Forum Schedule Fall 2019

I will provide an overview of populations of isolated (or not actively accreting) compact objects in our Galaxy. In recent years there has been significant progress in studying neutron stars across different windows of electromagnetic spectrum. The population of neutron stars turned out to be surprisingly diverse. The number of discovered neutron stars suggest either evolutionary links between different types of isolated neutron stars or calls for a revision of current stellar evolution models. Many supernova remnants are still missing compact objects requiring a large population of dim (fast-cooling?) young neutron stars, quiescent black holes, or, possibly, a larger than expected Type Ia supernova rate. I will discuss the connections between the known neutron stars and their host supernova remnant properties, prospects for identifying faint neutron stars and black holes in the vast amount of archival X-ray data, and the machine-learning approach to this challenging task.

In this talk, I will review the current status of galaxy formation studies at high- to intermediate redshift using cosmological hydrodynamic simulations for the upcoming ELT/JWST era. Recent ALMA observations have discovered many FIR line-emitting galaxies at z>~7, which provide unique probes of early galaxy formation epoch. In this context, I will present the latest results from our research group, and discuss implications on the ISM and CGM in high-z galaxies and dark matter halos.

The dynamics of circumplanetary disks (CPDs), mini disks formed around protoplanets embedded in their natal circumstellar disks, plays an essential role in planet formation. The limited parameter space explored and the lack of consensus on numerical approaches in previous work have led to disagreeing reports even on the basic properties of CPDs. Using the hydro codes PEnGUIn and Athena++, we perform a series of 3D hydrodynamics simulations to measure CPD sizes, masses, and rotation rates as functions of planet mass, in both the isothermal and adiabatic limits. These limits bracket the thermal evolution of planetary atmospheres, providing insight into their gas accretion rates. Additionally, the 3D flow dynamics from our simulations allow us to trace trajectories of solids and model their accretion onto planets. I will give an overview of these results and discuss their implicitions on gas giant formation.

The two most prolific techniques for detecting exoplanets are complementary: radial velocities yield planet masses, while transits yield their radii. Yet, to measure the density of a planet we need both quantities, from which we can "assay" the composition. However, these two techniques have only been applied to a single rocky, temperate planet (LHS 1140b, which is about six times as massive and 40% larger in radius compared with Earth).

I will introduce the TRAPPIST-1 system for which we can measure the masses *and* radii of *seven* temperate, *Earth-sized* exoplanets for the first time. The technique used is transit-timing variations caused by gravitational interactions of the planets, developed together with Jason Steffen.

I will describe the barriers to the applying this technique: degeneracies in the modeling, stellar variability, and high dimensionality. We are addressing these with new data from the Spitzer Space Telescope to measure "chopping", and with modern data analysis tools, including Gaussian Processes, Differentiable Programming, and Hamiltonian Monte Carlo.

I will conclude with the future prospects for studying this planet system with the James Webb Telescope and full photodynamical modeling.

Like in a magic trick, atomically thin layers of specific materials can be mixed and stacked in a well-defined way. Due to the inter-layer interaction and charge transfer, the heterostructure may exhibit unexpected behavior. In elemental boron, a previously unknown 2D ε–B allotrope converts stepwise to a stable honeycomb structure when doped heavily with electrons, resembling a magic conversion of boron to carbon with one extra valence charge [1]. As seen in Fig. 1(b), this extra charge may be provided by an adjacent 2D Ca2N electride layer. A different apparent example of magic involves the twist degree of freedom in 2D structures including bilayer graphene. Changing the twist angle θ changes the Moiré pattern, as seen in the left panel of Fig. 1(c). Recent evidence suggests that the electronic structure near the Fermi level of twisted bilayer graphene (TBLG) depends extremely sensitively on the twist angle θ. Near the magic angle value θm≈1.08°, a flat band emerges at EF, separated from conduction and valence states by energy gaps, with important consequences for 2D superconductivity and electron correlation. Even though TBLG and related non-periodic structures can not be treated by standard band structure theory, their electronic structure can be interpreted quantitatively using a parameterized model [2] that can be simply extended to consider also other deformations including shear [3]. A third example of apparent magic involves a transformation of elemental phosphorus to unexpected nested coil structures when inside nanometer-wide carbon nanotubes [4,5].

The flatness of the solar system led to the notion that the planets formed within a disk around the young Sun. Although this notion is confirmed by disk images, some exoplanet systems show departures from this flatness, presenting tilts relative to either their host stars’ equators (stellar obliquities) or other planets (mutual inclinations) . In this talk, I will discuss what physical processes can be responsible from these departures and what they teach about the history of planetary systems. First, I will focus on a population of sub-Neptune planets with large stellar obliquities and argue that their large tilts were likely imprinted early during the evaporation of their birth disks. Second, I will focus on the question of coplanarity in Kepler planetary systems and argue that most of these systems have been subject to dynamical heating, with the hottest systems leading to the formation of ultra-short-period planets. Finally, I will discuss the prospects of using astrometric measurements from Gaia to continue providing with new clues on the flatness of exoplanet systems.

Mass outflows of ionized and molecular gas may play a critical role in the coevolution of active galactic nuclei (AGN) and their host galaxies by regulating the supermassive black hole (SMBH) accretion rate and evacuating the bulge of star forming gas. Outflows that link the sub-parsec central engine to the kiloparsec scale galactic environment are found in the narrow emission line region (NLR), and detailed comparisons between simulations and observations are required to determine if NLR outflows carry enough energy to dynamically impact their host galaxies. Our goal is to quantify this feedback process using Hubble Space Telescope observations and multi- component photoionization models in order to precisely map the velocity and mass distribution of the ionized gas as a function of distance from the nucleus in a sample of nearby active galaxies. In this talk, I will highlight the observational and photoionization modeling techniques that we employ to constrain the physical conditions in the outflows and precisely calculate ionized gas masses. The discussion will emphasize modeling relevant to a range of astrophysical plasmas as well as emission line diagnostics for constraining the gas ionization, abundances, temperature, and density. Finally, I will summarize our recent findings in the context of previous studies.

What do Bach's compositions, Rubik's Cube, the way we choose our mates, and the physics of subatomic particles have in common? All are governed by principles of symmetry, which elegantly unify scientific and artistic principles. Yet the mathematical language of symmetry — known as "group theory" did not emerge from the study of symmetry at all, but from an equation that couldn't be solved.

For thousands of years mathematicians solved progressively more difficult algebraic equations, until they encountered the quintic equation, which resisted solution for three centuries. Working independently, two great prodigies ultimately proved that the quintic cannot be solved by a simple formula. These geniuses, a Norwegian named Niels Henrik Abel and a romantic Frenchman named Évariste Galois, both died tragically young. Their incredible labor, however, produced the origins of the language of all symmetries.