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

Abstract: HD141569 is a pre-main sequence star with a disc uniquely placed between protoplanetary and debris discs, similar to the older `hybrid' type discs. This work aims to place the mass and spatial structure of the disc midplane in the context of the debris, hybrid and protoplanetary discs. We observed HD~141569 with ALMA in 1.3~mm continuum and $^{13}$CO (2-1). This is the first detection and image of the optically thin gas emission from the midplane of this disc. In continuum emission, we detect a combination of an unresolved central peak and a ring of millimetre emission at 220$\pm$10~au, slightly interior to one of the rings discovered in scattered light. The minimum dust mass of the ring is 0.13$\pm$0.02~M${\oplus}$ while the unresolved millimetre peak at the stellar location is predominantly thermal emission due to a minimum of 1.2$\pm$0.2~M${\oplus}$ of dust. $^{13}$CO is distributed asymmetrically around the stellar position with a peak at 1.1" distance and a P.A. of -33$^\circ$. The gas is detected as far as 220$\pm$10~au, a radial separation the same as that of the mm ring. Assuming optically thin emission and standard ISM abundances, we use our $^{13}$CO data to derive the gas mass in the disc of (6.0$\pm$0.9) $\times 10^{-4}~$M$_\odot$. Comparison to published $^{12}$CO data shows that $^{12}$CO is optically thick, explaining why estimates based on $^{12}$CO underestimated the gas mass.