6.3

Question 1

Interstellar Gas Types

Answer 1

Different types of interstellar gas in the Milky Way disk include:

• Molecular cloud
• Diffuse atomic gas
• H II-regions
• Diffuse photo-ionized gas
• Diffuse collisionally ionized gas

Many Daring Heroes Defeat Dragons

Question 2

Molecular Cloud

Answer 2

• Temperature: 10 - 100 K
• Particle Density: 10⁹ - 10¹² m^-3
• Main Particles:
  – H₂
  – dust
  – CO

Question 3

Diffuse Atomic Gas

Answer 3

• Temperature: 100 - 1’000 K
• Particle Density: ≈ 10⁷ m^-3
• Main Particles:
  – H⁰
  – dust
  – C⁺
  – e^-
  – N⁰
  – O⁰

Question 4

H II-Regions

gas properties

Answer 4

• Temperature: ≈ 10’000 K
• Particle Density: 10⁷ - 10¹⁰ m^-3
• Main Particles (same as diffuse photo-ionized):
  – H⁺
  – e^-
  – dust
  – X⁺ⁱ

Question 5

Diffuse Photo-ionized Gas

Answer 5

• Temperature: ≈ 10’000 K
• Particle Density: ≈ 10⁵ m^-3
• Main Particles (same as H II):
  – H⁺
  – e^-
  – dust
  – X⁺ⁱ

Question 6

Diffuse Collisionally Ionized Gas

Answer 6

• Temperature: > 10⁶ K
• Particle Density: ≈ 10³ m^-3
• Main Particles:
  – H⁺
  – e^-
  – X⁺ⁱ

Question 7

Baryonic Mass of ISM

Answer 7

90% of baryonic mass of the Interstellar Medium (ISM) is composed of:

• molecular gas
• diffuse atomic gas

Question 8

Space Filling Components

Answer 8

Components 2 and 5 (Diffuse Atomic Gas and Diffuse Collisionally Ionized Gas) essentially fill the space in ISM.

are we sure he doesnt maybe mean 4 and 5 given their low densities and that 2nd component was already mentioned as part of 90%? (this is where the golden standard of Notes, Loulou’s Notes, come in)

I will check my notes and update hahaha

thank you very much!

Question 9

Physical Conditions in ISM (4)

Answer 9

ISM conditions are:

• Not in (very far from) thermal equilibrium
• Very low particle density (≈ 1 cm^-3)
• Weak radiation field
• Most particles in ground state

Question 10

Molecular Gas Properties

not the numbers

Answer 10

Cold, dense regions, molecular gas like H₂ and CO is shielded from UV-radiation by dust.

I believe there is a similar card to this but with temperature and density etc

Question 11

Atomic Diffuse Gas Properties

properties so not the numbers

Answer 11

• Composed of atoms
• UV-radiation (> 100 nm) dissociates molecules

i am so confused, I could have sworn the temperature and densities where on here, but i guess there must be another card like this, or ive started dreaming / hallucinating about astrophysics hahah :)

Question 12

Photoionized Gas Properties

a better title might be: what photoionises gas?

Answer 12

Hot stars (> 30’000 K) photoionize the gas, contributing to gas heating.

Question 13

Collisionally Ionized Gas Properties

properties so not the numbers

what heats it?

Answer 13

Hydrodynamic shocks heat gas to very high temperatures (> 10⁶ K), leading to collisional ionization.

Question 14

Interstellar Gas in Solar Neighborhood

properties and how is it observed

Answer 14

• “Hot Bubble” of collisionally ionised gas
• Diffuse distribution, observed as line absorption towards nearby (100 pc) background stars.
• Typical tracers include:
  – Ca^+
  – Na^0
  – H^0
  – C^+3
  – N^+4
  – O^+5
  – Si^+3

not sure if im learning all the tracers

Question 15

Interstellar Gas at Larger Distances > 300 pc

Answer 15

Line emission studies are difficult due to:
- Saturation
- Unclear location along line of sight

Question 16

Line Emissivity for Collisionally Excited Lines

Answer 16

Emissivity per steradian from decays of upper state to lower state, determined by the density of upper state.

steradian: area on sphere (i hope thats correct)

this might need some context

Question 17

Two Level Atom

how is equilibrium characterised?

Answer 17

Equilibrium for a two-level atom is characterized by:

• relationship between collisional rates
• energy differences between atomic states