List of Past Astronomy Colloquia : 01-Sep-2021 to 31-Dec-2021

Date:   Wednesday 08-Sep-2021
Speaker:   Dr. James Chibueze (NWU South Africa)
Title:  Unveiling the unseen magnetized universe with MeerKAT

Galaxy clusters are known to harbour magnetic fields, the nature of which remains unresolved. Intra-cluster magnetic fields can be observed at the density contact discontinuity formed by cool and dense plasma running into hot ambient plasma, and the discontinuity exists near the second brightest galaxy, MRC0600-399, in the merging galaxy cluster Abell 3376 (redshift 0.0461). Elongated X-ray emission in the east–west direction shows a comet-like structure that reaches the mega-parsec scale . Previous radio observations detected the bent jets from MRC 0600-399, moving in same direction as the sub-cluster, against ram pressure. Here we report radio observations of MRC 0600-399 that have 3.4 and 11 times higher resolution and sensitivity, respectively, than the previous results. In contrast to typical jets, MRC 0600-399 shows a 90-degree bend at the contact discontinuity, and the collimated jets extend over 100 kiloparsecs from the point of the bend. We see diffuse, elongated emission that we name ‘double-scythe’ structures. The spectral index flattens downstream of the bend point, indicating cosmic-ray re-acceleration. High-resolution numerical simulations reveal that the ordered magnetic field along the discontinuity has an important role in the change of jet direction. The morphology of the double-scythe jets is consistent with the simulations. Our results provide insights into the effect of magnetic fields on the evolution of the member galaxies and intra-cluster medium of galaxy clusters.

Date:   Wednesday 15-Sep-2021
Speaker:   Dr. Bill McDonough (UMD)
Title:  Why does Mercury have such a big iron core? Magnetism!

Terrestrial planet compositions follow a simple model of the magnetic field strength in the protoplanetary disk. Decreasing magnetic field strength outward from the Sun, leads to decreasing core size of the terrestrial planets with radial distance.

Date:   Wednesday 22-Sep-2021
Speaker:   Dr. Michelle Ntampaka (STScI)
Title:  The Importance of Being Interpretable

Cosmology is entering an era of data-driven science, due in part to modern machine learning techniques that enable powerful new data analysis methods. This is a shift in our scientific approach, and requires us to ask an important question: Can we trust the black box? In this talk, I will describe methods for building trust in machine learning models,focusing on models for interpreting cosmological large scale structure. I will show examples of how machine learning can be used, not just as a tool for getting better results and the expense of understanding, but as a partner that can point us toward physical discovery.

Date:   Wednesday 29-Sep-2021
Speaker:   Dr. Bekki Dawson (Penn State)
Title:  Inner Solar Systems

Over the past couple decades, thousands of extra-solar planets have been discovered orbiting other stars. The exoplanets discovered to date exhibit a wide variety of orbital and compositional properties; most are dramatically different from the planets in our own Solar System. Our classical theories for the origins of planetary systems were crafted to account for the Solar System and fail to account for the diversity of planets now known. We are working to establish a new blueprint for the origin of planetary systems and identify the key parameters of planet formation and evolution that establish the distribution of planetary properties observed today. The new blueprint must account for the properties of planets in inner solar systems, regions of planetary systems closer to their star than Earths separation from the Sun and home to most exoplanets detected to data. I present work combining simulations and theory with data analysis and statistics of observed planets to test theories of the origins of inner solars, including hot Jupiters, warm Jupiters, and tightly-packed systems of super-Earths. Ultimately a comprehensive blueprint for planetary systems will allow us to better situate discovered planets in the context of their systems formation and evolution, important factors in whether the planets may harbor life.

Date:   Wednesday 06-Oct-2021
Speaker:   Dr. Ilaria Caiazzo (Caltech)
Title:  Small But Mighty: Hunting for Extreme White Dwarfs

The advent of Gaia and of large photometric and spectroscopic surveys is changing the landscape of white dwarf studies. These incredible new data sets, together with improved models, have enabled tackling some unsolved mysteries concerning white dwarfs as a population, as well as discovering extremely peculiar objects that challenge our understanding of white dwarf formation and evolution. In my talk, I will show how the precise astrometry from Gaia has dramatically improved our capability of studying white dwarfs in young star clusters, and therefore probe the evolution of white dwarfs born from single progenitor stars. On the other hand, the Zwicky Transient Facility (ZTF) is shedding light on the evolution of white dwarfs in binary systems, substantially increasing the number of known eclipsing white dwarf binaries and finding the final products of such binaries. In fact, ZTF is discovering a large number of massive, rapidly rotating and highly magnetized white dwarfs whose extreme properties characterize them with high confidence as remnants of white-dwarf mergers. Finding a population of white dwarf merger remnants just below the Chandrasekhar mass can help constrain the number of mergers in the Galaxy and their contribution to the type Ia supernova rate, as well as help us understand the origin of strong magnetic fields in white dwarfs. I will present some early results of the search, including the discovery of ZTF J1901+1458, a moon-sized white dwarf that is extreme in almost every respect.

