Cygnus X - 2 , super - Eddington mass transfer , and pulsar binaries

We consider the unusual evolutionary state of the secondary star in Cygnus X-2. Spectroscopic data give a low mass (M 2 ≃ 0.5 − 0.7M ⊙) and yet a large radius (R 2 ≃ 7R ⊙) and high luminosity (L 2 ≃ 150L ⊙). We show that this star closely resembles a remnant of early massive Case B evolution, during which the neutron star ejected most of the ∼ 3M ⊙ transferred from the donor (initial mass M 2i ∼ 3.6M ⊙) on its thermal timescale ∼ 10 6 yr. As the system is far too wide to result from common-envelope evolution, this strongly supports the idea that a neutron star efficiently ejects the excess inflow during super–Eddington mass transfer. Cygnus X-2 is unusual in having had an initial mass ratio q i = M 2i /M 1 in a narrow critical range near q i ≃ 2.6. Smaller q i lead to long-period systems with the former donor near the Hayashi line, and larger q i to pulsar binaries with shorter periods and relatively massive white dwarf companions. The latter naturally explain the surprisingly large companion masses in several millisecond pulsar binaries. Systems like Cygnus X-2 may thus be an important channel for forming pulsar binaries.