Clouds in the Galactic Sky – The Gaseous Halos of Galaxies from the synergy of Observation and Theory
Cameron Liang (University of Chicago)

Abstract:
Undoubtedly, the Earth’s atmosphere is an integral part of its ecosystem.  Everyday weather and long-term climate of the atmosphere are directly linked to activities on the surface of the Earth and vice versa. Gaseous halos, known as the circumgalactic medium (CGM), are the equivalent atmosphere of galaxies. The galactic climate arises from infalling gas from intergalactic space, enriched materials launched from the interstellar medium and more. The CGM is one of the largest gas reservoirs with complex baryonic cycles. It is paramount to improve our understanding of the CGM to achieve a complete picture of galaxy formation and evolution.

In this talk, I will first focus on the observational efforts to place empirical constraints on the spatial extent and the metallicity of the CGM. I will then present some theoretical work on the baryonic cycles in cosmological zoom-in simulations and show that the CGM provides orthogonal constraints to star formation and feedback processes. Finally, I will present a new high-resolution (< 1pc) simulation study to model the CGM more systematically with radiative cooling, thermal conduction, and magnetic fields.

White Dwarf – Neutron Star Mergers: from Peculiar Supernovae to Pulsar Planets
Ben Tal Margalit (Columbia University)

Abstract:
The merger of binaries consisting of a white dwarf (WD) and a neutron star (NS), though much less studied than their NS-NS/WD-WD brethren, are relatively common astrophysical events which may contribute to the transient sky. I will review the background and motivation for studying WD-NS mergers. Dependent on the WD-to-NS mass ratio, mass transfer at Roche-lobe contact may become unstable, and the disrupted WD will be sheared into a hot-dense accretion torus surrounding the NS. I will present recent work modeling these accretion flows on both short (~min) and long (~kyr) timescales. Nuclear burning in the early hyper-Eddington accreting flow fuses matter up the alpha-chain, heating the geometrically thick disk to a marginally bound state prone to outflows. These outflows may power a rapidly-evolving (~week-long) optical transient, broadly consistent with the class of `Ca-rich gap transients’. Finally, by modeling the long-term disk evolution, I show that a WD-NS merger provides a natural mechanism for creating carbonaceous (“diamond”) planets orbiting the millisecond pulsar PSR B1257+12, providing new insight into the unusually high proper motion of the pulsar-planet system.