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

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

1998-12-18

arxiv.org/abs/astro-ph/9812343v2

Astronomy and Astrophysics

Astrophysics

9 pages, 4 encapsulated figures, LaTeX, revised version with a few typos corrected and an appendix added, accepted by MNRAS

Scientific paper

10.1046/j.1365-8711.1999.02862.x

We consider the unusual evolutionary state of the secondary star in Cygnus X-2. Spectroscopic data give a low mass ($M_2 \simeq 0.5 - 0.7\msun$) and yet a large radius ($R_2 \simeq 7\rsun$) and high luminosity ($L_2 \simeq 150\lsun$). We show that this star closely resembles a remnant of early massive Case B evolution, during which the neutron star ejected most of the $\sim 3\msun$ transferred from the donor (initial mass $M_{\rm 2i}\sim 3.6\msun$) on its thermal time-scale $\sim 10^6$ 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 $q_{\rm i} = M_{\rm 2i}/M_1$ in a narrow critical range near $q_{\rm i}\simeq 2.6$. Smaller $q_{\rm i}$ lead to long-period systems with the former donor near the Hayashi line, and larger $q_{\rm i}$ 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.

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