Solutions of Lane–Emden equation for n
= 0, 1, 2, 3, 4, 5
In astrophysics, the Lane–Emden equation is a dimensionless form of Poisson's equation for the gravitational potential of a Newtonian self-gravitating, spherically symmetric, polytropic fluid. It is named after astrophysicists Jonathan Homer Lane and Robert Emden. The equation reads
where is a dimensionless radius and is related to the density, and thus the pressure, by for central density . The index is the polytropic index that appears in the polytropic equation of state,
where and are the pressure and density, respectively, and is a constant of proportionality. The standard boundary conditions are and . Solutions thus describe the run of pressure and density with radius and are known as polytropes of index . If an isothermal fluid (polytropic index tends to infinity) is used instead of a polytropic fluid, one obtains the Emden–Chandrasekhar equation.
Physically, hydrostatic equilibrium connects the gradient of the potential, the density, and the gradient of the pressure, whereas Poisson's equation connects the potential with the density. Thus, if we have a further equation that dictates how the pressure and density vary with respect to one another, we can reach a solution. The particular choice of a polytropic gas as given above makes the mathematical statement of the problem particularly succinct and leads to the Lane–Emden equation. The equation is a useful approximation for self-gravitating spheres of plasma such as stars, but typically it is a rather limiting assumption.
From hydrostatic equilibrium
Consider a self-gravitating, spherically symmetric fluid in hydrostatic equilibrium. Mass is conserved and thus described by the continuity equation
where is a function of . The equation of hydrostatic equilibrium is
where is also a function of . Differentiating again gives
where the continuity equation has been used to replace the mass gradient. Multiplying both sides by and collecting the derivatives of on the left, one can write
Dividing both sides by yields, in some sense, a dimensional form of the desired equation. If, in addition, we substitute for the polytropic equation of state with and , we have
Gathering the constants and substituting , where
we have the Lane–Emden equation,
From Poisson's equation
Equivalently, one can start with Poisson's equation,
One can replace the gradient of the potential using the hydrostatic equilibrium, via
which again yields the dimensional form of the Lane–Emden equation.
For a given value of the polytropic index , denote the solution to the Lane–Emden equation as . In general, the Lane–Emden equation must be solved numerically to find . There are exact, analytic solutions for certain values of , in particular: . For between 0 and 5, the solutions are continuous and finite in extent, with the radius of the star given by , where .
For a given solution , the density profile is given by
The total mass of the model star can be found by integrating the density over radius, from 0 to .
The pressure can be found using the polytropic equation of state, , i.e.
Finally, if the gas is ideal, the equation of state is , where is the Boltzmann constant and the mean molecular weight. The temperature profile is then given by
In spherically symmetric cases, the Lane–Emden equation is integrable for only three values of the polytropic index .
For n = 0
If , the equation becomes
Re-arranging and integrating once gives
Dividing both sides by and integrating again gives
The boundary conditions and imply that the constants of integration are and . Therefore,
For n = 1
When , the equation can be expanded in the form
One assumes a power series solution:
This leads to a recursive relationship for the expansion coefficients:
This relation can be solved leading to the general solution:
The boundary condition for a physical polytrope demands that as .
This requires that , thus leading to the solution:
For n = 5
We start from with the Lane–Emden equation:
Rewriting for produces:
Differentiating with respect to ξ leads to:
Reduced, we come by:
Therefore, the Lane–Emden equation has the solution
when . This solution is finite in mass but infinite in radial extent, and therefore the complete polytrope does not represent a physical solution. Chandrasekhar believed for a long time that finding other solution for "is complicated and involves elliptic integrals".
In 1962, Sambhunath Srivastava found an explicit solution when . His solution is given by
and from this solution, a family of solutions can be obtained using homology transformation. Since this solution does not satisfy the conditions at the origin (in fact, it is oscillatory with amplitudes growing indefinitely as the origin is approached), this solution can be used in composite stellar models.
In general, solutions are found by numerical integration. Many standard methods require that the problem is formulated as a system of first-order ordinary differential equations. For example,
Here, is interpreted as the dimensionless mass, defined by . The relevant initial conditions are and . The first equation represents hydrostatic equilibrium and the second represents mass conservation.
It is known that if is a solution of the Lane–Emden equation, then so is . Solutions that are related in this way are called homologous; the process that transforms them is homology. If one chooses variables that are invariant to homology, then we can reduce the order of the Lane–Emden equation by one.
A variety of such variables exist. A suitable choice is
We can differentiate the logarithms of these variables with respect to , which gives
Finally, we can divide these two equations to eliminate the dependence on , which leaves
This is now a single first-order equation.
Topology of the homology-invariant equation
The homology-invariant equation can be regarded as the autonomous pair of equations
The behaviour of solutions to these equations can be determined by linear stability analysis. The critical points of the equation (where ) and the eigenvalues and eigenvectors of the Jacobian matrix are tabulated below.