Lecture 13 - Heating & Cooling of HI Gas Flashcards
1
Q
HI gas is mainly cooled by fine-structure excitation of…
A
CII
2
Q
What percentage of C is in the gas phas in form of C+, in HI gas?
A
About 50%
3
Q
In HI gas, can we neglect collisional de-excitation
A
Yes
4
Q
In HI gas, every collisonal excitation leads to…
A
radiative decay
5
Q
What are the four heating mechanisms for HI clouds?
A
Cosmic rays, X-rays, Starlight (photoionization) and Hydromagnetic waves
6
Q
H + CR –> ?
A
H + CR –> H+ + e- + CR
7
Q
X-rays can devided into two subclases. Explain them
A
Hard X-rays have energy over 250 eV. They penetrate clouds.
Soft X-rays have energy close to 100 eV. They don’t penetrate clouds
8
Q
What does the Two Phase model of the ISM claim?
A
That there are two stable phases of the ISM
