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Unlike around single stars where the HZ is a spherical shell with a distance
determined by the host star alone, in binary star systems, the radiation from
the stellar companion can influence the extent and location of the HZ of the
system. Especially for planet-hosting binaries with small stellar separations and/or
in binaries where the planet orbits the less luminous star, the amount of the flux
received by the planet from the secondary star may become non-negligible.
In addition, effects such as the gravitational perturbation of the secondary star
(see, e.g. Georgakarakos
2002
; Eggl et al.
2012
) can influence a planet's orbit in
the binary HZ and lead to temperature fluctuations if the planetary atmosphere
cannot buffer the change in the combined insolation. Since in an S-type system,
the secondary orbits more slowly than the planet, during one period of the binary,
the planet may experience the effects of the secondary several times. The latter,
when combined with the atmospheric response of the planet, defines the HZ of the
system.
Despite the fact that as a result of the orbital architecture and dynamics of the
binary, at times the total radiation received by the planet exceeds the radiation that
it receives from its parent star alone by a non-negligible amount, the boundaries
of the actual HZ of the binary cannot be obtained by a simple extrapolation of the
boundaries of the HZ of its planet-hosting star. Similar to the HZ around single stars,
converting from insolation to equilibrium temperature of the planet depends strongly
on the planet's atmospheric composition, cloud fraction and star's spectral type. A
planet's atmosphere responds differently to stars with different spectral distribution
of incident energy. Different stellar types will therefore contribute differently to
the total amount of energy absorbed by the planetary atmosphere (see, e.g. Kasting
et al.
1993
). A complete and realistic calculation of the HZ has to take into account
the spectral energy distribution (SED) of the binary stars as well as the planet's
atmospheric response. In this section, we address these issues and present a coherent
and self-consistent model for determining the boundaries of the HZ of S-type binary
systems.
13.7.1
Calculation of the Binary Habitable Zone
Habitability and the location of the HZ depend on the stellar flux at the planet's
location as well as the planet's atmospheric composition. The latter determines the
albedo and the greenhouse effect in the planet's atmosphere and as such plays a
strong role in determining the boundaries of the HZ. Examples of atmospheres with
different chemical compositions include the original CO
2
/H
2
O/N
2
model (Kasting
et al.
1993
;Selsisetal.
2007
; Kopparapu et al.
2013a
) with a water reservoir like
Earth's and model atmospheres with high H
2
/He concentrations (Pierrehumbert and
Gaidos
2011
) or with limited water supply (Abe et al.
2011
).
At present, the recent update to the Sun's HZ given by Kopparapu et al. (
2013a
,
b
)
presents the best model. According to this model, the HZ is an annulus around a
star where a rocky planet with a CO
2
/H
2
O/N
2
atmosphere and sufficiently large
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