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 m_v<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 with m_v<14.

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 R_e) 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 R_e to 1.3 R_e) and large terrestrial planets to be between 2 to 10 Earth masses (1.3 R_e to 2.2 R_e). Planets less than about 0.5 M_e 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 M_e (R>2.2 R_e) 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.

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