An ALMA/NOEMA study of gas dissipation and dust evolution in the 5 Myr-old HD 141569A hybrid disc

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 2.8 mm. 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-30 m. Results. The continuum emission is equally shared between a compact (≲50 au) and a smooth, extended dust component (~350 au). 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 τ13CO ~ 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 M12CO = 10−1M⊕. We confirm the presence of a small CO cavity (RCO = 17 ± 3 au), and find a possibly larger radius for the optically thin 13CO J = 2 → 1 transition (35 ± 5 au). We show that the observed CO brightness asymmetry is coincident with the complex ring structures discovered with VLT/SPHERE in the inner 90 au. The 12CO temperature T0(100 au) ~ 30 K 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.

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