Unlocking the secrets of the midplane gas and dust distribution in the young hybrid disc HD 141569

Context. HD 141569 is a pre-main sequence star with a disc uniquely placed between protoplanetary and debris discs, similar to the older "hybrid" type discs.

Aims: This work aims to place the mass and spatial structure of the disc midplane in the context of the debris, hybrid and protoplanetary discs.

Methods: We observed HD 141569 with ALMA in 1.3 mm continuum and 13CO (2-1). This is the first detection and image of the optically thin gas emission from the midplane of this disc.

Results: In continuum emission, we detect a combination of an unresolved central peak and a ring of millimetre emission at 220 ± 10 au, slightly interior to one of the rings discovered in scattered light. The minimum dust mass of the ring is 0.13 ± 0.02 M⊕ while the unresolved millimetre peak at the stellar location is predominantly thermal emission due to a minimum of 1.2 ± 0.2 M⊕ of dust. 13CO is distributed asymmetrically around the stellar position with a peak at 1ʺ&dotbelow;1 distance and a PA of -33°. The gas is detected as far as 220 ± 10 au, a radial separation the same as that of the mm ring. Assuming optically thin emission and standard ISM abundances, we used our 13CO data to derive the gas mass in the disc of (6.0 ± 0.9) × 10-4M⊙. Comparison to published 12CO data shows that 12CO is optically thick, explaining why estimates based on 12CO underestimated the gas mass.