Section 6 Flashcards
What happens if the core temperature of a cloud remains constant during the initial stages of the collapse?
The Jeans mass will decrease as density increases
Initial collapse is isothermal
What is the main heating process in molecular clouds?
Cosmic-ray ionisation of molecular hydrogen (H_2) (which generates electrons which heat molecular cloud)
What do cosmic rays consist of and why are they subjected to magnetic deflection?
They are made of relativistic protons, with a mixture of heavy elements and electrons
They are all charged
How are cosmic rays withe energies up to 10^9 GeV produced?
By particle acceleration within the magnetised shocks created by supernova remnants
What does a gyrating cosmic ray interact with in a molecular cloud, which causes heating?
Ambient nuclei and electrons through both Coulomb and nuclear forces
Excitations (Ep > 1GeV) cause the emission of gamma rays
What provides the heat after a proton scatters with hydrogen?
secondary electron which has subsequent collisions with H_2
How does an electron provide heating?
Through dissociation:
energy of electron big enough to dissociate H_2 and transfers kinetic energy to 2 hydrogen atoms (subsequent collisions disperses this energy to gas)
Why is there an inelastic collision of electrons?
So the electron carries most of the energy away
What is the net energy provided by a single 10MeV proton?
change in H_2 energy = 7eV
What is the ionisation rate?
Probability per unit time
Why are molecular clouds (10K) warmer than the Cosmic Microwave Background (2.7K)?
The heating (through proton scatter and electron dissociation) is balanced by cooling, giving an equilibrium temperature of around 10K in a typical molecular cloud
What is the main cooling mechanism due to?
CO rotational emission
What does the rate of loss of thermal energy of Co depend on?
Number density of CO molecules in the cloud
Energy of transition
Optical depth of emitted lines
What is the typical rate of loss of thermal energy of CO?
10^-23 Js^-1m^-3
What is the cooling rate sensitive to?
Temperature
Doubling T increases the rate by more than an order of magnitude
What is the cooling time?
thermal energy density divided by the rate of energy loss
For a typical molecular cloud what is the cooling time?
7000 years
What is the cooling time shorter than?
The free fall time
What happens to the cooling time with the respect to the free fall time when density is increased?
Ratio increases (faster) as it is proportional to n^3/2
What causes the cosmic ray ionisation to vary?
Distance from galactic centre due to it being denser and a sight of supernova remnants
What leads to fragmentation?
When smaller subunits in molecular cloud become unstable and collapse
It helps understand how solar mass stars are formed
When does a large cloud begin to collapse?
When it becomes sufficiently dense, leading to fragmentation
How is a protostar formed?
When a fragment can collapse independently without further disruption
When does the molecular cloud remain isothermal during collapse?
When the grav potential energy released during the collapse is efficiently radiated away i.e cloud must remain optically THIN
What happens if cloud is optically thick?
It can no longer cool efficiently and fragmentation is halted
What happens once a fragment is opaque?
It will radiate almost as a black body (at T=10K)
For minimum fragment mass, what does the rate of energy lost due to radiation equate to?
Rate of gain in gravitational potential energy (for typical cloud = 2 Jupiter masses)
At what mass does collapse occur if Jeans mass is considered?
5 M_o
