Lane-Emden Equation ***
Problem: Use the Lane-Emden equation to create a basic model for the white dwarf Sirius B. Calculate and provide plots of density and pressure. Note that a polytropic index of 3 applies to all white dwarfs. (A method devised by Larson (2012) is employed in the calculations.)
Hints:
Solve the Lane-Emden equation for n = 3.
Boundary conditions for the Lane-Emden equation for Sirius B: at the centre of the star, ξ = 1 and its first derivative is 0.
Data: (SI units)
| (1) |
| (2) |
| (3) |
| (4) |
| (5) |
| (6) |
Define variables in the Lane-Emden equation:
Useful Equation:
Solution:
The Lane-Emden equation may be written as.
| (7) |
Boundary conditions. At the centre of Sirius B, ξ = 1, and its first derivative is 0.
| (8) |

Solve the equation:
| (9) |
| (10) |
Since the variable x (see above) ranges from 0 to 1, we can try 0.7 to test the solution:
| (11) |
We are concerned here with the value of ξ(x). Select this value and plot it from 0 to 150.
| (12) |
Calculate the density (ξ(r) = ξ(x* and the pressure (from the equation of state) and plot them:
| (13) |
| (14) |
Plot rho over a range to determine where the value at the surface will be found:
The surface value (0) will be found between x = 0 and x = 10. Calculate the value and assign it to Xsurf:
| (15) |
Plot the density from the centre of Sirius B to the surface:
Plot the pressure from the centre of Sirius B to the surface:
Now adjust the plot so that density and pressure appear as functions of Sirius B's radius (R):
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Reference
Larson, S. (2012). Stellar Structure with Maple. http://sciencejedi.com/professional/classes/astrophysics/lectures/maple_laneEmden.pdf (Accessed 2016-08-29).
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