to compact object binary merger events without electromagnetic counterparts using galaxy catalogs, for which I will present the latest results. In the last part of the seminar, I will talk about some interesting possibilities for the formation of the most massive binary black hole mergers detected so far which are related to galaxies’ central black holes, in particular those in disks of Active Galactic Nuclei. I will show how the various standard siren measurements presented could be promising tools to shed light on the Hubble constant tension in the coming years.
Date: Wednesday 04-Oct-2023
Speaker: Dr. Kishalay De (MIT)
Title: The transient mid-infrared sky: Revealing the enshrouded births, deaths and afterlives of stars through cosmic time
The growth of mass in stars and supermassive black holes (SMBHs) underpins every area of astrophysics. It is now becoming increasingly clear that short-lived transient outbursts caused by eruptive mass transfer episodes likely dominate the growth processes. Yet, direct observations remain severely limited either due to their intrinsically red emission, or absorption due to intervening dust and gas which make these phenomena invisible in optical/X-ray time domain surveys. In this talk, I will present the WISE transient pipeline -- a novel effort aimed at a systematic all-sky census of the dynamic mid-infrared sky using more than a decade of data from the NEOWISE space mission. Utilizing the highest spatial resolution images together with state-of-the art image subtraction algorithms, we have created a statistically complete sample of 15 million infrared transients in NEOWISE data. Embarking on a large ground-based follow-up program, I will present results from multi-wavelength characterization of the brightest mid-infrared transients, revealing i) a complete census of proto-stellar growth through transient outbursts, ii) a new class of dust obscured stellar mergers in low mass stars, iii) new insights into the fiery fates of close planetary systems, iv) missed tidal disruption events by SMBHs in nearby galaxies and iv) highly variable accretion outbursts in Active Galactic Nuclei in the near and distant universe.
Date: Wednesday 11-Oct-2023
Speaker: No Colloquium
Title:
Date: Wednesday 18-Oct-2023
Speaker: Dr. Bharat Ratra (Kansas State University)
Title: The "Standard" Model of Cosmology ... and Open Questions
Experiments and observations over the last two decades have provided strong support for a "standard" model of cosmology that describes the evolution of the universe from an early epoch of inflation to the complex hierarchy of structure seen today. I review the basic physics, astronomy, and history of ideas, on which this model is based. I describe the data which persuade cosmologists that (as yet not directly detected) dark energy and dark matter are by far the main components of the energy budget of the universe. I conclude with a list of open cosmological questions.
Date: Wednesday 25-Oct-2023
Speaker: Dr. Megan DeCesar (George Mason University)
Title: "The Search for Nanohertz Gravitational Waves with the NANOGrav Pulsar Timing Array"
Pulsar timing arrays (PTAs) are kiloparsec-scale gravitational wave (GW) detectors composed of many millisecond pulsars (MSPs) timed over many years. A detection of quadrupolar spatial correlations in the variations in pulse arrival times would indicate the presence of a nanohertz-frequency gravitational wave (GW) signal in the timing residual data. In particular, the correlations between MSP pairs’ timing residuals are predicted to follow the Hellings-Downs correlation curve in the presence of an isotropic stochastic GW background (GWB). The most likely origin of such a GWB is thought to be a population of coalescing supermassive black hole binaries SMBHBs), but other more exotic sources like cosmic strings have been proposed. PTAs are predicted to first detect the GWB, followed by continuous waves (CWs) from individual foreground SMBHBs. GW detection by PTAs requires timing many tens, to hundreds, of MSPs with ~10-100 ns timing precision over years to decades; adequate sky coverage to sufficiently populate the Hellings-Downs curve; and for CW detection, high (~weekly) observing cadence. In this talk, I will focus primarily on the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) PTA and the International Pulsar Timing Array (IPTA). I will describe NANOGrav's analysis methods, our successful campaign to dramatically increase the size of our PTA and thus increase our GWB sensitivity, and the evidence for a GWB found in the NANOGrav 15-year Data Release as well as in our international partners' recent data releases. Finally I will discuss the ongoing efforts to produce the next fully-combined IPTA data set, which will be the most sensitive data set for nHz GWs to date, and NANOGrav's plans for further improving our PTA with future facilities, in particular the DSA-2000 and the next-generation VLA.
Date: Wednesday 01-Nov-2023
Speaker: Ms. Jillian Rastinejad (Northwestern University)
Title: Observing the sites of R-Process Nucleosynthesis
Thanks to the landmark discovery of the first binary neutron star (NS) merger detected via gravitational waves (GWs) and its accompanying thermal explosion, a so-called ``kilonova'', it is widely accepted that NS mergers contribute significantly to the Universe's production of heavy elements. However, it remains an open question whether kilonovae are the only astronomical site of heavy element formation in our Universe. In this talk, I will discuss our work expanding our view of the diversity of kilonovae and their heavy element yields using observations following bright, high-energy, cosmological flashes called gamma-ray bursts (GRBs) and GW signals. I will highlight our recent discovery of a nearby kilonova that provides a striking counterexample to a decades-long held GRB paradigm. I will also discuss a potential secondary source of heavy element nucleosynthesis and our efforts to leverage the Hubble Space Telescope to observe their formation. Finally, I will show that JWST and, soon, Roman observations can provide an unprecedented view of these sites and answer numerous open questions about the creation of heavy elements.
