Computing accretion rates from Phantom data

There are several ways to do this.

Use the .ev file output

For a single or binary system the cumulative accreted mass onto the first two sink particles, or of the two stars if using an external binary potential, is printed to the .ev file. The advantage of using the .ev file is that the data is written frequently – every few timesteps or so, so you get a very fine-grained output compared to just using the dump files.

To get accretion rates from the .ev data, use the ev2mdot utility. To compile it, use:

make ev2mdot

which compiles the following program into the phantom/bin directory:

$ cd ~/phantom/bin/
$ ls ev*

then run this with the .ev file(s) as arguments:

$ ev2mdot disc*.ev
disc01.ev --> disc.mdot
disc02.ev --> disc.mdot
disc03.ev --> disc.mdot
disc04.ev --> disc.mdot
disc05.ev --> disc.mdot
disc06.ev --> disc.mdot
disc07.ev --> disc.mdot

the .mdot files are regular ascii file with columns as follows:

time mdot macc mdot1 macc1 mdot2 macc2

where mdot is the instantaneous mass accretion rate and macc the cumulative mass accreted, and the 1 and 2 referring to the values on the two stars (if present).

ev2mdot assumes the shortest possible time interval by default, but to get smoother accretion rates you can average over a longer time by specifying an optional argument:

$ ev2mdot 0.1 disc*.ev
disc01.ev --> disc01.mdot

Write an analysis module

More generally, to have full access to all of the sink particle information in the dump files, write yourself a module for the phantomanalysis utility. Then you can just import the sink particle arrays directly and perform whatever analysis you desire.

You can access the mass accreted by a sink particle by first importing the main sink arrays:

subroutine do_analysis(dumpfile,num,xyzh,vxyzu,particlemass,npart,time,iunit)
 use part,     only:xyzmh_ptmass, nptmass, imacc

then get the accreted mass using the imacc tag. For example, the accreted mass on sink particle 1 would be

mass_accreted_by_sink1 = xyzmh_ptmass(imacc,1)

converting to physical units

To get an accretion rate in Msun/year, import the mass and time units as follows:

use units, only:umass,utime

multiplying by the relevant unit converts to cgs units. For example, to get the accreted mass in g we would use

mass_accreted_by_sink1 = xyzmh_ptmass(imacc,1)*umass

to get this in solar masses, just divide by the mass of the Sun

use physcon, only:solarm
mass_accreted_by_sink1 = xyzmh_ptmass(imacc,1)*umass/solarm

the same procedure applies to the time scaling, i.e.

use physcon, only:years
time_in_years = time_in_code*utime/years