Common Envelope Evolution

Abstract: We present simulations done on the onset of common envelope evolution with giant donors by taking into account the mass loss through the outer Lagrange point and the forming of the common envelope. In addition we apply proper stellar models and evolve our simulations using an Runge-Kutta integrator with adaptive step-size. We show that each of these processes are significant in the onset of common envelope evolution. The mass loss through the outer Lagrange point has significant effect, if the atmospheric scale height is high(as is the case for red giants $H_p=0.01 R_'$). In addition to this we find that subgiants with a scale height of about $H_p=0.1 R_'$ can have a stable mass transfer period during its evolution and can as a result exhaust its envelope before starting Roche lobe overflow. The nature of the mass transfer is found to be heavily dependent on the atmospheric scale height of the star. As different scale heights lead to very different evolutionary paths for the mass transfer. We also make comparisons with the conservative case and show that simple stability analysis of unstable mass transfer do not concur with the non-conservative case. We discuss the standard application of the alpha formalism and show that there are severe flaws in using it at the start of Roche lobe overflow, as opposed to at the start of common envelope evolution for giant donor stars.