In astrophysics, the Bondi accretion (also called Bondi–Hoyle–Lyttleton accretion), named after Hermann Bondi, is spherical accretion onto a compact object traveling through the interstellar medium. It is generally used in the context of neutron star and black hole accretion. To achieve an approximate form of the Bondi accretion rate, accretion is assumed to occur at a rate
- .
where:
- is the ambient density
- is the object's velocity or the sound speed in the surrounding medium if
- is the Bondi radius, defined as .
The Bondi radius comes from setting escape velocity equal to the sound speed and solving for radius. It represents the boundary between subsonic and supersonic infall.[1] Substituting the Bondi radius in the above equation yields:
.
These are only scaling relations rather than rigorous definitions. A more complete solution can be found in Bondi's original work and two other papers.
Application to accreting protoplanets
[edit]When a planet is forming in a protoplanetary disk, it needs the gas in the disk to fall into its Bondi sphere in order for the planet to be able to accrete an atmosphere. For a massive enough planet, the initial accreted gas can quickly fill up the Bondi sphere. At this point, the atmosphere must cool and contract (through the Kelvin–Helmholtz mechanism) for the planet to be able to accrete more of an atmosphere.
Bibliography
[edit]- Bondi (1952) MNRAS 112, 195, link
- Mestel (1954) MNRAS 114, 437, link
- Hoyle and Lyttleton (1941) MNRAS 101, 227
References
[edit]- ^ Edgar, Richard (21 Jun 2004). "A Review of Bondi-Hoyle-Lyttleton Accretion". New Astronomy Reviews. 48 (10): 843–859. arXiv:astro-ph/0406166. Bibcode:2004NewAR..48..843E. doi:10.1016/j.newar.2004.06.001. S2CID 17638601. Retrieved 19 February 2018.
Well, that’s interesting to know that Psilotum nudum are known as whisk ferns. Psilotum nudum is the commoner species of the two. While the P. flaccidum is a rare species and is found in the tropical islands. Both the species are usually epiphytic in habit and grow upon tree ferns. These species may also be terrestrial and grow in humus or in the crevices of the rocks.
View the detailed Guide of Psilotum nudum: Detailed Study Of Psilotum Nudum (Whisk Fern), Classification, Anatomy, Reproduction