摘要：Context. The recently detected gravitational wave signals (GW150914 and
GW151226) of the merger event of a pair of relatively massive stellar-mass black holes
(BHs) calls for an investigation of the formation of such progenitor systems in
general.
Aims. We analyse the common-envelope (CE) stage of the traditional
formation channel in binaries where the first-formed compact object undergoes an in-spiral
inside the envelope of its evolved companion star and ejects the envelope in this
process.
Methods. We calculated envelope binding energies of donor stars with
initial masses between 4 and 115M⊙ for metallicities of
Z =
ZMilky Way ≃ Z⊙/
2 and Z =
Z⊙/ 50, and derived minimum masses of
in-spiralling objects needed to eject these envelopes.
Results. In addition to producing double white dwarf and double neutron
star binaries, CE evolution may also produce massive BH-BH systems with individual BH
component masses of up to ~50 −
60M⊙, in particular for donor stars
evolved to giants beyond the Hertzsprung gap. However, the physics of envelope ejection of
massive stars remains uncertain. We discuss the applicability of the energy-budget
formalism, the location of the bifurcation point, the recombination energy, and the
accretion energy during in-spiral as possible energy sources, and also comment on the
effect of inflated helium cores.
Conclusions. Massive stars in a wide range of metallicities and with
initial masses of up to at least 115M⊙ may shed their envelopes and
survive CE evolution, depending on their initial orbital parameters, similarly to the
situation for intermediate- and low-mass stars with degenerate cores. In addition to being
dependent on stellar radius, the envelope binding energies and λ-values also depend on the
applied convective core-overshooting parameter, whereas these structure parameters are
basically independent of metallicity for stars with initial masses below 60M⊙. Metal-rich
stars ≳60M⊙ become
luminous blue variables and do not evolve to reach the red giant stage. We conclude that
based on stellar structure calculations, and in the view of the usual simple energy budget
analysis, events like GW150914 and GW151226 might be produced by the CE channel.
Calculations of post-CE orbital separations, however, and thus the estimated LIGO
detection rates, remain highly uncertain.
关键词：stars: evolution;binaries: close;X-rays: binaries;stars: black holes;gravitational waves
