Massive Star Formation

Question 1

What is the stellar birthline?

Answer 1

line in HR diagram (L vs Teff) which below which the young Pre-Main Sequence stars become visible

Question 2

What do the massive stars seem to coincide with?

Answer 2

The Main sequence

Question 3

What is the main sequence?

Answer 3

a series of types to which most stars belong, represented on a Hertzsprung–Russell diagram as a continuous band extending from the upper left (hot, bright stars) to the lower right (cool, dim stars).

Question 4

What is the Kelvin-Helmholtz timescale?

Answer 4

• The timescale for contraction.
• Protostar needs to contract further before it can begin H2 burning and join the main sequence
• t k-h = GPE/Luminosty = GM^2/RL
• M=m bc star is attracting itself

Question 5

How does the protostar contract?

Answer 5

Radiates away energy released by gravitational energy

Question 6

what is the Mass Luminosity Relation?

Answer 6

L proportional M^4

Question 7

Does a massive star have a visible pre-main sequence phase?

Answer 7

• Kevin-Helmholtz timescale &laquo_space;freefall time scale
  ie contraction is rlly quick
  -massive stars arrive on the Main Sequence while still embedded in their molecular clouds
  -so still accreting material and radiating in IR so invisble in MPS phase

Question 8

What is the typical output of massive stars?

Answer 8

UV radiation

Question 9

what do energetic photons do in the massive stars?

Answer 9

• dissociate HII (ionised atomic hydrogen) regions and ionize HI (atomic hydrogen)
• HI is recreated by proton electron recombination
• ionization absorbs a photon
• stars with a fixed output in UV radiation can only ionize a limited region in surrounding cloud

Question 10

what can bet determined from the equilibrium state of the Strömgren sphere?

Answer 10

The volumetric photo ionization rate = p e- recombination rate

Question 11

State the formula for the volumetric rate free e- and p combine

Answer 11

R = nenpa(T) = n^2 *a rec (T)

can integrate R for total rate of recombinations

Question 12

What is the total rate of recombinations?

Answer 12

can integrate volumetric rate, R for total rate of recombinations.
N = (4/3pi)n^2a rec (T)Rs^3

Question 13

what can we same the number density of electrons is equal to the number density of H atoms external to Rs?

Answer 13

Ionization spreads so quickly to the Strömgren Radius its original cloud density will no change appreciably

Question 14

what do HII regions emit?

Answer 14

Ionized gas emits radio continuum, free-free radiation. This radiation is bright, not absorbed by dust

Question 15

What us the Strömgren Sphere?

Answer 15

The size of the ionized region (HI) found from the ionization balance

Question 16

Why does the ionization front expand?

Answer 16

• Due to overpressure

- caused by at Rs there are different pressures (ionized HI has 2x n and T=10,000K from UV photons)

Question 17

What is the sequence for embedded massive stars?

Answer 17

1) InfraRed Dark Cloud
2) Hot Core
3) Massive Young Stellar Object
4) Hyper and Ultra compact HII region
5) Compact and Classical HII region

Question 18

Describe the Massive Young Stellar Object phase

Answer 18

• emits in Mid to IR
• Luminous >10^4 L0
• radio quiet as UCHII has not formed
• Bipolar molecular outflow as accretion still takes place

Question 19

Describe UCHII regions

Answer 19

• smaller L (L < 10^17) than optically visible HII regions (L > 10^18)
• larger ne (ne > 10^5 cm^-3) than optically visible HII regions (ne < 10^4 cm^-3)
• most luminous objects in Galaxy in Far IR