Home > Astronomical notes > Pulsating stars
Pulsating stars III: Polytropic model
Michel Bonnardeau
6 March 2006
Revised 13 July 2012
Abstract
The solutions to the differential equation of a radially pulsating
star are presented for a polytropic model.
Introduction
Part I was written down the differential
equation for the oscillations of a pulsating star;
Part II this equation was solved in the case where
the star density is constant;
This part, a more realistic model of the star is used.
Differential equation for the pulsations from the Lane-Emden function
Part I was established the differential equation
for the pulsations:
where r0 is the distance from the star center,
r1 is the amplitude of the oscillation and X=r1/r0,
prime ' is the derivation d/dr0,
rho0 is the density at r0,
P0 is the pressure at r0,
g0 is the gravitational acceleration
at r0 (see I-(6)),
gamma describes the P response to a change of density (gamma=5/3
for an adiabatic ideal gaz),
omega is the pulsation of the oscillation.
In the astronomical note Star structure (referred
as "S" here) the density and pressure are computed as a function of the
distance r0 from the star center for a polytropic
model, using the Lane-Emden function u(z):
where PHIc is the gravitational potential
at the star center,
and n is the polytropic index:
The density and the pressure are given by:
Although both gamma and n relate the pressure to the density,
they are not to be confused:
gamma is for the response of the gaz to a change of pressure or of density
(or of temperature). gamma=1 for an isothermal gaz, 5/3 for an adiabatic gaz
(no heat exchange), 4/3 for a radiation pressure dominated gaz;
n is for the internal structure of the star and its nuclear heating.
n=0 for a liquid star model, 3/2 for a convective
star model, 3 for a standard star model.
One has, with u'=du/dz:
Putting these into the differential equation I-(15) gives:
OMEGA depends only on the indexes n and gamma (the Lane-Emden function
u(z) depends only upon n). It does not depend upon the other parameters
of the star such as its mass, etc.
The star is unstable for gamma<4/3
and the period of the fundamental mode is infinite when gamma=4/3. (gamma=4/3
is when the pressure is dominated by the radiative pressure). The periods
depend only on the density at the center of the star:
| (2) |
| (2a) |
The Q-value or pulsation constant: The density at the center
is expected to be proportional to the average density rhom
which may be directly observed from the star mass and radius. Pulsating
stars are then quite often characterized by their observed Q-value:
| (3) |
with the period PI in days and rhomo the average Sun density.
Schwarzschild solution
Equation (1) was solved numerically by Schwarzschild (1941) for the
polytropic index n=3, up to the fourth overtone, and for different values
of gamma. The ratios of the overtone periods to the fundamental period
are:
gamma | 10/7 = 1.43 | 20/13 = 1.54 | 5/3 = 1.67 adiabatic |
PI1/PI0 | 0.554 | 0.687 | 0.738 |
PI2/PI0 | 0.402 | 0.515 | 0.570 |
PI3/PI0 | 0.317 | 0.412 | 0.461 |
PI4/PI0 | 0.262 | 0.343 | 0.387 |
and the ratios of the successive overtone periods (for gamma=4/3 the fundamental
period is infinite):
gamma |
4/3 = 1.33
radiative pressure |
10/7 = 1.43 | 20/13 = 1.54 |
5/3 = 1.67 adiabatic |
PI1/PI0 | 0.000 | 0.554 | 0.687 | 0.738 |
PI2/PI1 | 0.700 | 0.724 | 0.749 | 0.772 |
PI3/PI2 | 0.778 | 0.789 | 0.799 | 0.810 |
PI4/PI3 | 0.822 | 0.828 | 0.833 | 0.839 |
The dependance of the pulsations with the polytropic index n for gamma=5/3
(Cox (1967)):
More
Non radial oscillations: They are characterized by their degree and azimuthal
orders:
the degree, denoted l, is the number of nodal lines parallel to
the equator;
the azimuthal, denoted m, is the number of nodal lines that intersect
the equator.
The radial oscillations are then denoted n (not be confused with
the polytropic index), with n=1 the fundamental, n=2 the first overtone.
The l,m,n oscillations of the Sun are well observed (although they have
much smaller amplitudes than for a "variable" star) and much studied (helioseismology).
This allows a good understanding of the structure and of what is going
on inside the Sun. This brought up the "solar neutrino problem":
early neutrino telescopes detected much less neutrinos than predicted
by these Sun models. The explanation was found recently (2002 Nobel Prize):
the neutrinos switch from one specie into another.
p-mode and g-mode: the restoring force for the oscillations
studied here is the pressure: these oscillations are p-mode waves. Other
oscillations may be due to buoyancy: they are gravity mode or g-mode waves.
g-mode waves cannot travel through convection zones; the external layer
of a star being usually convective (because of the strong temperature
gradient), they are then trapped in the deep interior.
References
Cox J.P. (1967) IAU Symposium n°28 p3
Schwarzschild M. (1941) ApJ 94 245.