ON THE FORMATION OF DOUBLE WHITE DWARFS THROUGH STABLE MASS TRANSFER AND A COMMON ENVELOPE

Abstract

Although several dozen double white dwarfs (DWDs) have been observed, for\nmany the exact nature of the evolutionary channel(s) by which they form remains\nuncertain. The canonical explanation calls for the progenitor binary system to\nundergo two subsequent mass-transfer events, both of which are unstable and\nlead to a common envelope (CE). However, it has been shown that if both CE\nevents obey the standard alpha-prescription (parametrizing energy loss), it is\nnot possible to reproduce all of the observed systems. The gamma-prescription\nwas proposed as an alternative to this description, instead parametrizing the\nfraction of angular momentum carried away in dynamical-timescale mass loss. In\nthis paper, we analyze simultaneous energy and angular-momentum conservation,\nand show that the gamma-prescription cannot adequately describe a CE event for\nan arbitrary binary, nor can the first phase of mass loss always be understood\nin general as a dynamical-timescale event. We consider in detail the first\nepisode of mass transfer in binary systems with initially low companion masses,\nwith a primary mass in the range 1.0--1.3 solar masses and an initial mass\nratio between the secondary and primary stars of 0.83--0.92. In these systems,\nthe first episode of dramatic mass loss may be stable, non-conservative mass\ntransfer. This strips the donor's envelope and dramatically raises the mass\nratio; the considered progenitor binary systems can then evolve into DWDs after\npassing through a single CE during the second episode of mass loss. We find\nthat such a mechanism reproduces the properties of the observed DWD systems\nwhich have an older component with less than approximately 0.46 solar masses\nand mass ratios between the younger and older WDs greater than 1.\n