Comments on the final orbital separation in common envelope evolution

Abstract

I study some aspects of common envelope evolution, where a compact star enters the envelope of a giant star. I show that in some binary systems under a narrow range of parameters, a substantial fraction of the giant stellar envelope is lost before the onset of the common envelope. The reduced envelope mass at the onset of the common envelope implies that the binary system emerges from the common envelope with a relatively large orbital separation. I therefore caution against a simple treatment, which omits this process, in the study of systems that evolved through a common envelope phase and ended with a relatively large orbital separation, e.g., PG1115+166. A fraction of the envelope that is lost while the companion is still outside the giant stellar envelope is accreted by the companion. The companion may form an accretion disk and blow two jets. I propose this scenario for the formation of the bipolar planetary nebula NGC 2346, which has a binary nucleus with an orbital period longer than that of any other known binary system in planetary nebulae. Because this scenario, of a late common envelope phase, occurs for a narrow range of parameters, I expect it to be applicable to a small, but non-negligible, number of systems.