2009ApJ...698.1907M

The formation of ultracompact X-ray binaries (UCXBs) has not been well understood. Previous works show that ultrashort orbital periods (less than 1 hr) may be reached through mass transfer driven by magnetic braking in normal low/intermediate-mass X-ray binaries (L/IMXBs) only for an extremely small range of initial binary parameters, which makes it difficult to account for the rather large population of UCXBs. In this paper, we report the calculated results on mass transfer processes in L/IMXBs with a circumbinary (CB) disk. We show that when the orbital angular momentum loss due to a CB disk is included, ultrashort orbital periods could be reached for a relatively wide range of initial binary parameters. The results of our binary models suggest an alternative formation channel for UCXBs.