ALMA resolves CI emission from the $β$ Pictoris debris disk

Abstract: The debris disk around $\beta$~Pictoris is known to contain gas. Previous ALMA observations revealed a CO belt at $\sim$85 au with a distinct clump, interpreted as a location of enhanced gas production. Photodissociation converts CO into C and O within $\sim$50 years. We resolve CI emission at 492 GHz using ALMA and study its spatial distribution. CI shows the same clump as seen for CO. This is surprising, as C is expected to quickly spread in azimuth. We derive a low C mass (between $5\times10^{-4}$ and $3.1\times10^{-3}$ M$\oplus$), indicating that gas production started only recently (within $\sim$5000 years). No evidence is seen for an atomic accretion disk inwards of the CO belt, perhaps because the gas did not yet have time to spread radially. The fact that C and CO share the same asymmetry argues against a previously proposed scenario where the clump is due to an outward migrating planet trapping planetesimals in an resonance; nor can the observations be explained by an eccentric planetesimal belt secularly forced by a planet. Instead, we suggest that the dust and gas disks should be eccentric. Such a configuration, we further speculate, might be produced by a recent tidal disruption event. Assuming that the disrupted body has had a CO mass fraction of 10%, its total mass would be $\gtrsim$3 $M\mathrm{Moon}$.