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Virial Analysis
Hydrodynamic equation of motion using the Lagrangean time derivative [eq.(A.3)] is
|
(4.17) |
Multiplying the position vector r and integrate over a volume of a cloud, we obtain the Virial relation as
|
(4.18) |
where
|
(4.19) |
is an inertia of the cloud,
|
(4.20) |
is a term corresponding to the thermal pressure plus turbulent pressure,
|
(4.21) |
comes from a surface pressure, and
|
(4.22) |
is a gravitational energy.
To derive the last expression in each equation, we have assumed the cloud is spherical and uniform.
Here we use a standard notation as the radius , the volume
,
the average pressure , and the mass .
To obtain a condition of mechanical equilibrium, we assume .
Equation (4.18) becomes
|
(4.23) |
Assuming the gas is isothermal
, the average pressure is written as
|
(4.24) |
Using equation (4.24) to eliminate from equation (4.18),
the external pressure is related to the mass and the radius as
|
(4.25) |
Keeping constant and increasing from zero, increases first, but it takes a maximum,
, and finally declines.
This indicates that the surface pressure must be smaller than
for the cloud
to be in an equilibrium.
In other words, keeping and changing , it is shown that has a maximum value to
have a solution.
The maximum mass is equal to
|
(4.26) |
The cloud massive than cannot be supported against the self-gravity.
This corresponds to the Bonnor-Ebert mass [eq.(4.9)],
although the numerical factors are slightly different.
Subsections
Next: Magnatohydrostatic Clouds
Up: Local Star Formation Process
Previous: Equilibria of Cylindrical Cloud
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Kohji Tomisaka
2007-07-08