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tokens: suited for tracing the very inner regions of a molecular disc ( Hirota et al. 2014 ; Ginsburg et al. 2018 ). To our sensitivity and resolution limit, the H 2 O emission appears devoid of any structure and does not extend beyond the outer radius of the disc as traced by the dust continuum emission (grey contours Fig. 2 b). The peak of emission is away from the centre of the dust disc in all velocity channels, while the H 2 O emission peaks between the enhanced continuum dust ring and the central location of G17.64 at the highest blue- and red-shifted velocities. The zero-moment contours in Fig. 2 a show that the H 2 O emission might appear like a disc with a central hole or like a single wide ring ranging from approximately half of the beam (∼30 au) to 120 au in radius (e.g. like the SO emission in Yen et al. 2014 ). The cut for the PV diagram was taken at a PA of 25.9° using a width of 5 pixels to encompass one synthesised beam (grey dashed line in Figs. 2 a,b). The PA was established by fitting a line through the centres of Gaussian fits to independent zero-moment maps of the blue- (3.2−21.4 km s −1 ) and red-shifted (22.7−44.8 km s −1 ) emission and the central position of G17.64 from the dust continuum. The PV diagram depicts the clear pattern of Keplerian rotation, particularly the red-shifted emission > 30 km s −1 , where the highest velocities peak closer towards the central source. The H 2 O undoubtedly traces a rotating disc. Fig. 2. Panel a : integrated intensity (zero-moment) map of the H 2 O 5 5, 0 −6 4, 3 ν 2 = 1 (232.68670 GHz) molecular line emission. The contours trace emission at the 3, 5, 7, 9, and 11 σ levels, where 1 σ = 7.13 mJy beam −1 km s −1 . Panel b : velocity-weighted integrated intensity (first-moment) map overlaid with the contours of the continuum emission from Fig. 1 a in grey. The outer extent of the H 2 O emission matches that of the dust continuum. Panel c : PV diagram for the H 2 O emission extracted from a 5 pixel wide slice along the dashed line at PA = 25.9° as indicated in (a) and (b). The contours of the data are plotted at the 20–80% levels in steps of 10%, where 10% corresponds to ∼1 σ (0.65 mJy beam −1 ). Panel d : same as (c), but the contours indicate the best representative model using inner and outer disc radii of 30 and 120 au, respectively, a stellar mass of 45 M ⊙ , and a disc inclination angle of 40°. The spatial (28 mas) and velocity resolution (1.3 km s −1 ) are indicated by the black cross at the bottom right. 4. Discussion 4.1. Stellar mass Based on its luminosity, G17.64 is firmly positioned as an O star (e.g. Vacca et al. 1996 ). However, previous estimates of the stellar mass, which are essential for understanding the disc stability, were not well constrained because the kinematics were poorly resolved ( Maud et al. 2018 ). Building from our previous representative models presented in Maud et al. (2018) , we matched the H 2 O PV diagram with only very minor changes. We used