Custom population synthesis

The modularity of the POSYDON code allows the user to easily change the default evolutionary tree dictated by the flow chart tree. The flow chart tree is a list of 4D points composed of binary.star_1.state, binary.star_2.state, binary.state, binary.event which map to a corresponding evolutionary step. Here we show how the user can replace part or the entire POSYDON flow chart tree and customize the evolution with user specified steps.

Custom step

The user can develop a POSYDON step which is a python class with a __call__ method which updates the current properties of the binary objects (see reference binary object). As an example, let’s create our own treatment for a common envelope. For simplicity sake we will halve the orbital separation and set the post-CE donor star mass to the star’s helium core mass.

from posydon.utils import common_functions as cf

class my_CE_step(object):
  """Compute a fake CE event."""

  def __init__(self, verbose):
    self.verbose = verbose

  def __call__(self, binary):

    if self.verbose:
      print('The orbital separation post CE is half the pre CE orbital separation!')

    # Determine which star is the donor and which is the companion
    if binary.event in ["oCE1", "oDoubleCE1"]:
        donor_star = binary.star_1
        comp_star = binary.star_2
    elif binary.event in ["oCE2", "oDoubleCE2"]:
        donor_star = binary.star_2
        comp_star = binary.star_1
        raise ValueError("CEE does not apply if `event` is not "
                         "`oCE1`, 'oDoubleCE1' or `oCE2`, 'oDoubleCE1'")

    binary.separation /= 2.
    m1 = donor_star.he_core_mass
    m2 = comp_star.mass
    binary.separation = cf.orbital_period_from_separation(binary.separation, m1, m2)

The new custom step can be used by importing the new function into the simulation properties as follows (see ref. the POSYDON API)

from custom_CE_step import my_CE_step

CE_STEP = dict(verbose=False)

# pass the simulation properties to each step
sim_kwargs = dict(
    flow = (flow_chart, {}),
    step_HMS_HMS = (MS_MS_step, MESA_STEP),
    step_CO_HeMS = (CO_HeMS_step, MESA_STEP),
    step_CO_HMS_RLO = (CO_HMS_RLO_step, MESA_STEP),
    step_detached = (detached_step, DETACHED_STEP),
    step_CE = (my_CE_step, CE_STEP), # <--- WE ONLY EDITED THIS LINE HERE
    step_SN = (StepSN, SN_STEP),
    step_dco = (DoubleCO, DCO_STEP),
    step_end = (step_end, END_STEP),
    extra_hooks = [(StepNamesHooks, {})]

Custom flow charts

Let us now see how one can alter the default flow chart links. Let us assume that you have a custom step that maps the evolution of Roche lobe overflowing HMS-HeMS binaries called HMS_HeMS_RLO_step. With the CHANGE_FLOW_CHART option of the flow_chart method we can replace the the tree links which are currently mapped to the step_end as follows.

from posydon.binary_evol.flow_chart import flow_chart, POSYDON_FLOW_CHART, STAR_STATES_H_RICH, STAR_STATES_HE_RICH


  for s1 in STAR_STATES_H_RICH:
      for s2 in STAR_STATES_HE_RICH:
          NEW_FLOW_CHART_LINKS[(s1, s2, 'RLO1', 'oRLO1')] = 'step_HMS_HeMS_RLO'
          NEW_FLOW_CHART_LINKS[(s2, s1, 'RLO2', 'oRLO2')] = 'step_HMS_HeMS_RLO'


We can load the new flow chart into the simulation properties as.

from custom_HMS_HeMS_step import HMS_HeMS_RLO_step

HMS_HeMS_RLO_STEP = dict(verbose=False)

sim_kwargs = dict(
    flow = (new_flow_chart, {}), # <--- WE ONLY EDITED THIS LINE HERE
    step_HMS_HMS = (MS_MS_step, MESA_STEP),
    step_CO_HeMS = (CO_HeMS_step, MESA_STEP),
    step_CO_HMS_RLO = (CO_HMS_RLO_step, MESA_STEP),
    step_detached = (detached_step, DETACHED_STEP),
    step_CE = (StepCEE, CE_STEP),
    step_SN = (StepSN, SN_STEP),
    step_dco = (DoubleCO, DCO_STEP),
    step_end = (step_end, END_STEP),
    extra_hooks = [(StepNamesHooks, {})]