Abstract
While most of the known debris discs present cold dust at tens of AU, a few young systems exhibit hot dust analogous to the Zodiacal dust. $\eta$ Corvi is particularly interesting as it is old and it has both, with its hot dust significantly exceeding the maximum luminosity of an in-situ collisional cascade. Previous work suggested that this system could be undergoing an event similar to the Late Heavy Bombardment (LHB) soon after or during a dynamical instability. Here we present ALMA observations of $\eta$ Corvi with a resolution of 1."2 (~22au) to study its outer belt. The continuum emission is consistent with an axisymmetric belt, with a mean radius of 152au and radial FWHM of 46au, which is too narrow compared to models of inward scattering of an LHB-like scenario. Instead, the hot dust could be explained as material passed inwards in a rather stable planetary configuration. We also report a 4sigma detection of CO at ~ 20au. CO could be released in situ from icy planetesimals being passed in when crossing the H$_2$O or CO$_2$ ice lines. Finally, we place constraints on hidden planets in the disc. If a planet is sculpting the disc's inner edge, this should be orbiting at 75-100au, with a mass of 3-30 M$_\oplus$ and an eccentricity < 0.08. Such a planet would be able to clear its chaotic zone on a timescale shorter than the age of the system and scatter material inwards from the outer belt to the inner regions, thus feeding the hot dust.
Highlights
As a byproduct of planet formation, discs of kilometre-sized bodies or planetesimals can form, analogous to the Asteroid and Kuiper belts in the Solar system (Lissauer 1993)
4.1.1 Belt or self-stirred disc In Section 3, we found that the disc continuum emission is consistent with a belt at 150 au and given the visibility uncertainties and insufficient short baselines in our Atacama Large Millimeter/submillimeter Array (ALMA) data, the Gaussian belt model with three free parameters to describe the surface density, gives the best fit and it is not necessary to invoke a more complex model such as the self-stirred disc with two more free parameters
We have presented the first ALMA observations of the debris disc around η Corvi at 0.88 mm, obtaining the most detailed image of its outer belt up to date
Summary
As a byproduct of planet formation, discs of kilometre-sized bodies or planetesimals can form, analogous to the Asteroid and Kuiper belts in the Solar system (Lissauer 1993) Destructive collisions between these bodies can give rise to debris discs, grinding down the largest bodies producing a wide size distribution of solids, from micrometre-sized dust grains up to the size of asteroids or comets, in a so-called collisional cascade (Wyatt et al 2007b). The last decade of debris disc surveys has shown that Kuiper belt analogues are quite common, with detection rates of at least 20 per cent around AFGK stars (Su et al 2006; Hillenbrand et al 2008; Carpenter et al 2009; Eiroa et al 2013; Thureau et al 2014; Matthews et al 2014) These surveys have shown that the levels of infrared excess decay with stellar age, as expected from collisional models
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