Spring 2023 Schedule:
February 2nd:
Lucia Perez (Future Faculty in the Physical Sciences Fellow, Princeton)
March 23rd:
Burcin Mutlu-Pakdil (Dartmouth)
March 30th:
Yang Zhang (Graduate Student, Caltech)
Title: Neutral-charged-particle Collisions as the Mechanism for Accretion Disk Angular Momentum Transport
Abstract: The matter in an accretion disk must lose angular momentum when moving radially inwards but how this works has long been a mystery. In this talk, I will present a new angular momentum transport mechanism based on the Lagrangian mechanics of colliding charged and neutral particles in combined gravitational and magnetic fields. As predicted by Lagrangian mechanics, the fundamental conserved global quantity in an axisymmetric system is the total canonical angular momentum, not the ordinary angular momentum. When the Kepler angular velocity and the magnetic field have opposite polarity, collisions between neutrals and charged particles cause: (i) ions to move radially inwards, (ii) electrons to move radially outwards, (iii) neutrals to lose ordinary angular momentum, and (iv) charged particles to gain canonical angular momentum. Neutrals thus spiral inward due to their decrease of ordinary angular momentum while the accumulation of ions at small radius and accumulation of electrons at large radius produces a radially outward electric field. In 3D, this radial electric field would drive an out-of-plane poloidal current that produces the magnetic forces that drive bidirectional astrophysical jets. Because this neutral angular momentum loss depends only on neutrals colliding with charged particles, it should be ubiquitous. Quantitative scaling of the model using plausible disk density, temperature, and magnetic field strength gives an accretion rate of 3 × 10−8 solar mass per year, which is in good agreement with observed accretion rates.