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.