A New Paradigm of Binary-Disk Interaction

Colloquium with Dr. Diego J. Muñoz, Research Assistant Professor, Northwestern University, Assistant Professor, Universidad Adolfo Ibañez

In the last five years, we have witnessed a renewed interest in the classical problem of binary-disk interaction, which describes the gravitational coupling between two point masses and a surrounding accretion disk. In addition to binary growth rate, this process determines the exchange of angular momentum between the gas disk and the central bodies, and thus, it dictates the evolution of the binary’s orbital elements. Binary-disk interaction plays a pivotal role in a wide range of astrophysical contexts, such as  the formation and migration of stellar binaries, the formation of circumbinary planets, or the interaction between massive black hole binaries (MBHBs) in gaseous environments. But despite the critical importance of understanding binary-disk interaction, and despite the monumental advances in computational models of accretion disks, some of the most basic aspects of this decades-old problem remain unclear. In particular, a  ``dogma’’ of binary-disk interaction has been recently challenged: binaries might expand (or ``soften’’) due to their coupling to the gas rather than contract (or ``harden’’), as it was assumed to be the case for the last three decades. This finding has important repercussions for the evolution of MBHBs and for the gravitational wave signatures of MBHB inspirals and mergers.

In this talk, Dr. Muñoz will go over this new  ‘outward migration’ conundrum of binary-disk interaction, and will discuss other ``dogmas’’ that might be require a second look.  I will explain why we missed this unexpected effect for 30 years, and discuss what combination of physical ingredients could still make binaries migrate inward.