The Plummer model or Plummer sphere is a density law that was first used by
H. C. Plummer to fit observations of
globular cluster
A globular cluster is a spheroidal conglomeration of stars. Globular clusters are bound together by gravity, with a higher concentration of stars towards their centers. They can contain anywhere from tens of thousands to many millions of membe ...
s. It is now often used as
toy model
In the modeling of physics, a toy model is a deliberately simplistic model with many details removed so that it can be used to explain a mechanism concisely. It is also useful in a description of the fuller model.
* In "toy" mathematical models ...
in
N-body simulation
In physics and astronomy, an ''N''-body simulation is a simulation of a dynamical system of particles, usually under the influence of physical forces, such as gravity (see n-body problem, ''n''-body problem for other applications). ''N''-body ...
s of stellar systems.
Description of the model
The Plummer 3-dimensional density profile is given by
:
where ''
'' is the total mass of the cluster, and ''a'' is the Plummer radius, a scale parameter that sets the size of the cluster core. The corresponding potential is
:
where ''G'' is
Newton's
gravitational constant. The velocity dispersion is
:
The distribution function is
:
if
, and
otherwise, where
is the
specific energy
Specific energy or massic energy is energy per unit mass. It is also sometimes called gravimetric energy density, which is not to be confused with energy density, which is defined as energy per unit volume. It is used to quantify, for example, sto ...
.
Properties
The mass enclosed within radius
is given by
: