Constraints to Uranus? great collision IV The origin of Prospero

M. G. PARISI, G. CARRARO, M. MARIS, A. BRUNINI

ASTRONOMY AND ASTROPHYSICS

EDP SCIENCES S A

Lugar: Paris; Año: 2008 vol. 482 p. 657 - 664

0004-6361

Context. It is widely accepted that the large obliquity of Uranus is the result of a great tangential collision (GC) with an Earth sizeproto-planet at the end of the accretion process. The impulse imparted by the GC affected the Uranian satellite system. Nine irregularsatellites (irregulars) have been discovered around Uranus. Their orbital and physical properties, in particular those of the irregularProspero, set constraints on the GC scenario.Aims. We attempt to set constraints on the GC scenario as the cause of Uranus? obliquity as well as on the mechanisms able togenerate the Uranian irregulars.Methods. Different capture mechanisms for irregulars operate at different stages on the giant planet formation process. The mechanismsable to capture before and after the GC the Uranian irregulars are analysed. Assuming that they were captured before theGC, we calculate the orbital transfer of the nine irregulars by the impulse imparted by the GC. If their orbital transfer is dynamicallyimplausible, they should have originated after the GC. We then investigate and discuss the dissipative mechanisms able to operatelater.Results. Very few transfers exist for five of the irregulars, which makes their existence unlikely before the GC. In particular Prosperocould not exist at the time of the GC. Different capture mechanisms for Prospero after the GC are investigated. Gas drag byUranus?envelope and pull-down capture are not plausible mechanisms. Capture of Prospero through a collisionless interaction seemsto be difficult. The GC itself provides a mechanism of permanent capture. However, the capture of Prospero by the GC is a lowprobability event. Catastrophic collisions could be a possible mechanism for the birth of Prospero and the other irregulars after theGC. Orbital and physical clusterings would then be expected.Conclusions. Either Prospero originated after the GC or the GC did not occur. In the former case, the mechanism for the origin ofProspero after the GC remains an open question. An observing program able to look for dynamical and physical families is needed.In the latter case, another theory to account for Uranus? obliquity and the formation of the Uranian regular satellites on the equatorialplane of the planet would be needed.