A comprehensive population synthesis study of post-common envelope binaries

We apply population synthesis techniques to calculate the present-day population of post-common envelope binaries (PCEBs) for a range of theoretical models describing the common envelope (CE) phase. Adopting the canonical energy budget approach, we consider models where the ejection efficiency alpha(CE) is either a constant or a function of the secondary mass. We obtain the envelope binding energy from detailed stellar models of the progenitor primary, with and without the thermal and ionization energy, but we also test a commonly used analytical scaling. We also employ the alternative angular momentum budget approach, known as the gamma-algorithm. We find that a constant, global value of alpha(CE) greater than or similar to 0.1 can adequately account for the observed population of PCEBs with late spectral-type secondaries. However, this prescription fails to reproduce IK Pegasi (IK Peg), which has a secondary with spectral type A8. We can account for IK Peg if we include thermal and ionization energy of the giant's envelope, or if we use the gamma-algorithm. However, the gamma-algorithm predicts local space densities that are 1 to 2 orders of magnitude greater than estimates from observations. In contrast, the canonical energy budget prescription with an initial mass ratio distribution that favours unequal initial mass ratios (n(q(i)) proportional to q-0.99(i)) gives a local space density which is in good agreement with observations, and best reproduces the observed distribution of PCEBs. Finally, all models fail to reproduce the sharp decline for orbital periods, P(orb) greater than or similar to 1 d in the orbital period distribution of observed PCEBs, even if we take into account selection effects against systems with long orbital periods and early spectral-type secondaries.