1998/12/31 by A. R. King, H. Ritter · 2 citations
Physics and Astronomy · #Astronomy and Astrophysical Research #Astrophysical Phenomena and Observations #Pulsars and Gravitational Waves Research #astro-ph
paper · pdf · doi:10.1046/j.1365-8711.1999.02862.x
9 pages, 4 encapsulated figures, LaTeX, revised version with a few typos corrected and an appendix added, accepted by MNRAS
arxiv created 1999/07/20 · openalex publication_date 1999/10/11 · arxiv updated 2009/12/01 · openalex created_date 2016/06/24 · openalex updated_date 2026/07/28
We consider the unusual evolutionary state of the secondary star in Cygnus X-2. Spectroscopic data give a low mass (M2≃0.5-0.7 M⊙) and yet a large radius (R2≃7 R⊙) and high luminosity (L2≃150 L⊙). We show that this star closely resembles a remnant of early massive Case B evolution, during which the neutron star ejected most of the ∼3 M⊙ transferred from the donor (initial mass M2i∼3.6 M⊙) on its thermal time-scale ∼106 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 qiM2iM1 in a narrow critical range near qi≃2.6. Smaller qi lead to long-period systems with the former donor near the Hayashi line, and larger qi 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.