3.1 Hydrogen in SFR

Question 1

What is the source of radiation in the SFR?

Answer 1

Hot young stars emitting mostly UV photons

Question 2

What is the equation for the ionisation rate?

Answer 2

N_ion = Energy output * ε / E needed to ionise
N_UV = εL / 13.6 eV

Question 3

What is ε in the ionisation rate equation, and what is it equal to for a hot star?

Answer 3

ε is the fraction of the energy output emitted in the UV

- Roughly = 1 for a hot star

Question 4

What is the recombination rate equal to?

Answer 4

N_rec = V_HII * n_p * n_e * α_2

= Number of available protons, number density of electrons and the recombination constant

Question 5

If Hydrogen atoms recombine quickly, what is the relationship between the ionisation rate and the recombination rate?

Answer 5

N_rec = 4/3 pi r^3 * n_p * n_e * α_2 = N_ion

Question 6

For a typical star, what is the radius of the volume that it can ionise?

Answer 6

Roughly 1pc

- Distance to the nearest star from the sun

Question 7

What two things does the distance travelled by radiation depend on?

Answer 7

The absorption coefficient (-κ * I) and the emissivity (η)

Question 8

What does the intensity of the radiation depend on?

Answer 8

The composition of the medium (through the source function S)
The total absorption of the medium (through optical depth τ)

Question 9

What is the source function S?

Answer 9

A ratio of the emissivity η and the absorption coefficient κ

Question 10

What is the optical depth, dτ?

Answer 10

dτ = -κ dx

Question 11

What is an optically thick medium, and describe its properties?

Answer 11

When τ&raquo_space; 1 (MW centre) and e^-τ = 0 so I ~ S

• Medium does not preserve information about original path of radiation
• Only the surface is accessible e.g. forest

Question 12

What is an optically thin medium, and describe its properties?

Answer 12

When τ &laquo_space;1 (MW from any direction not from centre)
e^-τ ~ (1-τ), I ~ ( S - I(0) ) τ
- Intensity of radiation is proportional to total absorption along l.o.s

Question 13

Describe the spectrum on a flux wavelength graph that would be observed when directly looking at stellar cores?

Answer 13

Would see a continuous negative skewed parabola

Question 14

Describe the spectrum on a flux wavelength graph that would be observed when directly looking at stellar cores through a gas cloud?

Answer 14

Would see the continuous negative skewed parabola, but you would see defecits due to the absorption of photons at H lines

Question 15

Describe the spectrum on a flux wavelength graph that would be observed when not directly looking at stellar cores?

Answer 15

You would only see the emission lines from photons that have re-emitted isotropically
- No continuous spectrum, just lines

Question 16

What effect dominates when observing stellar cores through a cloud of gas - emission or absorption?

Answer 16

Emission always dominates as it can occur in any random direction. Absorption is over one line of sight

Question 17

What do we expect the stellar spectra of a galaxy to be made up of?

Answer 17

A composite of the old stars and the young stars

- Depends on the recent star formation and galaxy type etc.

Question 18

What is the spectrum of SFR made up of?

Answer 18

Hot cores (BB) and emission lines (H series)
- Emission lines come from the recombination of H

Question 19

What do we expect the stellar spectra of an elliptical galaxy to be made up of?

Answer 19

cD gE E
Old stellar populations with little gas - composite spectrum similar to G and K stars
- Peak in the optical and absorption lines from H and metals

Question 20

What do we expect the stellar spectra of a spiral galaxy to be made up of?

Answer 20

SB S
Young stellar populations like OBA stars + gas emission lines (H II)
- Old stars contribute, but out shone by young stars