2022 Spring Schedule

Title: Snails Across Scales or Fun with Phase-Mixing

Abstract: Results from ESA’s Gaia mission paint a detailed picture of our Galaxy in 6-D - revealing abundant signatures of departures from purely random samples drawn from equilibrium distributions. This talk outlines some fun connections back to basic dynamical concepts, and forward to interpretation and implications. 

Title:  Messengers from Cosmos

Abstract: Neutrinos are fascinating elementary particles heralding the dawn of the multi-messenger astronomy era. Neutrinos affect the stellar dynamics, drive the formation of the heavy elements, and carry signatures of the yet mysterious physics ruling the most powerful cosmic fireworks. Recent developments on the role of neutrinos in cosmic sources will be reviewed together with the most exciting detection prospects. 

Title:  Multiwavelength Variability in Young Stars

Abstract:  Young stars accrete material from their surrounding disks. Simulations predict that matter accretes via the stellar magnetic field lines, leaving a hot spot with a density gradient. We present observational evidence of this density gradient utilizing a comprehensive, coordinated multi-epoch multi-wavelength observing campaign. UV and optical light curves of GM Aur display periodicity, but do not peak at the same time. The offset peaks are due to a hot spot with a density gradient; different density regions of the hot spot emit at distinct wavelengths and have separate physical locations leading to periodic, offset peaks in the UV and optical as the hot spot rotates along with the star. These observations confirm theoretical predictions and demonstrate the insights gained from coordinated multi-epoch multi-wavelength observations.

Title: Stability of relativistic AGN jets

Abstract:  Active galactic nuclei accelerate jets that extend well beyond the vicinity of the host galaxy. The morphology of these jets is described within the Fanaroff-Riley classification, with the FR-I sources being brighter near their origin and less energetic whereas the FR-II sources are filling hot cocoons, have hotspots at their edges and are, overall, more energetic. Injecting a relativistic jet into an environment with some pressure profile, leads to reconfinement shocks where their cylindrical radius expands and contracts, while the flow lines converge into a narrow region and rebounce, provided that the confining external pressure does not drop faster than r^-2. I will present 3-D simulations of such relativistic jets, starting from an initial state that corresponds to an equilibrium. It is found that hydrodynamical jets are susceptible to the centrifugal instability, as the Rayleigh criterion is identically satisfied. The inclusion of an azimuthal field leads to a more stable jet and a magnetisation of σ~0.01 is sufficient for the complete stabilization of the flow. Moreover, the appearance of the instability in the hydrodynamical and weakly magnetised cases depends on the environment. Jets confined by a high density, cold medium, become unstable by flaring and decelerating. On the contrary, jets confined by a low density, hot medium develop their instability by generating hotspots and without decelerating. This leads to substantial qualitative differences between the two types of jets which is possibly related to the Fanaroff-Riley dichotomy. 

Title: Particle acceleration in astrophysical plasmas: Radiation belts and collisionless shocks

Abstract:  Tenuous plasmas that comprise the vast majority of volume in the cosmos are responsible for the acceleration of individual particles to exceptional energies, which we know as cosmic rays. Particle acceleration processes are as diverse and varied as the space environments around the Milky Way, from planetary and stellar systems to supernovae shocks and remnants. Here, I focus on two particular types of systems that serve as exceptionally efficient particle accelerators: planetary radiation belts and collisionless shocks. Starting first with radiation belt systems, I quickly introduce some of the most important fundamentals of radiation belt physics (including particle trapping, adiabatic invariants, quasi-linear diffusion, and nonlinear wave-particle interactions) before walking through some of the most impactful insights on the nature of Earth's radiation belts developed over the past decade. In particular, I will focus on rapid acceleration of relativistic electrons by i) nonlinear and quasi-linear interactions with electromagnetic "chorus" waves (an electron whistler mode), ii) non-diffusive inward radial transport, and iii) exceptionally efficient and rapid acceleration by magnetic reconnection in Earth's magnetotail. Next, I shift focus to two other radiation belt systems in the Solar System: Jupiter and Uranus. Jupiter is exceptional, as it boasts by far the strongest radiation belts in the Solar System, and by some metrics, the jovian magnetosphere is the most effective particle accelerator in the Solar System. In the context of what we know from Earth's radiation belts, we examine some of the key aspects of why Jupiter's radiation belts are so intense, discuss how – thanks to it boasting electron synchrotron emissions – Jupiter is an ideal stepping stone to understanding more extreme exoplanetary and astrophysical radiation belt systems, and highlight some of the outstanding mysteries of that enormous magnetospheric system. Having only ever been visited once in a flyby from Voyager-2, Uranus remains shrouded in mystery. It is an exceptional world considering its extreme dipole tilt, offset rotational axis, and confoundingly strong radiation belts. Venturing further into the cosmos, I then will shift focus to particle acceleration at collisionless shocks in space plasmas. Highlighting what we know now from detailed in situ observations at Earth's and other planetary bow shocks, I will put those results into context with shocks in other plasmas, such as solar and interplanetary, the heliospheric termination shock, and ultimately astrophysical shocks in more extreme systems. In summary, we are genuinely gifted with an incredible variety of natural space plasma laboratories around the Solar System, and we should be using those laboratories more effectively and often to explore what we don't know about other astrophysical systems that we can only explore via limited remote sensing and loosely bounded theory.

Title:  From Dwarf Galaxies to “Wanderers”: Simulations on the Most Elusive Populations Supermassive BH

Abstract:   Discussions on supermassive black holes (SMBHs) are often focused on the most massive, rapidly growing SMBHs residing in the centers of the largest galaxies because these are the ones we are most likely to detect. However, recent advancements in observations - along with critical improvements to modeling SMBHs in simulations - have allowed us to expand our studies to more elusive populations of SMBHs. I will present new results from cosmological hydrodynamic simulations of galaxy formation on the population of SMBHs in low-mass galaxies. In particular, I will discuss the influence of SMBH feedback and the effect of environment on the likelihood for dwarfs to host SMBHs. I will also examine off-center, `wandering’ SMBHs that are predicted to exist in large numbers in the outskirts of massive galaxies. I will discuss the origins of these wandering SMBHs and how we might be able to observe them (and possible already have) in the real Universe.