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Abstract
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)1and the Atacama Large Millimeter/ submillimeter Array (ALMA) in12CO,13COJ= 2 → 1, and13COJ= 1 → 0 transitions. We incorporate ALMA archival data of the12COJ= 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. The12CO is optically thick, while13CO is optically thin withτ13CO~ 0.15 (C18O is not detected). The13CO surface density is constrained to be one order of magnitude smaller than around younger Herbig Ae stars, and we derive a gas massM12CO= 10−1M⊕. We confirm the presence of a small CO cavity (RCO= 17 ± 3 au), and find a possibly larger radius for the optically thin13COJ= 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. The12CO temperatureT0(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.
Highlights
The evolution of circumstellar discs from the initial reservoir of the interstellar medium (ISM)–dust and gas mixture towards planetary systems is governed by the growth and coagulation of dust grains, viscous spreading and accretion onto the central star, and dissipation of the primordial gas (Williams & Cieza 2011)
In Péricaud et al (2017), we reported a correlation between the CO flux density and the mm continuum emission for a broad variety of systems including T Tauri, Herbig Ae, transition discs, and even the proto-typical young debris disc β Pic
The continuum emission at mm wavelength is clearly much more compact than the 250–400 au broad rings of the debris disc revealed at optical/NIR wavelengths, and is more compact and centrally peaked than the gas emission characterised
Summary
The evolution of circumstellar discs from the initial reservoir of the interstellar medium (ISM)–dust and gas mixture towards planetary systems is governed by the growth and coagulation of dust grains, viscous spreading and accretion onto the central star (or on proto-planets), and dissipation of the primordial gas (Williams & Cieza 2011). The gas-rich debris discs revealed by CO lines simultaneously display a weak dust mm continuum and an IR excess characteristic of more evolved, optically thin debris discs These systems are thought to witness a short-lived transient phase at the interface between proto-planetary and debris discs, when the material dissipates through photo-evaporation and accretion. In Péricaud et al (2017), we reported a correlation between the CO flux density and the mm continuum emission for a broad variety of systems including T Tauri, Herbig Ae, transition discs, and even the proto-typical young debris disc β Pic. The position of hybrid discs in the diagram, systematically above the correlation line, suggests a faster evolution of the dust in these systems, leading to a transient and unusual increase of the gas-to-dust ratio. Assuming a similar line width to that of the CO lines (7 km s−1 FWHM), the 3σ detection threshold on the integrated line flux is 45 mJy km.s−1 around 90 GHz, and 60 mJy km.s−1 around 145 GHz
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