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A protoplanetary disk is a rotating circumstellar disc of dense gas and dust surrounding a young newly formed star, a T Tauri star, or Herbig Ae/Be star. The protoplanetary disk may not be considered an accretion disk ; while the two are similar, an accretion disk is hotter and spins much faster.
An accretion disk is a structure (often a circumstellar disk) formed by diffuse material [a] in orbital motion around a massive central body. The central body is most frequently a star . Friction , uneven irradiance, magnetohydrodynamic effects, and other forces induce instabilities causing orbiting material in the disk to spiral inward toward ...
[2] [3] Alternatively, planetesimals may form in a very dense layer of dust grains that undergoes a collective gravitational instability in the mid-plane of a protoplanetary disk—or via the concentration and gravitational collapse of swarms of larger particles in streaming instabilities. [4]
ALMA image of HL Tauri, a protoplanetary disk. In astrophysics, accretion is the accumulation of particles into a massive object by gravitationally attracting more matter, typically gaseous matter, into an accretion disk. [1] [2] Most astronomical objects, such as galaxies, stars, and planets, are formed by accretion processes.
Disk Planet Planet distance (in AU) Notes HD 100546: Disk extends out to 17: Diameter, 721 AU [1] HD 100546 b: 6.5 – GM Aurigae: 0–300 – – – Disc is warped Lynds 1551: 0.1 – – – Binary system 45 AU apart OTS 44 – – – – Second smallest brown dwarf TW Hydrae: 80: 440 [2]
In astronomy, a disk wind is a particle outflow observed around accretion disks, mainly near protoplanetary disks [1] [2] and active galactic nulei (AGN). [3] [4] The disk wind is made up of a gaseous and a dusty component. [1] Especially in edge-on protoplanetary disks this disk wind can be directly imaged. [5] [6]
The protoplanetary disk is sometimes referred to as an accretion disk, because while the young T Tauri-like protostar is still contracting, gaseous material may still be falling onto it, accreting on its surface from the disk's inner edge. [40] In an accretion disk, there is a net flux of mass from larger radii toward smaller radii. [23]
Photoevaporation is the process where energetic radiation ionises gas and causes it to disperse away from the ionising source. The term is typically used in an astrophysical context where ultraviolet radiation from hot stars acts on clouds of material such as molecular clouds, protoplanetary disks, or planetary atmospheres.