Dr. Joel Leja, Penn State University

Title: Alien oceans: hot springs, phosphorus, and the search for life in the solar system

Abstract: The discovery of liquid water oceans on Saturn’s moon Enceladus in 2006 and as many as a dozen other moons in the solar system (most notably Jupiter’s moon Europa) has greatly changed our understanding of our solar system’s "habitable zone." Rather than simply searching for liquid water, planetary scientists now need a framework to assess the relative likelihood of different planetary targets to host the physical and chemical ingredients required to support detectable biology. 

In this talk, I will discuss the planetary (hydrothermal) and exogenous (meteoritic sedimentation) processes leading to nutrients and bioavailable energy on ocean moons. I show how even a relatively tectonically quiescent seafloor can lead to hydrothermal circulation. I will then focus on a specific bioessential nutrient produced by this circulation: phosphorus that was recently discovered on Enceladus. Our published results anticipated those observations to within analytical error of the satellite. I show how models integrating stellar stoichiometry and an understanding of thermodynamics and fluid dynamics can lead to testable predictions of icy moon ocean chemistry. Finally, I discuss the implications for exoplanet and future planetary science missions. 

Dr. Akash Gupta, Princeton University

Title: Connecting the Dots: From Electrons to Planets

Abstract: The most common planets observed to date fall between the sizes of Earth and Neptune. I will present how my work, together with recent studies, demonstrates that most of these planets accreted hydrogen-helium envelopes from their protoplanetary disks. Over millions to billions of years, some lose these primordial atmospheres through escape processes and emerge as rocky super-Earths, while those that retain their envelopes correspond to the population observed today as sub-Neptunes.

This evolutionary pathway has profound implications for interpreting the diversity of atmospheres now being revealed by the James Webb Space Telescope and for designing future surveys. In particular, these hydrogen-helium-dominated atmospheres are expected to interact strongly with their molten or supercritical interiors over much of their lifetime. Yet, despite their central role in planetary evolution, our fundamental understanding of these atmosphere-interior interactions remains limited, largely because they occur under extreme pressures and temperatures that are difficult to access experimentally.

I will present new quantum-mechanical insights into how key planetary materials — hydrogen, helium, water, silicates and iron — interact and transform under conditions relevant to Earth-to- Neptune-mass planets. I will discuss how these results challenge long-held views on the evolution and structure of planets and outline their implications for interpreting current and future observations, including those from JWST, as well as upcoming missions and facilities such as the Uranian Orbiter and Probe, the Habitable Worlds Observatory and ELTs.

Title: Exact Transition Amplitudes in Liouville CFT and the Cigar

Abstract: We study exact transition amplitudes in certain 2D CFTs — Liouville theory and the SL(2,R)/U(1) coset model describing Witten's 2D black hole. Using bootstrapped conserved currents and a generating functional approach based on canonical transformations, we compute these transitions both recursively and via functional integral representations. The tree-level evaluation of the generating functional reproduces exact results, while providing a diagrammatic framework for loop corrections. We also find that the statistics of Liouville transition elements at higher oscillator levels exhibit level repulsion.

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Zoom Link (password 114038)

Title: Quasars in Cosmic Reionization: Environment and Impact

Abstract: The epoch of reionization (EoR) is when the first galaxies form and ionize the neutral gas in the intergalactic medium (IGM). Due to the vast distance, direct observations of EoR were limited until the past decade. Data from quasars, the brightest nontransient objects, have opened windows to study structure formation at the earliest stages. 

The regions around the quasars, called quasar proximity zones, are particularly exciting because they offer insights into a wide range of interesting physics. In this talk, I will first demonstrate how to interpret the Lyα spectra corresponding to quasar proximity zones. Then, I will show how to use absorption features to recover the density and further constrain cosmological parameters and quasar properties. 

I will also present my suite of quasar proximity zone simulations and show how galaxy formation is affected in this radiation-dominated environment. I will conclude with an outlook on synergizing JWST and ground-based observations of quasar proximity zones to advance our understanding of reionization.

Title: Star formation and evolution in AGN disks, with application to Little Red Dots

Abstract: Study of stellar objects embedded in AGN accretion disks has been motivated by i) the disk(s) of stars that possibly formed in-situ in the galactic center; ii) the super-solar metallicity of classical quasars independent of redshift, as well as possible AGN+star origin of Little Red Dots; iii) quasi-periodic eruptions connected to star-disk collisions; and iv) LIGO-Virgo gravitational wave sources potentially born in gas rich environments. 

In this talk, I will introduce some recent progress on radiation hydrodynamic simulations of stellar evolution in AGN disks, focusing on their formation from fragmentation of a gravitationally unstable disk and their accretion process in a stratified gas-rich background. We argue that a population of such stars is able to power the extended, optically thick and marginally gravitationally stable disk to generate a big red bump of universal Teff~5000K in the disk SED, which can be invoked to explain continuum features of Little Red Dots.