code: def _density_par(self,dangle,tdisrupt=None):
          
          if tdisrupt is None: tdisrupt= self._tdisrupt
          dOmin= dangle/tdisrupt
          # Normalize to 1 close to progenitor
          return 0.5\
              *(1.+special.erf((self._meandO-dOmin)\
                                   /numpy.sqrt(2.*self._sortedSigOEig[2])))
docs: The raw density as a function of parallel angle