ON THE BINDING ENERGY PARAMETER λ OF COMMON ENVELOPE EVOLUTION

The binding energy parameter lambda plays an important role in common envelope evolution. Previous works have already pointed out that lambda varies throughout the stellar evolution, though it has been adopted as a constant in most of the population synthesis calculations. We have systematically calculated the binding energy parameter lambda for both Population I and Population II stars of masses 1-20M(circle dot), taking into account the contribution from the internal energy of stellar matter. We present fitting formulae for lambda that can be incorporated into future population synthesis investigations. We also briefly discuss the possible applications of the results in binary evolutions.