Date: Wednesday 08-Nov-2023
Speaker: Dr. Natasha Hurley-Walker (Curtin University-Australia)
Title: Long-period radio transients
Pulsars have long been known to produce periodic pulsed radio emission as they rotate with periods of milliseconds to seconds, generating radio waves that sweep across our line-of-sight. They allow us to probe extremes of magnetism, gravity, neutron star composition, and have been used to test general relativity to high precision. While the exact radio emission mechanism is not well- understood, they are thought to be jointly powered by their magnetic fields and fast rotation, persisting for millions of years before slowing to rotation rates (~minutes) that no longer generate radio waves (crossing the “death lines”). Magnetars, which are highly magnetized, young neutron stars, are known for their sporadic periodic bursts of X-rays, gamma-rays, and radio waves, believed to be triggered by intense magnetic activity.
Using the Murchison Widefield Array, we have detected two long-period radio transients, with periods of 18 and 22 minutes. Timing estimates place the most recently discovered source below the conventional death lines of physical mechanisms that produce radio emission from pulsars, or rescaled death lines for white dwarf pulsars. However, its long-lived activity makes it challenging to interpret as a magnetar. The sources' bright radio emission is therefore a puzzle which new observations with MeerKAT and new physical modelling is beginning to illuminate. I will describe the sources, show our current thinking on the origin of the radio emission, and conclude with future survey plans across a range of telescopes.
Date: Wednesday 15-Nov-2023
Speaker: Dr. Pieter van Dokkum (Yale University)
Title: Did JWST break the Universe? New insights into the origins of massive galaxies
The first JWST images of the distant Universe showed a hoped-for, yet unexpected, richness of galaxies that are beyond the reach of HST. One of the most puzzling results was an apparent population of Milky Way-mass galaxies when the Universe was only 500 Myr old. The discovery of these "Universe breakers" led to intense discussion and analysis, as such objects should not exist in most models of galaxy formation in a LCDM cosmology. The talk will review these and other JWST results on early massive galaxies, as well as HST spectroscopy of their putative descendants in the nearby Universe. Taken together, the currently-available information suggests that the central few 100 pc of massive galaxies had a unique formation history, stretching back to extremely early times. There are still major uncertainties, such as a lack of direct dynamical measurements of the earliest galaxies. Fortunately we can expect rapid progress over the next few years as we are only at the very beginning of JWST's era of exploration.
Date: Wednesday 22-Nov-2023
Speaker: THANKSGIVING BREAK
Title: NO COLLOQUIUM
Date: Wednesday 29-Nov-2023
Speaker: Dr. Michael Kelley (University of Maryland)
Title: JWST reveals water ice in the asteroid belt
Nearly 30 years ago, comet 133P/Elst-Pizarro was discovered in the asteroid belt. Despite having a stable orbit seemingly no different from any other outer main belt asteroid, this object had a tail. Three months and several telegrams later, the cometary nature was confirmed based on a morphology that was best described by continuous mass loss over a period of weeks to months. Fast forward to today, and a variety of mass-loss mechanisms have been identified in asteroid belt members, including impacts and rotational instability. But only the so-called "main-belt comets" like Elst-Pizarro repeatedly produce dust near perihelion, suggesting activity driven by water ice sublimation. These objects present the exciting possibility that outer belt asteroids can preserve water ice in their interiors over the ~4.5 Gyr lifetime of the Solar System. However, 15 years of attempts to detect gas around main-belt comets have only returned upper-limits. That is, until JWST observed comet 238P/Read in 2022. I will review the JWST observations of comet Read and the findings based on them, and share a few other cometary results from the first year of JWST's science operations.
Date: Wednesday 06-Dec-2023
Speaker: Dr. Karen Meech (University of Hawaii)
Title: Small Bodies: Primitive Witnesses to the Birth of a Habitable Solar System
No one knows if our solar system, with a planet possessing the necessary ingredients for life within the “habitable zone’’, is a cosmic rarity. We also don’t know if the gas giants in our solar system played a role in delivering the materials essential to life to the habitable zone. Our solar system does not have a common arrangement of planets. Does this have implications for Earth’s habitability? The answers to these questions are contained in the leftovers of the process of forming our solar system. Today, we observe these remnants of the birth of our solar system as comets and icy asteroids. Decades of small body observations from the ground and in space, along with contributions of all-sky surveys are both confirming old theories and uncovering new information that will help piece together how at least one inhabited solar system became habitable. In this talk I will explore how recent discoveries and interdisciplinary investigations are addressing the question of how Earth became habitable, and what we can expect using new tools such as JWST, the LSST and future missions.
