Sink particles in phantom

Sink particles (Bate et al. 1995) are used to represent point masses (usually stars or planets) in phantom calculations. These particles are treated separately to the SPH particles and have the following properties:

  • sink-sink gravity is always computed by direct sum

  • sink-gas gravity is mutual and computed by direct sum

  • sinks evolve on a separate timestep to the gas

  • sinks can accrete gas in a momentum and angular-momentum conserving manner

  • can be used to represent stars, or the cores of stars, or planets

  • there is no density field computed on sink particles

  • no gravitational softening is applied to sink-sink interaction by default

  • can emit winds, radiation or other feedback onto the simulation

Sink particles compared to external forces

The main advantage of a sink particle over a fixed external potential is that they satisfy the conservation laws. For example, a disc of gas particles orbiting in a 1/r potential will not conserve momentum, but a disc of gas particles orbiting a sink particle will.

Sink particles compared to fixed binary potential

A circumbinary disc around a sink particle pair will similarly satisfy the conservation of linear and angular momentum, unlike a prescribed binary potential. The backreaction of the gas disc onto the sink means that the binary can naturally gain or lose angular momentum to the disc, and will gain mass due to accretion.

Sink particle properties

As well as position, velocity, mass and acceleration, sink particle have the following properties:

  • hacc: accretion radius, inside of which gas particles are tested for accretion

  • hsoft: softening length for gas-sink interaction (zero by default)

  • macc: total accreted mass (to avoid round-off error, as this is often a small fraction of total mass)

The full list of extended properties (extracted from part.F90) is as follows:

Index

Description

ihacc

accretion radius

ihsoft

softening radius

imacc

accreted mass

ispinx

spin angular momentum x

ispiny

spin angular momentum y

ispinz

spin angular momentum z

i_tlast

time of last injection

ilum

luminosity

iTeff

effective temperature

iReff

effective radius

imloss

mass loss rate

imdotav

accretion rate average

i_mlast

accreted mass of last time

imassenc

mass enclosed in sink softening radius

iJ2

2nd gravity moment due to oblateness

These are stored in the following arrays:

  • xyzmh_ptmass (positions, mass, accretion radius and extended properties)

  • vxyz_ptmass (velocities)

  • fxyz_ptmass (accelerations)

  • dsdt_ptmass (time derivative of spin, where oblateness is used)

Where to look in the code

Sink particle functionality is implemented in the ptmass module.