Last giant impact on the Neptunian System. Constraints on oligarchic masses in the trans-Saturnian region

PARISI, M. GABRIELA AND DEL VALLE, LUCIANO

ASTRONOMY AND ASTROPHYSICS

EDP SCIENCES S A

Año: 2010

0004-6361

  Context. Current models of the formation of ice giants attempt to ccount for the formation of Uranus and Neptune within the protoplanetary disk lifetime. Many of these models calculate the     formation of Uranus and Neptune in a disk that may be several  times  the minimum mass solar nebula model (MMSN). Modern core  accretion  theories assume the formation of the ice giants either in situ,  or between  10 and 20 AU in the framework of the Nice model.    However,  at present, none of these models account for the spin  properties of the ice giants. Aims. Stochastic impacts by large bodies are, at present, the usually     accepted mechanisms able to account for the obliquity of the ice giants. We attempt to set constraints on giant impacts as the cause of Neptune current obliquity in the framework of modern theories. We also use the present orbital properties of the Neptunian irregular  satellites (with the exception of Triton) to set constraints on the scenario of giant impacts at the end of Neptune formation. Methods. Since stochastic collisions among embryos are assumed to  occur beyond oligarchy, we model the angular momentum transfer to proto-Neptune and the impulse transfer to its irregular  satellites by the last stochastic collision (GC) between the protoplanet and an oligarchic mass at the end of Neptune’s formation. We assume a   minimum oligarchic mass  of 1 Earth masses . Results. From angular momentum considerations, we obtain that an   oligarchic mass  between 1 and 4 Earth masses would be required at the GC to reproduce the present rotational properties of Neptune. An impact with a higher mass is not possible, unless the impact parameter of the collision were very small. This result is invariant either  Neptune had formed in situ or between 10 and 20 AU and does not depend on the occurrence of the GC after or during the possible  migration of the planet. From impulse considerations,  we obtain that an oligarchic mass  between 1 and 1.4 Earth masses at the GC is required to keep or capture the present population of irregular satellites. If the impactor mass had been higher, the present Neptunian irregular  satellites had to  be  formed or captured after the end of stochastic impacts.  Conclusions. The upper bounds on the oligarchic masses (4 Earth  masses from the obliquity of Neptune and 1.4 Earth masses from the  Neptunian irregular satellites) are independent of unknown parameters, such as  the  mass and distribution of the planetesimals, the location at  which Uranus and Neptune were formed, the Solar Nebula initial   surface mass density, and the growth regime. If stochastic impacts had occurred, these results should be understood as upper constraints  on the oligarchic masses in the trans-Saturnian region at the end of ice planet formation and may be used to set constraints on planetary  formation scenarios.