Bondi accretion

{{Short description|Accretion of matter within the interstellar medium}}

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

:$\backslash dot\{M\}\; \backslash simeq\; \backslash pi\; R^2\; \backslash rho\; v$.

where:

• $\backslash rho$ is the ambient density
• $v$ is the object's velocity $v\_o$ or the sound speed $c\_s$ in the surrounding medium if
• $R$ is the Bondi radius, defined as $2\; G\; M\; /\; c\_s^2$.

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.{{cite journal|last1=Edgar|first1=Richard|title=A Review of Bondi-Hoyle-Lyttleton Accretion|url=https://ned.ipac.caltech.edu/level5/March09/Edgar/Edgar2.html|journal=New Astronomy Reviews|access-date=19 February 2018|language=en|doi=10.1016/j.newar.2004.06.001|date=21 Jun 2004|volume=48 |issue=10 |pages=843–859 |arxiv=astro-ph/0406166 |bibcode=2004NewAR..48..843E |s2cid=17638601 }} Substituting the Bondi radius in the above equation yields:

$\backslash dot\{M\}\; \backslash simeq\; \backslash frac\{\; \backslash pi\; \backslash rho\; G^2\; M^2\; \}\{c\_s^3\}$.

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.