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Collisional evolution of eccentric planetesimal swarms

2009; Oxford University Press; Volume: 402; Issue: 1 Linguagem: Inglês

10.1111/j.1365-2966.2009.15930.x

1365-2966

M. C. Wyatt, Mark Booth, Matthew J. Payne, L. J. Churcher,

Stellar, planetary, and galactic studies

Models for the steady-state collisional evolution of low eccentricity planetesimal belts identify debris discs with hot dust at 1 au, such as η Corvi and HD 69830, as anomalous since collisional processing should have removed most of the planetesimal mass over their >1 Gyr lifetimes. This paper looks at the effect of large planetesimal eccentricities (e≫ 0.3) on their collisional lifetime and the amount of mass that can remain at late times Mlate. Assuming an axisymmetric planetesimal disc with common pericentre distances and eccentricities e, we find that Mlate∝e−5/3(1 +e)4/3(1 −e)−3. For a scattered disc-like population (i.e. with common pericentre distances but range of eccentricities), in the absence of dynamical evolution, the mass evolution at late times would be as if only planetesimals with the largest eccentricity were present in the disc. Despite the increased remaining mass, higher eccentricities do not increase the amount of hot emission from the collisional cascade until e > 0.99, partly because most collisions occur near pericentre, thus increasing the dust blow-out diameter. However, at high eccentricities (e > 0.97) the blow-out population extending outwards from pericentre may be detectable above the collisional cascade; higher eccentricities also increase the probability of witnessing a recent collision. All of the imaging and spectroscopic constraints for η Corvi can be explained with a single planetesimal population with pericentre at 0.75 au, apocentre at 150 au and mass of 5 M⊕; however, the origin of such a high eccentricity population remains challenging. The mid-infrared excess to HD 69830 can be explained by the ongoing destruction of a debris belt produced in a recent collision in an eccentric planetesimal belt, but the lack of far-infrared emission would require small bound grains to be absent from the parent planetesimal belt, possibly due to sublimation. The model presented here is applicable wherever non-negligible planetesimal eccentricities are implicated and can be readily incorporated into N-body simulations.