ALMA observations of the η Corvi debris disc: inward scattering of CO-rich exocomets by a chain of 3–30 M_⊕planets?

Abstract

While most of the known debris discs present cold dust at tens of astronomical unit (au), a few young systems exhibit hot dust analogous to the Zodiacal dust. &eegr; Corvi is particularly interesting as it is old and it has both, with its hot dust significantly exceeding the maximum luminosity of an <it>in situ</it> 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 &eegr; Corvi with a resolution of 1.2 arcsec (∼22 au) to study its outer belt. The continuum emission is consistent with an axisymmetric belt, with a mean radius of 152 au and radial full width at half-maximum of 46 au, 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 4σ detection of CO at ∼20 au. CO could be released <it>in situ</it> from icy planetesimals being passed in when crossing the H<inf>2</inf>O or CO<inf>2</inf> 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 75–100 au, with a mass of 3–30 M<inf>⊕</inf> and an eccentricity <0.08. Such a planet would be able to clear its chaotic zone on a time-scale shorter than the age of the system and scatter material inwards from the outer belt to the inner regions, thus feeding the hot dust.