2020A&A...635A..94D

Context. The study of gas-rich debris discs is fundamental to characterising the transition between protoplanetary discs and debris discs.
Aims. We determine the physical parameters of the brightest gas-rich debris disc orbiting HD 141569A.
Methods. We analyse images from the NOrthern Extended Millimeter Array (NOEMA)¹ and the Atacama Large Millimeter/ submillimeter Array (ALMA) in ¹²CO, ¹³CO J=2-1, and ¹³CO J=1-0 transitions. We incorporate ALMA archival data of the ¹²CO J=3-2 transition and present continuum maps at 0.87, 1.3, and 2.8mm. We use simple parametric laws with the Diskfit code and MCMC exploration to characterise the gas disc parameters and report a first attempt to characterise its chemical content with IRAM-30m.
Results. The continuum emission is equally shared between a compact (≤50au) and a smooth, extended dust component (∼350au). Large millimetre grains seem to dominate the inner regions, while the dust spectral index is marginally larger in the outer region. The ¹²CO is optically thick, while ¹³CO is optically thin with τ_13CO∼0.15 (C¹⁸O is not detected). The ¹³CO surface density is constrained to be one order of magnitude smaller than around younger Herbig Ae stars, and we derive a gas mass M_12CO=10^–1M_⊕. We confirm the presence of a small CO cavity (R_CO=17±3au), and find a possibly larger radius for the optically thin ¹³CO J=2-1 transition (35±5au). We show that the observed CO brightness asymmetry is coincident with the complex ring structures discovered with VLT/SPHERE in the inner 90au. The ¹²CO temperature T₀(100au)∼30K is lower than expected for a Herbig A0 star, and could be indicative of subthermal excitation.
Conclusions. With the largest amount of dust and gas among hybrid discs, HD 141569A shows coincident characteristics of both protoplanetary discs (central regions), and debris discs at large distance. Together with its morphological characteristics and young age, it appears to be a good candidate to witness the transient phase of gas dissipation, with an apparently large gas-to-dust ratio (G/D>100) favouring a faster evolution of dust grains.