Planets and Binary Stars
Number of Stars Monitored
About half of the stellar systems monitored
are expected to be multiple systems. Doppler spectroscopy observations
have already shown the presence of planets orbiting individual
stars in multiple star systems (Cochran et al., 1997). Also proto-planetary
disks exist in binary systems such as HR4796A (Jayawardhana et
al 1998, and Koerner, et al., 1998). Numerical integrations have
shown that there is a range of orbital radii (between about 1/3
and 3.5 times the stellar separation) for which stable orbits
are not possible (Wiegert
and Holman, 1997; Holman and Wiegert, 1999).
We expect to be able to determine the range of binary separations
for which planetary orbits do exist.
Based on observations (Heacox
and Gathright 1994, figure below), about 23% will not have stable
orbits between 0.4 and 2 AU. However, this factor is more than
compensated for by noting that about half of the binary stars
are so widely separated (>2 AU) that planetary systems could
form around both stars. For transits in binary systems, where
both stars are similar in brightness, the transit depth is approximately
one-half that for a transit occurring in a single-star system.
The fraction of G-dwarf binaries whose companions are too dim
to appreciably degrade the statistical significance by more than
20% is estimated to be 85%, based on the brightness distribution
of companions to G-dwarf binaries tabulated by Duquennoy and
Mayor, (1991). The combination of these factors suggests that
the average frequency of planets around binary stars could be
similar to that around single stars.
Distribution of Binary Star Separations
From the target list of 100,000 dwarf stars,
the number of each spectral type for which a planet of a given
size can be detected is shown below assuming a single near-grazing
6.5 hour transit with an SNR=4.
A model of the Galaxy for the selected FOV
was developed using the luminosity function of Wielen, Jahreiss
and Kruger (1983) to obtain the stellar distribution. The galactic
model is the same used by Bahcall and Soneira (Bahcall, 1986).
It defines the number of stars per pc3 per magnitude. This model was normalized to the star
density in the FOV from the USNO-A1.0 data base (Monet,1996)
to provide the number of stars per magnitude interval, spectral
type and luminosity class. The model was cross-checked with the
spectral distribution of all stars with |b|<10° in the
catalog of Positions and Proper Motions (Röser and
Bastion, 1988); against the distribution of dwarfs and giants
of the Bahcall and Soneira model; and against the number of M-dwarfs
in the Catalog of Nearby Stars (Gliese and Jahreiss, 1991).
Of the 223,000 stars in the FOV with mv<14,
an estimated 61% or 136,000 are dwarfs. In the first year of
operation about 25% of these are identified and excluded as being
too young, rotating too fast, or too variable to be useful, resulting
in 100,000 usable target stars.
Based on the model of stellar distribution
and dependence of detectable planet size on stellar type and
brightness, the number and type of stars monitored as a function
of planet size is shown in the figure.
Number of Dwarf Stars for Which Planets Can Be Detected.
The solid lines show the number of
dwarf stars of each spectral type for which a planet of a given
radius can be detected at >8 sigma. The conservative numbers
are based on 4 near-grazing transits with a 1 yr period and stars
The symbols along each solid line
indicate the approximate apparent magnitude of the stars contributing
to the integral number of stars.
The dashed lines show a significant
increase in the number of stars (a factor of 2 at R=1.0 Re) when assuming 4 near-central transits with a 1-yr
period. An even greater increase is realized for 8 near-grazing
transits with a 0.5-yr period.
We define Earth-size to be between
0.5 and 2.0 Earth masses (0.8 Re
to 1.3 Re) and large terrestrial
planets to be between 2 to 10 Earth masses (1.3 Re to 2.2 Re). Planets
less than about 0.5 Me that
reside in or near the HZ are likely to lose their life-supporting
atmospheres because of their low gravity and lack of plate tectonics.
Planets of more than about 10 Me (R>2.2 Re) are
considered to be giant cores like Uranus and Neptune.
They are likely to attract a hydrogen-helium atmosphere and become
gas giants like Jupiter and Saturn.