Date:   Wednesday 13-Oct-2021
Speaker:   Dr. Mia Bovill (TCU)
Title:  Ending the Cosmic Dark Ages: Exploring the First Stars with JWST

The first, Population III stars ignited several hundred million years after the Big Bang. While we know they formed from primordial metallicity gas in 10^5 - 10^7 solar mass dark matter halos, and are thought to be more massive than later generations of stars, their detailed properties remain unconstrained. Current unknowns include; when the first Pop III stars ignited, how massive they were, and when and how the era of the first stars ended. Investigating these questions requires a exploration of a multi-dimensional parameter space including, the slope of the Pop III stellar initial mass function (IMF), and the strength of the non-ionizing UV background. We have developed a novel model which treats these relative unknowns as true free parameters. Our simple model reproduces the results from hydrodynamic simulations, but with a computational efficiency which allows us to investigate the observable differences between a wide range of potential Pop III IMFs. The upcoming launch of the James Webb Space Telescope (JWST) provides us with a unprecedented opportunity to marry theoretical predictions to observations and determine the astrophysics which governs the formation of the first stars. Here we suggest constraining the masses of the first stars with JWST will require a multi-pronged approach. In addition to direct detection via gravitation lensing, this also includes understanding how the Pop III IMF affects observed rates of highly luminous pair instability supernova.

Date:   Wednesday 20-Oct-2021
Speaker:   Dr. Kyle Helson (GSFC)
Title:  The Status and Outlook of Cosmic Microwave Background Polarization Measurements

Measurements of the Cosmic Microwave Background (CMB) over the last 50+ years have yielded a wealth of information about the universe and LambdaCDM cosmology. Modern CMB instruments continue to search for the B-mode polarization signal from gravitational waves produced during the inflationary epoch. In this talk I will cover a brief history of measurements of the CMB and give a survey of the current status of the field. I will also talk about my work at Goddard Space Flight Center with the Cosmology Large Angular Scale Surveyor (CLASS) collaboration. CLASS is a ground-based series of telescopes in the Atacama desert in northern Chile, designed to measure the CMB and large angular scale B-mode polarization.

Date:   Wednesday 27-Oct-2021
Speaker:   Dr. Ronald Ballouz (APL)
Title:  The strengths of near-Earth asteroid surfaces from remote and in situ measurements

Spacecraft exploration of near-Earth asteroids (NEAs) has shown that their surfaces are composed of both fine regolith and larger meter-scale boulders. An asteroids surface history is determined, in large part, by its strength against collisions with other objects. Furthermore, for asteroid sample-return missions, characterizing the strength of the surface and near surface is critical for ensuring spacecraft safety and sampling success. Prior to the Hayabusa2 and OSIRIS-Rex missions to the asteroids Ryugu and Bennu, it had not been possible to assess directly the impact strength of the regolith and boulders that constitute the building blocks of a rubble-pile asteroid.

In this talk, I will discuss how we used a combination of remote observations of asteroid Bennu and the OSIRIS-REx spacecraft's surface interaction on October 2020 to gain knowledge of the strength of both the regolith layer and meter-scale boulders. I will then place these results in the context of the surface evolution and interior structure of Bennu and NEAs, in general. Finally, I will show how these findings may be used to interpret upcoming next-generation population surveys of NEAs, providing a glimpse into the strength and interior properties of the least discovered and most populous Potentially Hazardous Asteroids.

Date:   Wednesday 03-Nov-2021
Speaker:   Dr. Jan Eldridge (University of Auckland)
Title:  "New insights into black hole ecology"

Since the detection of the first gravitational wave transient in 2015 there has been a small revolution in our understanding of the mass distribution of black holes. But there are still many unknowns and theory is still struggling to understand how the observed distribution is determined. There are also other windows onto black holes that are showing that the GW transient masses are only have the story. I'll present the main samples of black holes that are available to us today and in the not-too-distant future. Then describe how theoretical work will allow us to understand what physics is in play in determining the birth and evolution of black holes, their ecology within galaxies and the Universe.

