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# Long description

In the figure, a central yellow sphere labelled ‘M subscript asterisk’ is shown. A horizontal line is drawn across the sphere, such that the sphere lies in the middle of the line. This line is labelled ‘midplane (z equals 0)’. A right arrowhead is drawn at the right end of the line. This arrowhead is labelled ‘r-axis’. An upward arrow labelled ‘z-axis’ is drawn from the sphere. The protoplanetary disc is shown as a semicircular ring around the yellow sphere, lying on the horizontal plane facing away from the observer. The two cross-sections of the ring, lying on either side of the sphere are symmetrical about the z-axis. The height of the disc increases on either side of the horizontal line towards the periphery, forming two triangular cross-sections. A thin green layer covers the upper and lower surfaces of the disc, and the interior portion of the disc is shown as a blue region with several black dots. The black dots lying closest to the horizontal line are bigger, and the size decreases the further the dots are from the horizontal line. A point labelled ‘A’ is taken near the top-right corner of the right disc. A line of length d joins A and the centre of the sphere. This line forms an angle theta with the horizontal line. From A, a vertical line of length z is drawn to the horizontal line.