### An Introduction

Stability properties of a physical system refers to how the system responds to some perturbation; whether the system can recover on its own after being perturbed or whether it goes haywire. Stability analysis helps us to understand what happens when we perturb a system.

### Stability Properties of a Physical System

*Phase Transitions Between Massive Stars and Molecular Hydrogen in a Model Galaxy*

By MPF (originally written November 2006)

**Abstract**

The stability properties of a physical system, phase transitions between massive stars and molecular hydrogen in a galaxy, can be determined by performing a stability analysis on a system of ordinary differential equations that govern the mass of the system.

**Introduction**

Stability analysis of systems allows us to determine whether or not a system is stable or will be stable if perturbed. This is important in a wide range of applications since many behaviours observed in the real world can be described using differential equations.

The stability analysis of phase transitions between massive stars and molecular hydrogen in a model galaxy allow us to determine whether or not the mass of molecular hydrogen and the mass of stars in a galaxy is in an equilibrium, if it ever was, or if it ever will be. This lets us model the future behaviour of an observed galaxy or lets us work backwards and theorize what might have occurred in a galaxy’s past, for example, if a galactic collision had once occurred and if so, when?

While stability analysis has many wide-ranging uses not necessarily related to astronomy, this report will examine the stability of a model galaxy.

**Procedure and Results**

The phase transitions between massive stars and molecular gas in a model galaxy are described by a system of ordinary differential equations. In this case, the equations are:

*dM*_{s}*/dt = -rM*_{s}* + kaM*_{s}*M*_{H2}* * and* dM*_{H2}*/dt = -aM*_{s}*M*_{H2}* + A* (1)

Where *M*_{s} and *M*_{H2} are the total masses of massive stars and molecular hydrogen, respectively, *a* is the rate of induced star formation, *k* is the efficiency of star formation, 1/*r* is an average lifetime of massive stars and *A* is the mass accretion rate onto the galaxy.

In order to derive an equilibrium for both masses, both equations of system (1) must be set equal to 0. The equilibrium mass of massive stars, *M*_{s}^{0}, can be expressed as:

*M*_{s}^{0}* = Ak/r*

While the equilibrium mass of molecular hydrogen, *M*_{H2}^{0}, can be expressed as:

*M*_{H2}^{0} = *r/ak*

System (1) can be expressed in the following form:

*M*_{s}* = M*_{s}^{0}* + m*_{s}* * exp*(λt) *and* M*_{H2}* = M*_{H2}^{0}* + m*_{H2}* * exp*(λt)* (2)

Where *m*_{s} and *m*_{H2} are small perturbations in the masses of massive stars and molecular hydrogen, respectively. In order to determine if the system is stable or not, we must look at *λ. * When we solve for λ (see appendix), the resulting expressions are:

*λ = 0.5(kaM*_{H2}^{0}* – r – aM*_{s}^{0}* ± √(( -kaM*_{H2}^{0}* + r + aM*_{s}^{0}*)*^{2}* – 4arM*_{s}^{0 } *))*

If any of the following conditions are met, the system will be stable:

*[(-kaM*_{H2}^{0}* + r + aM*_{s}^{0}*)*^{2}* *>* 4arM*_{s}^{0} AND (*kaM*_{H2}^{0}* – r – aM*_{s}^{0}*) *<* 0]*

OR [0 < *0.5(kaM*_{H2}^{0}* – r – aM*_{s}^{0}* ± √(( -kaM*_{H2}^{0}* + r + aM*_{s}^{0}*)*^{2}* – 4arM*_{s}^{0 }*))]*

Since the mass functions are proportional to exp(λt), stability can only occur when either λ < 0 or λ is complex with a real part < 0 and an imaginary part ≠ 0.

If we make the following assumptions:

*1/r = 1, k = 0.02, M*_{H2}^{0}*, A = 3, a = 0.5 *and* M*_{s}^{0}* = 0.06* (3)

We get the following roots:

*λ = -0.0150 + 0.172554i*

*λ = -0.0150 - 0.172554i*

Since λ is a complex number and the real part of λ < 0 and the imaginary part of λ ≠ 0, we know that the system is oscillatory with a maximum amplitude that dampens over time and is therefore stable.

When the substitutions in (3) are made, and a non-equilibrium occurs with a small perturbation in the mass of molecular hydrogen, *m*_{H2}, over time we should expect amplitudes of the masses for both the massive stars and the molecular hydrogen to oscillate and eventually dampen out into an equilibrium state. An increase in the amount of molecular hydrogen in the system from, say, a collision with a small galaxy, would increase the total molecular hydrogen of the system and induce star formation. The induced star formation would then decrease the amount of molecular hydrogen in the system and thus decrease the rate of induced star formation, etc. This cycle would continue until an equilibrium in the mass of massive stars and the mass of molecular hydrogen is reached.

If we assume the system is initially in a slightly non-equilibrium state where

*M*_{H2 = } *1.1M*_{H2}^{0}* * and *M*_{s = }*M*_{s}^{0 } and use the same values as (3) and adopt the units of mass 10^{7}M and time 10^{7} yr for a numerical simulation model we observe the behaviour of the masses of massive stars and molecular hydrogen in the system over a period of 5 billion years as follows:

The masses exhibit an oscillatory behaviour. The model shows that the initial non-equilibrium caused induced massive star formation. This led to a decrease in the amount of molecular hydrogen, which in turn, led to a decrease in the rate of induced star formation which continues to oscillate and dampen until equilibrium is reached.

The predictions of the stability analysis agree with the numerical simulation.

**Conclusion**By considering the ordinary differential equations that govern the phase transitions between massive stars and molecular hydrogen in a model galaxy, the stability of the system can be determined by knowing and/or assuming certain characteristics of the system.

In a case where * 1/r = 1, k = 0.02, M*_{H2}^{0}*, A = 3, a = 0.5 *and* M*_{s}^{0}* = 0.06, *and an initial non-equilibrium of *M*_{H2 = } *1.1M*_{H2}^{0}* * and *M*_{s = }*M*_{s}^{0}, the system is oscillatory and becomes stable over time.

**Appendix**

Matlab code:

**l = -0.0150 - 0.1726i;**

r = 1;

k = 0.02;

A = 3;

Ms0 = 0.06;

Mh0 = 100;

dMsdt = 0;

dMhdt = 0;

Ms = Ms0;

Mh = Mh0*1.1;

a = 0.5;

ms = 0;

mh = 0;

for t = 1:500;

dMsdt = -r*Ms+k*a*Ms*Mh;

dMhdt = -a*Ms*Mh+A;

ms = Ms - Ms0 + dMsdt;

mh = Mh - Mh0 + dMhdt;

a = real(-(dMhdt-A)/(Ms*Mh));

Ms = real(Ms0 + ms*exp(l));

Mh = real(Mh0 + mh*exp(l));

Msf(t) = Ms;

Mhf(t) = Mh;

end;