Open AccessCygnus X‐2, super‐Eddington mass transfer, and pulsar binaries
Author(s) -
King A. R.,
Ritter H.
Publication year - 1999
Publication title -
monthly notices of the royal astronomical society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.058
H-Index - 383
eISSN - 1365-2966
pISSN - 0035-8711
DOI - 10.1046/j.1365-8711.1999.02862.x
Subject(s) - physics , astrophysics , neutron star , millisecond pulsar , pulsar , white dwarf , luminosity , astronomy , radius , x ray pulsar , pulsar planet , accretion (finance) , stars , binary pulsar , computer security , galaxy , computer science
We consider the unusual evolutionary state of the secondary star in Cygnus X‐2. Spectroscopic data give a low mass ( M 2 ≃0.5−0.7 M ⊙ ) and yet a large radius ( R 2 ≃7 R ⊙ ) and high luminosity ( L 2 ≃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 M 2i ∼3.6 M ⊙ ) on its thermal time‐scale ∼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.