arXiv subjects

R. Brasser

Publications and source records attributed to R. Brasser.

38 records · Page 3Linked to original sources

arXiv2011

Reassessing the formation of the inner Oort cloud in an embedded star cluster

We re-examine the formation of the inner Oort comet cloud while the Sun was in its birth cluster with the aid of numerical simulations. This work is a continuation of an earlier study (Brasser et al., 2006) with several substantial modifications. First, the system consisting of stars, planets and comets is treated self-consistently in our N-body simulations, rather than approximating the stellar encounters with the outer Solar System as hyperbolic fly-bys. Second, we have included the expulsion of the cluster gas, a feature that was absent previously. Third, we have used several models for the initial conditions and density profile of the cluster -- either a Hernquist or Plummer potential -- and chose other parameters based on the latest observations of embedded clusters from the literature. {These other parameters result in the stars being on radial orbits and the cluster collapses.} Similar to previous studies, in our simulations the inner Oort cloud is formed from comets being scattered by Jupiter and Saturn and having their pericentres decoupled from the planets by perturbations from the cluster gas and other stars. We find that all inner Oort clouds formed in these clusters have an inner edge ranging from 100 AU to a few hundred AU, and an outer edge at over 100\,000 AU, with little variation in these values for all clusters. All inner Oort clouds formed are consistent with the existence of (90377) Sedna, an inner Oort cloud dwarf planetoid, at the inner edge of the cloud: Sedna tends to be at the innermost 2% for Plummer models, while it is 5% for Hernquist models. We emphasise that the existence of Sedna is a generic outcome. We define a `concentration radius' for the inner Oort cloud and find that its value increases with increasing number of stars in the cluster, ranging from 600 AU to 1500 AU for Hernquist clusters and from 1500 AU to 4000 AU for Plummer clusters...

astro-ph.EP↗

arXiv2011

Reassessing the origin of Triton

Agnor & Hamilton (2006) demonstrated that the disruption of a binary was an effective mechanism to capture Triton. The subsequent evolution of Triton's post-capture orbit could have proceeded through gravitational tides. The study by Agnor & Hamilton (2006) is repeated in the framework of the Nice model to determine the post-capture orbit of Triton. After capture it is then subjected to tidal evolution. The perturbations from the Sun and the figure of Neptune are included. The perturbations from the Sun acting on Triton cause it to spend a long time in its high-eccentricity phase, usually of the order of 10 Myr, while the typical time to circularise to its current orbit is some 200 Myr. The current orbit of Triton is consistent with an origin through binary capture and tidal evolution, even though the model prefers Triton to be closer to Neptune than it is today. The probability of capturing Triton in this manner is approximately 0.7%. Since the capture of Triton was at most a 50% event -- since only Neptune has one, but Uranus does not -- we deduce that in the primordial trans-Neptunian disc there were 100 binaries with at least one Triton-sized member. Morbidelli et al. (2009) concludes there were some 1000 Triton-sized bodies in the trans-Neptunian proto-planetary disc, so the primordial binary fraction with at least one Triton-sized member is 10%. This value is consistent with theoretical predictions, but at the low end. If Triton was captured at the same time as Neptune's irregular satellites, the far majority of these, including Nereid, would be lost. This suggests either that Triton was captured on an orbit with a small semi-major axis a < 50 R_N (a rare event), or that it was captured before the dynamical instability of the Nice model, or that some other mechanism was at play. The issue of keeping the irregular satellites remains unresolved.

astro-ph.EP↗