Talks & Events
Astro Tuesday Series
Astro Tuesday Series:
Past Astro Tuesday Series
The origin of the ultra-high energy cosmic-ray dipole
Although their astrophysical sources remain a mystery, new measurements brought by experiments such as the Pierre Auger Observatory and Telescope Array, have radically improved our knowledge of the ultra-high energy cosmic-rays (UHECRs). I will review the current observational status (spectrum, composition, and arrival directions) and present new results on the interpretation of the first 5 sigma anisotropy in the UHECR sky: a "dipole" for UHECRs with energies above 8 EeV, reported in 2017 by the Pierre Auger Observatory.
Astro2020 Decadal Survey Update and Discussion
Black-Hole Energy Extraction via General-Relativistic Kinetic Plasma Simulations
Black holes of all masses drive powerful relativistic jets, using magnetic fields dragged in by their accretion flows. The jets' plasma should be so diffuse as to be effectively collisionless, and self-consistently supplied by pair creation near the horizon. I will present the first general-relativistic kinetic plasma simulations of collisionless black-hole magnetospheres, showing the launching of electromagnetic jets by the Blandford-Znajek mechanism. The simulations reveal a population of particles with negative effective energy, which can contribute significantly to black-hole rotational-energy extraction. The kinetic approach will be useful for studying the accretion flows of the primary Event Horizon Telescope targets, M87 and Sgr A*, where the plasma is likewise of low density and collisionless, and for probing black holes' nonthermal X-ray and gamma-ray emission from first principles.
Transformative advances in post-main-sequence planetary system science
Connecting planetary systems at different stages of stellar evolution helps us understand their formation, evolution, and fate, and provides us with exclusive and crucial insights about their dynamics and chemistry. Post-main-sequence white dwarf and giant branch stars host planetary systems which include a variety of observed objects and phenomena, such as planetary debris discs, disintegrating and embedded asteroids, exo-comets, and photospheric metal pollution. Here, I provide a review of both our current knowledge of these systems and models which have been used to explain them. I also highlight the transformative advances expected in upcoming years with the current and next generation of ground-based and space-based initiatives. Looming orders-of-magnitude increases in available data must be accompanied by novel theories and simulations in order to understand the results from this interdisciplinary and expanding research field.
Clues to Globular Cluster Formation, and Contributions to the Field Population
Globular clusters are now well-established to host "Second-generation" stars, which show anomalous abundances in some or all of He, C, N, O, Na, Al, Mg, etc. The simplest explanations for these phenomena typically require the globular clusters to have been ~20x more massive at birth, and to have been enriched by processes which are not consistent with the theoretical predictions of massive star chemical synthesis models. The library of observations is now a vast one, yet there has been comparatively little progress in understanding how globular clusters could have formed and evolved. In this talk I discuss two new insights into the matter. I first report on a meta-analysis of globular cluster abundances that combined APOGEE and literature data for 42 globular clusters, new trends with globular cluster mass are identified. I then discuss the chemical properties of former globular cluster stars that are now part of the field population, and what can be learned. I demonstrate that globular clusters were the predominant sites of star formation in the first epoch of star formation.
Magnetic fields and cosmic rays in disk galaxies
Only very recently cosmological zoom simulations of galaxies have started to be able to include the evolution of magnetic fields and the treatment of relativistic cosmic ray particles. I will present recent results from the Auriga simulations of high resolution cosmological zoom simulations of Milky Way-like galaxies and show how the magnetic field in these galaxies is amplified and ordered during cosmic evolution. Moreover, I will discuss how the inclusion of cosmic rays in the cosmological simulations changes the properties of disk galaxies and their circumgalactic medium.