Peter K. G. Williams
   Harvard-Smithsonian Center for Astrophysics
   News From the Fourth Dimension: Radio Astronomy and the Time Domain

Many recent breakthroughs in astrophysics, from exoplanet characterization to supernova cosmology, have been driven by an ongoing and dramatic expansion of our ability to perform "time domain" astronomy. Due to the particular technical needs of radio astronomy, it may be the single sub-field that is seeing the largest changes in this time-domain astronomical revolution. I will discuss my work on several topics at the forefront of time-domain radio astronomy: fast radio bursts, untargeted surveys for radio transients, and magnetism in the lowest-mass stars and brown dwarfs. These "ultracool dwarfs" host dynamos that -- unexpectedly -- generate strong, organized magnetic fields, and their magnetospheric phenomenologies are surprisingly similar to those of the Solar System planets. Time-domain studies at shorter wavelengths, such as the first ALMA detections of ultracool synchrotron emission and optical monitoring to measure rotation periods, combine with this work to help develop a comprehensive understanding of (sub)stellar dynamo physics and its implications for the spin-down and drop in magnetic activity as stars age. With the advent of a range of new and improved radio telescopes as well as LSST, time-domain techniques promise to yield further astrophysical breakthroughs in the decades to come.

Raffaella Margutti

Xuening Bai
   Harvard-Smithsonian Center for Astrophysics
   The Microphysics of Astrophysics: Adventures in Computational Magnetohydrodynamics

Astronomical observations yield rich data of phenomenology on macroscopic scales, while they are typically consequences of microphysical processes on much smaller scales. Many of such processes involve magnetic fields, instabilities and turbulence, and are the subject of computational magnetohydrodynamics (MHD). By properly incorporating the microphysics, I will draw three examples demonstrating that MHD simulations can greatly improve our understandings towards reality, and sometimes lead to surprises. First, I discuss how to build a “magnetically arrested disk” (MAD), characterized by having large amount of magnetic flux threading the central black hole, which has recently been realized to be able to launch powerful jets from AGNs and X-ray binaries. Second, I discuss the gas dynamics in protoplanetary disks (PPDs) and show that disk evolution is largely MHD-wind-driven, governed by the amount of magnetic flux threading the disk. I will further discuss magnetic flux evolution in PPDs. Finally, I describe an MHD-Particle-in-Cell method for coupling cosmic-rays with a thermal plasma, and discuss its applications to particle acceleration and cosmic-ray transport via cosmic-ray-driven instabilities.

Sourav Chatterjee

Natalia Storch
   Caltech
   Aero-Resonant Migration: a New Disk Migration Mechanism for Small Planets

Detailed numerical simulations have shown that Type I migration is not a reliable mode of inward migration for small exoplanets, as its magnitude and direction are extremely sensitive to the thermodynamics of the protoplanetary disk. We discuss an alternate migration mechanism, termed Aero-Resonant Migration (ARM), in which small planetesimals undergo orbital decay due to aerodynamic drag and resonantly shepherd planets ahead of them. Using a combination of analytical and numerical calculations, we show that this is a viable migration mechanism, and discuss the conditions under which it dominates. We suggest that ARM may be key to assembling compact exoplanet systems such as Kepler-80.

Fabio Antonini