Section 7 Flashcards
When does fragmentation stop?
The collapsing core becomes so dense that it becomes opaque to its own radiation so it can no longer efficiently radiate energy released during collapse
What happens when fragmentation stops?
The internal temperature and therefore pressure increases
Eventually an equilibrium is reached between internal pressure and gravity
Halts free fall collapse in centre
What is formed as a result of the halt of free fall collapse?
Hydrostatic core
What does a hydrostatic core cause to develop?
A shock front develops at interface between the core and the envelope
Infall velocity at shock = free fall velocity
How is the free fall velocity derived?
Equating KE to GE
What happens when the hydrostatic core heats up?
There is potential for further collapse
What happens if the heat capacity ratio is greater than 4/3?
Jeans mass increases as T increases and core is STABLE
What happens if the heat capacity ratio is less than 4/3?
Jeans mass decreases as T increases and core is UNSTABLE leading to further collapse
What is γ?
The ratio of specific heat capacity at constant pressure and constant volume
f + 2/f
What happens within the protostar as time goes on that might disrupt its stability?
There is a hydrostatic core that is accreting molecular material from a free-falling outer envelope
Temperature and density are both increasing
A point will be reached at which molecules and dust will dissociate
What are degrees of freedom?
The threshold for a given type of particle in gas e.g atomic
What are the degrees of freedom for monatomic gas?
f = 3 (translational)
What are the degrees of freedom for diatomic molecules?
f = 5 (3 translational, 2 rotational)
What happens when T = 2000K under high pressure conditions in the core?
Collisions lead to molecular hydrogen and dust becoming dissociated and atomised (into its constituent atoms) so f goes from 5 to 3
What happens as a result of every dissociation of H_2 event in the core?
4.5 eV of energy is absorbed that goes into breaking that bond (removing energy from core and pressure, affecting the hydrostatic equilbrium)
What does the dissociation of the gas from molecular to atomic hydrogen cause?
Energy from gas is removed and core becomes dynamically unstable and undergoes second collapse
What is the size of the second core?
a few R_o (2R_o) and hydrostatic equilibrium is reached but accretion of mass continues
What does the small mass of the secondary core mean?
It is not sufficient enough to increase the density to the point where nuclear fusion can start
What happens to the density throughout?
Outer edges of core have same density as molecular cloud that collapsed (seen in graph)
There is a jump in density due to first shock front in first core
When molecular hydrogen dissociates, second phase of collapse causes density to increase again and leads to second shock front
What are the conditions for nuclear fusion?
density = 100 gcm^-3
temperature = 1 million K
What is the size of the first hydrostatic core?
5 AU