Date:   Wednesday 10-Nov-2021
Speaker:   Dr. Miguel Montargès
Title:  The Great Dimming of Betelgeuse spatially resolved by the VLT/VLTI

During winter 2019-2020, the nearby red supergiant star Betelgeuse (alpha Ori) an unprecedented drop in brightness, reaching its historic magnitude maximum of V~1.6 during February 2020. I will present the outcome of our VLT/VLTI that spatially resolved the star at the time of the Great Dimming. This event, visible to the unaided eye, caused a media frenzy but it also allowed us to better understand the mass-loss process of red supergiants. The rate of this outflow has a crucial impact on the fate of the star: it defines its final mass - hence the mass of the compact object that will remain after the supernova - and also the nature of the supernova progenitor. During the Great Dimming, we may have directly witnessed an enhanced mass ejection in the environment of Betelgeuse.

Date:   Wednesday 17-Nov-2021
Speaker:   Dr. Edwin Bergin (University of Michigan)
Title:  Making a Habitable Planet

Today we stand on the cusp of characterizing potentially living worlds, or habitable planets, planets with orbits where liquid water would exist on a rocky surface. This concept of habitability implicitly assumes water in some form is present on rocky planets but is it? More broadly are rocky planets generally chemically habitable do they contain the elements needed for life, such as carbon and nitrogen, on their surfaces? In this talk, I will review what we know about the chemical habitability of forming planets. I will follow the most abundant volatile elements needed for life (C, H, O, N) from the vast cold and low pressure environs of the interstellar space to their presence on planetary surfaces of worlds such as our own. I will outline our current understanding of star and planetary birth while discussing the fate of primary carriers of lifes elements, both volatile species (e.g. H2O, CO, CO2) and more refractory materials (e.g. silicates, aliphatic/aromatic hydrocarbons). Some of these carriers can be characterized via existing facilities (e.g. the Atacama Large Millimeter Array), planned facilities (the James Webb Space Telescope), with others require future options (such as the Origins Space Telescope). Finally, within a young planet, the ultimate fate of delivered material is not set until the hot young terrestrial world solidifies and core formation ceases thus the process of planet formation and evolution itself influences whether a mature planet is habitable or uninhabitable. Looking forward, the astrochemical study of lifes materials in space, and the astronomical characterization of terrestrial exoplanets, must be intimately linked to grounding knowledge from the planetary sciences. A fascinating interdisciplinary future awaits, where we will seek to ascertain the origin of our own biosphere and provide crucial chemical context for habitability. Final note: if time allows I will share new results from the ALMA large program Molecules at Planet-forming Scales.

Date:   Wednesday 01-Dec-2021
Speaker:   Dr. Peter Gao (Carnegie Institute for Science)
Title:  Lifting the Veil on Exoplanet Clouds and Hazes

Clouds and hazes are common in the atmospheres of exoplanets across a wide swath of temperatures, masses, and ages. These aerosols strongly impact observations of transmitted, reflected,and emitted light from exoplanets, obfuscating our understanding of exoplanet thermal structure and composition. In this talk, I will discuss how aerosol microphysics modeling has helped advance our understanding of exoplanet aerosols and atmospheres. By constraining an aerosol microphysics model with trends in exoplanet transmission spectra, we found that silicates dominate aerosol opacity above planetary equilibrium temperatures of 950 K due to low nucleation energy barriers and high elemental abundances, while hydrocarbon hazes dominate below 950 K due to a rapid increase in methane abundance and photodissociation rate. In addition, spatial heterogeneity of exoplanet clouds can explain the observed homogeneity in the nightside temperatures of hot Jupiters, while also leading to asymmetry between the dawn and dusk limbs of exoplanets in transits. Meanwhile, high altitude photochemical hazes may strongly impact our understanding of mini-Neptune mass-radius relationships. I will conclude by describing how we can further characterize exoplanet clouds and hazes in the years to come.

Date:   Wednesday 08-Dec-2021
Speaker:   Dr. Chris Mankovich (CalTech)
Title:  Unlocking planetary seismology using Saturn's rings

Gas giants -- even the ones in our own back yard -- still have poorly understood internal structures, hampering our understanding of the accretion process and subsequent mixing in the planet. Planetary seismology holds tremendous promise for revealing these planets' interior composition, buoyancy, and rotation via their normal mode oscillation power spectra, akin to the fields of global helioseismology or asteroseismology. At Saturn's rings, stellar occultations viewed by the Cassini spacecraft have revealed 30+ ring waves excited at resonances with Saturn's nonradial normal modes, confirming a hypothesis dating to the 1980s and yielding the most comprehensive dataset ever for seismological study of a planet other than Earth. I will present on the lessons we have managed to learn so far about Saturn's internal structure and rotation using this one-of-a-kind technique.

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