The velocity structure kinematics of a Be star disc are dominated by its rotational velocity. This can be clearly seen from the symmetry of emission lines formed in the disc.
Dachs et al. (1988) are able to conclude from a study of Balmer emission line profiles, (see also Dachs et al. 1986, Hummel and Vrancken 2000) that the envelopes surrounding Be stars are most easily explained if it is assumed that there are Keplerian motions within the disc. The rotational velocity, vrot(R), as a function of radius is therefore governed by Keplerian rotation. From Newton’s laws of motion:
 | =  | (1.3) |
| => vrot(R) |  R-1/2. | (1.4) |
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