2017MNRAS.469.4726N

We present high-resolution Atacama Large Millimeter/submillimeter Array (ALMA) observations of the CO(3-2) and 350 GHz continuum emissions of the debris disc of 49 Ceti, known to be particularly rich in molecular gas in spite of its age. The main new results are as follows: i) both CO and dust discs share the same position angle and the same inclination but the gas disc is more homogeneous, more central and thinner than the dust disc; (ii) evidence is obtained for a significant deficit of observed CO(3-2) emission at Doppler velocities differing from the star systemic velocity by less than 1 km s^–1; (iii) gas velocities are accurately measured and found Keplerian over a broad range of disc radii and (iv) the observed CO(3-2) line width is dominated by Keplerian shear and upper limits are obtained to the intrinsic line width. Simple phenomenological models of both CO(3-2) and 350 GHz continuum emissions are presented, requiring the use of only very few parameters. The results are discussed in the frame of currently favoured models.