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Disk migration

My work on migration is divided between protoplanetary disks, to explain the survival of the Earth against angular momentum loss to the gas disk; and AGN disks, to explain the recent LIGO observations of black hole binary mergers. These works are detailed below.

Planet migration in evolutionary disk models

 
In this series of works, I am interested in the final orbit of planets, as a result of migration and disk evolution. As the gas accretes and photo-evaporates, the density (LHS figure) and temperature (RHS picture) fall steadily.

 
Planet migration occurs towards equilibrium radii with zero torque. These radii themselves migrate inwards as the disk evolves. We see that as the surface density and temperature fall, the planet orbital migration and disk depletion timescales eventually become comparable, with the precise timing depending on the mass of the planet. When this occurs, the planet decouples from the equilibrium radius. At this time, however, the gas surface density is already too low to drive substantial further migration. A higher mass planet, of 10 Earth masses, can open a gap during the late evolution of the disk, and stops migrating. Low mass planets, with 1 or 0.1 Earth masses, released beyond 1 AU in our models, avoid migrating into the star.

 
In the video below we take this model one step further by implementing multiple interacting planets. We incorporate the torques from an evolving non-isothermal disk into an N-body simulation to examine the behavior of planets or planetary embryos interacting in the convergence zone. We find that mutual interactions do not eject objects from the convergence zone. Small numbers of objects in a laminar disk settle into near resonant orbits that remain stable over the 10 Myr periods that we examine. However, either or both increasing the number of planets or including a correlated, stochastic force to represent turbulence drives orbit crossings and mergers in the convergence zone. These processes can build gas giant cores with masses of order ten Earth masses from sub-Earth mass embryos in 2-3 Myr. This simulation and video were done by Brandon Horn, the graduate student at Columbia University, with my co-supervision.

Publications:
Horn et al. (2012), ApJ, 750, 34. [arXiv] [ADS]
Lyra et al. (2010), ApJ, 715, L68. [arXiv] [ADS]

Black hole migration and merger in AGN disks

I have been participating in a collaboration with the goal of explaining the LIGO observations as merger events in AGN, similarly to the core accretion model in protoplanetary disks. The idea is based on the realization that the mass ratio of compact objects or stellar mass black holes (1-10 solar masses) to a supermassive black hole (1e6-1e8 solar masses) is about the same as that of planetesimals/protoplanets to the Sun. Thus, in the same way that planetesimals in a protoplanetary disk collide to form protoplanets and planets, stellar mass black holes orbiting in an AGN disk would collide to form larger black holes. Essentially, the core accretion model of planet formation can be scaled up to AGN disks. The presence of gas in the AGN makes the motion dissipative, so black holes will migrate in the same way planets migrate in circumstellar disks. Hefty black holes like seen by LIGO are difficult to explain by stellar evolution, but would be natural outcomes of this model. We have published a series of works dealing with key aspects of the model.

The figures above show (left, McKernan et al. 2018) a cartoon of an AGN disk, with stellar mass compact objects orbiting a supermassive black hole. At disk crossing the orbiters lose energy and angular momentum, and slowly circularize with the AGN disk rotation. Once in the disk, the situation proceeds like the core accretion model of planet formation. In the right panel (Bellovary et al. 2016) the migration traps that we described for protoplanetary disks (Lyra et al 2010) were computed for an AGN disk model. In these traps, binary formation and merger is favored.  

 

Publications:
Secunda et al. (2018) ApJ, submitted.[arXiv]
McKernan et al. (2018), ApJ, 866, 66. [arXiv] [ADS]
Leigh et al. (2018), MNRAS, 474, 5672. [arXiv] [ADS]
Bellovary et al. (2016), ApJL, 819, L17. [arXiv] [ADS]
McKernan et al. (2014), MNRAS, 441, 900. [arXiv] [ADS]
McKernan et al. (2012), MNRAS, 425, 460. [arXiv] [ADS]
McKernan et al. (2011), MNRAS, 417L, 103. [arXiv] [ADS]