Dynamical Simulations of the Planetary System HD69830

The star HD 69830 exhibits radial velocity variations attributed to three planets as well as infrared emission at 8 − 35μm attributed to a warm debris disk. Previous studies have developed models for the planet migration and mass growth (Alibert et al. 2005, 2006) and the replenishment of warm grains (Wyatt et al. 2007). In this paper, we perform n-body integrations in order to explore the implications of these models for: 1) the excitation of planetary eccentricity, 2) the accretion and clearing of a putative planetesimal disk, 3) the distribution of planetesimal orbits following migration, and 4) the implications for the origin of the infrared emission from the HD 69830 system. We find that: i) It is not possible to explain the observed planetary eccentricities (∼ 0.1) purely as the result of planetary perturbations during migration unless the planetary system is nearly face-on. However, the presence of gas damping in the system only serves to exacerbate the problem again. ii) The rate of accretion of planetesimals onto planets in our n-body simulations is significantly different to that assumed in the semi-analytic models, with our inner planet accreting at a rate an order of magnitude greater than the outer ones, suggesting that one cannot successfully treat planetesimal accretion in the simplified manner of Alibert et al. (2006). iii ) We find that the eccentricity damping of planetesimals does not act as an insurmountable obstacle to the existence of an excited eccentric disk: All simulations result in a significant fraction (∼15%) of the total planetesimal disk mass, corresponding to ∼ 25M⊕, remaining bound in the region ∼1-9 AU, even after all three planets have migrated through the region iv) This swarm of planetesimals has orbital distributions that are size-sorted by gas drag, with the largest planetesimals (∼ 1, 000km), which may contain a large proportion of the system mass, preferentially occupying the highest eccentricity (and thus longest-lived) orbits. Although such planetesimals would be expected to collide and produce a disk of warm dust, further work will be required to understand whether these eccentricity distributions are high enough to explain the level of dust emission observed despite mass loss via steady state collisional evolution.