An Etymological Dictionary of Astronomy and Astrophysics
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A physical process that is contrary to accretion, referring to the outward direction of the mass flow from a star to its environment.

From → de- "reversal, removal," + -cretion, from L. accretionem (nom. accretio, gen. accretionis) "a growing larger," from stem of accrescere, → accretion.

A disk that would form around a star when the star injects matter into a close orbit. This is in contrast to an → accretion disk, which transfers matter from outside to the star. The mass gets injected into the decretion disk by a not yet well-known mechanism, most probably a combination of → non-radial pulsation, fast rotation, and possibly small-scale → magnetic fields. See also → viscous decretion disk.

→ decretion; → disk.

A model for explaining several observational features of → circumstellar disks around → Be stars. According to this model, the central star provides → angular momentum to the disk at the innermost radius, and then it is redistributed over the whole disk via → viscosity. Thus, in isolated Be stars, the equatorial disk can spread out to a large distance as long as the star can give angular momentum to the disk. The VDD model, first introduced by Lee et al. (1991, MNRAS, 250, 432) and further developed by several other researchers, is now widely accepted as the best physical model for describing the circumstellar disks of Be stars. Among the growing evidence supporting the VDD model is the confirmation that the disks rotate in a Keplerian way (→ Keplerian orbit), allowing for the identification of viscosity as the mechanism that makes the disk grow (see, e.g., Klement et al., 2015, A&A 584, A85).

→ viscous; → decretion; → disk.