THE GROWTH OF JUPITER AND SATURN AND THE CAPTURE OF TROJANS

We have studied the capture of planetesimals in Trojan-type orbits by a growing proto-planet. The change of the gravity field due to the mas s growth causes a significant fraction of planetesimals orbiting nearb y to be trapped as Trojans of the proto-planet. After a planetesimal i s captured on a Trojan-type orbit, the libration amplitude of its crit ical argument is consistently reduced by the further mass growth of th e proto-planet. The dynamical mechanism is discussed and the character istics of the Trojan population captured by Jupiter during its growth are analysed. We find an interesting mechanism which could explain the observed high inclination Trojans. The synergy of a Kozai sec ular re sonance with the growth of Jupiter's mass generates high inclination T rojans from low inclination-high eccentricity planetesimals orbiting n ear the growing proto-planet. The libration amplitudes of the model Tr ojans trapped by the mass-growth of Jupiter are higher compared to tho se of the observed Trojans. A possible mechanism that decreases the li bration amplitudes of the model population is collisional evolution. W e also show that the simultaneous formation of Jupiter and Saturn stro ngly inhibits the capture of planetesimals as Saturn Trojans. The inte rference of the 1:1 resonance with a secular resonance and, in some ca ses, also with the 5:2 resonance with Jupiter (Innanen and Mikkola, 19 89), generates instability and causes the ejection of most Saturn Troj ans out of resonance before the end of Saturn's mass growth.