Letter to the Editor

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

¹ School of Physics and Astronomy, University of Leeds, Woodhouse Lane, Leeds LS2 9JT, UK
e-mail: py12jm@leeds.ac.uk
² Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK
³ Department of Physics, University of Warwick, Gibbet Hill Road, Coventry CV4 7AL, UK
⁴ Centre for Exoplanets and Habitability, University of Warwick, Gibbet Hill Road, Coventry CV4 7AL, UK

Received: 6 May 2018
Accepted: 20 June 2018

Abstract

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 1ʺ̣1 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.