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Astron. Astrophys. 326, L21-L24 (1997)

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4. Discussion

It is possible to produce EGPs by gravitational interactions between cluster stars starting from giant planets in nearly circular orbits. They can be generated after a four-body interaction among two single stars and a planetary system in which one of the single stars carries away the excess energy, allowing the formation of a hierarchical system. Such a scattering event enables a substantial momentum transfer which permits to change a circular planetary orbit to one with significant eccentricity. From our computations it is clear that a system like 16 Cygni is easily formed by the processes described in this Letter. Observational properties of the 16 Cygni-like systems formed in our calculations appear in Table 1.


Table 1. Properties of the 16 Cygni-like systems

The systems studied could be easily detected in a long-term Doppler shift survey in nearby open clusters; the minimum period is about 12 yr. The results provide dynamical mechanisms to produce EGPs but they cannot explain the small orbital radius of many of them. Our present calculations do not include the effects of tidal dissipation processes; they could also play an important role as regards diminution of the orbital separation because the periastron of the planet in our calculations can be as small as 0.2 AU. Another interesting fact to point out is the percentage of EGPs from the current planet detections may be about 25% but the percentage of EGPs formed in our computations is about 3%, although our systems have periods greater than 10 yr. This percentage is a lower limit because life-times of poorly populated clusters, as our models, are short. A higher percentage for richer clusters with extended life-times is expected. On the other hand, all the planets discovered until now orbit G or F stars. If we only consider the G-dwarf stars (0.85-1.1 [FORMULA]), our percentage increases (at least) to 20% in the mean. However, the observational statistics is currently too low to make solid conclusions because strong selection effects could be at work. In any case, our numerical work provides significant evidence of a high survival probability of planetary systems in open clusters. Our present results do not depend significantly on stellar evolution because our models disintegrate before significant mass loss from the stars of the MPSs. The Galactic tidal field (i.e. the initial cluster radius) only affects them indirectly, through the cluster life-time.

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© European Southern Observatory (ESO) 1997

Online publication: April 8, 1998