3 Testing Bimodal Planet Formation Dale

We suggest that the observed break in giant-planet frequency as a function of host metallicity at Z = 0.02 may be a reflection of bimodal planet formation. We search for signatures of this bimodality in the distributions of the planet eccentricities, periods, masses, and multiplicity. However, the low-metallicity sample is at present too small to test for any but the most severe differences in these two putative populations. Subject headings: planetary systems – planetary systems: formation – stars: abundances Fischer, Valenti & Marcy (2003) have demonstrated that the frequency of extra-solar giant planets is a strong function of metallicity Z. Santos, Israelian & Mayor (2004) have confirmed this result and have further shown that there is a sharp break in frequency at Z = 0.02, which can be represented algebraically by, f = 0.025 + 16(Z − 0.02)Θ(Z − 0.02), (1) where Θ is a step function. See their figure 7. We suggest that the two terms of this equation correspond to two different modes of giant-planet formation, the first being metallicity-independent and the second being strongly dependent on metallicity. For example, in its simplest form, the disk-instability model of Boss (1995) would not be expected to depend on metallicity. By contrast, the more conventional picture of gas accretion onto rock-ice cores might well be very sensitive to metallicity. If the two mechanisms generated planets with substantially different distributions in eccentricity, period, or mass, then these should be revealed in the observed distributions of these properties as functions of metallicity. That is, the planets with Z < 0.02 should entirely reflect one distribution, while those with Z ≥ 0.02 should predominantly reflect the other.