The process by which an object increases its mass under the influence of its
→ gravitational attraction.
Accretion plays a key role in a wide range of astrophysical phenomena.
In particular stars result from the accretion of material by a
→ protostar from a surrounding
→ molecular cloud.
The accumulation of mass on the protostar involves
the formation of an → accretion disk.
Theoretical and observational investigations of protostars and newborn stars
indicate the important role of → magnetic fields
in this process. They favor the magnetospheric accretion model for mass transfer
from the circumstellar disk onto the newborn star. In this model, the stellar magnetosphere
→ truncates the disk at a few stellar radii. Gas from the disk
accretes onto the star along the magnetic field lines and hits the stellar surface at
approximately the → free fall velocity, causing a strong accretion
shock. Various → emission lines, such as the hydrogen
→ Balmer series, He I 5876 Å, Brγ 2.17 μm,
and so forth are formed in the infalling magnetospheric flow. Moreover, optical/ultraviolet
excess continuum emission is produced in the → accretion shocks.
The accretion is accompanied
by mass ejection through collimated → bipolar jets.
Accumulation of dust and gas onto larger bodies by → coalescence
under the influence of their mutual → gravitational attraction
or as a result of chance collisions.
See also:
→ accretion column,
→ accretion disk,
→ accretion flow,
→ accretion rate,
→ accretion shock,
→ accretion time.
Etymology (EN): L. accretionem (nom. accretio, gen. accretionis)
“a growing larger,” from stem of accrescere, from ad- “to” +
crescere “to grow”.
Etymology (PE): Farbâl from prefix → far- which conveys “increase,
abundance” + bâl, from bâlidan “to grow, to
wax great,” Mid.Pers. vâlitan, Av. varəd-, varədait-
“to increase, augment, strengthen, cause to prosper,” Skt. vrdh-,
vardhati.
