Simulation of self-gravitating and gravitationally unstable accretion discs
In PHANTOM, it is possible to perform simulations of accretion discs taking into account the role of the disc self-gravity and, possibly, to trigger gravitational instability.
Self-gravity in (vertically isothermal) accretion discs
Normally in the disc and dustydisc environments the disc self-gravity is not taken into account. Indeed, usually the disc to star mass ratio is so low that the disc contribution to the gravitational potential is negligible. The setups isosgdisc and dustyisosgdisc allow simulating vertically isothermal discs with self-gravity and disc viscosity ($α_{SS}$), and work as the disc and dustydisc ones.
Gravitationally unstable accretion discs
The environments sgdisc and dustysgdisc allow the user to simulate a gravitationally unstable accretion disc. Gravitational instability is triggered by cooling, using an adiabatic equation of state and without disc viscosity. Usually, a cooling law it is prescribed, and the simplest one has been proposed by Gammie (2001): the cooling timescale $t_{cool}$ is assumed to be proportional to the dynamical time of the disc $t_{dyn}$ so that $t_{cool} = β t_{dyn}$.
In the setup file there are the disc parameters, and in the input file it is possible to prescribe the cooling law. In particular, the variables to pay attention to are:
ieos = 2 , to choose an adiabatic equation of state
icooling = 3, to choose a β cooling prescription with constant β (icooling = 7 prescribes a varying β cooling with the radius)
beta_cool = #, to choose the value of β cooling