Section 6

Question 1

What happens if the core temperature of a cloud remains constant during the initial stages of the collapse?

Answer 1

The Jeans mass will decrease as density increases

Initial collapse is isothermal

Question 2

What is the main heating process in molecular clouds?

Answer 2

Cosmic-ray ionisation of molecular hydrogen (H_2) (which generates electrons which heat molecular cloud)

Question 3

What do cosmic rays consist of and why are they subjected to magnetic deflection?

Answer 3

They are made of relativistic protons, with a mixture of heavy elements and electrons

They are all charged

Question 4

How are cosmic rays withe energies up to 10^9 GeV produced?

Answer 4

By particle acceleration within the magnetised shocks created by supernova remnants

Question 5

What does a gyrating cosmic ray interact with in a molecular cloud, which causes heating?

Answer 5

Ambient nuclei and electrons through both Coulomb and nuclear forces
Excitations (Ep > 1GeV) cause the emission of gamma rays

Question 6

What provides the heat after a proton scatters with hydrogen?

Answer 6

secondary electron which has subsequent collisions with H_2

Question 7

How does an electron provide heating?

Answer 7

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)

Question 8

Why is there an inelastic collision of electrons?

Answer 8

So the electron carries most of the energy away

Question 9

What is the net energy provided by a single 10MeV proton?

Answer 9

change in H_2 energy = 7eV

Question 10

What is the ionisation rate?

Answer 10

Probability per unit time

Question 11

Why are molecular clouds (10K) warmer than the Cosmic Microwave Background (2.7K)?

Answer 11

The heating (through proton scatter and electron dissociation) is balanced by cooling, giving an equilibrium temperature of around 10K in a typical molecular cloud

Question 12

What is the main cooling mechanism due to?

Answer 12

CO rotational emission

Question 13

What does the rate of loss of thermal energy of Co depend on?

Answer 13

Number density of CO molecules in the cloud
Energy of transition
Optical depth of emitted lines

Question 14

What is the typical rate of loss of thermal energy of CO?

Answer 14

10^-23 Js^-1m^-3

Question 15

What is the cooling rate sensitive to?

Answer 15

Temperature

Doubling T increases the rate by more than an order of magnitude

Question 16

What is the cooling time?

Answer 16

thermal energy density divided by the rate of energy loss

Question 17

For a typical molecular cloud what is the cooling time?

Answer 17

7000 years

Question 18

What is the cooling time shorter than?

Answer 18

The free fall time

Question 19

What happens to the cooling time with the respect to the free fall time when density is increased?

Answer 19

Ratio increases (faster) as it is proportional to n^3/2

Question 20

What causes the cosmic ray ionisation to vary?

Answer 20

Distance from galactic centre due to it being denser and a sight of supernova remnants

Question 21

What leads to fragmentation?

Answer 21

When smaller subunits in molecular cloud become unstable and collapse

It helps understand how solar mass stars are formed

Question 22

When does a large cloud begin to collapse?

Answer 22

When it becomes sufficiently dense, leading to fragmentation

Question 23

How is a protostar formed?

Answer 23

When a fragment can collapse independently without further disruption

Question 24

When does the molecular cloud remain isothermal during collapse?

Answer 24

When the grav potential energy released during the collapse is efficiently radiated away i.e cloud must remain optically THIN

Question 25

What happens if cloud is optically thick?

Answer 25

It can no longer cool efficiently and fragmentation is halted

Question 26

What happens once a fragment is opaque?

Answer 26

It will radiate almost as a black body (at T=10K)

Question 27

For minimum fragment mass, what does the rate of energy lost due to radiation equate to?

Answer 27

Rate of gain in gravitational potential energy (for typical cloud = 2 Jupiter masses)

Question 28

At what mass does collapse occur if Jeans mass is considered?

Answer 28

5 M_o