Section 8

Question 1

What happens when a cloud core is only supported by thermal pressure?

Answer 1

kinetic energy = thermal energy

Question 2

Why are there emissions over a range of velocities?

Answer 2

Due to doppler shifting

Question 3

What is the shape of the emission line?

Answer 3

Gaussian shaped with a dispersion on the order of 0.1km s^-1

Full width half maximum is about 2.3 times the dispersion

Question 4

How are molecular sizes in clouds mapped out?

Answer 4

By their element components

Question 5

What happens when the size of the cloud is increased?

Answer 5

The measured velocity dispersion increases almost linearly in log log space
Larger width measured

Question 6

What do the smallest clouds approach and what does this mean?

Answer 6

Thermal velocity dispersion which means that the objects have a FWHM larger than expected if gas only had thermal motion
They are moving faster than they should so other mechanism is causing them to move this way

Question 7

What do non-thermal velocities observe dominate over?

Answer 7

Thermal component

Question 8

What is Larson’s law?

Answer 8

An empirical relationship between line width and cloud (core ) size

(holds for single molecular cloud complexes)

Question 9

What is the timescale for large changes in the structure of a molecular cloud?

Answer 9

The crossing time and it around 2 times the free-fall time

The crossing time disturbs star formation due to dispersion

Question 10

What is the timescale of observed internal motion?

Answer 10

Time taken for pressure waves to propagate through cloud

Question 11

What occurs during the timescale?

Answer 11

Dissipation of turbulent motions and gravitational collapse and star formation in some parts of the cloud

Question 12

What occurs in the crossing time?

Answer 12

The cloud is partially or completely dispersed or restructured by effects of stellar winds or HII regions

Large structural changes take place

Question 13

What does Larson’s law show?

Answer 13

Even the largest molecular cloud complexes must be rather transient and will completely restructure if not completely disperse after a few times 10^7 years

Question 14

Why do larger clouds live longer?

Answer 14

It takes longer for pressure wave to travel through them

Question 15

What does combining Jeans mass and the free-fall time of a typical cloud show?

Answer 15

That our galaxy should be highly unstable to gravitational collapse and that star formation rates must be around 200-400 M_o but in reality only 3M_o per year

Question 16

What does the low rate of actual star formation and timescale of dispersion show?

Answer 16

That molecular clouds cannot be supported by thermal pressure alone

Free-fall timescale can only act as a lower limit to the cloud collapse time

Question 17

What is another potential source of cloud support?

Answer 17

Magnetic fields and turbulence

Question 18

Where does rotation source of support come from?

Answer 18

from Galactic rotation or cloud-cloud collisions

small compared to gravitational energies

Question 19

Are magnetic fields a source of support?

Answer 19

Magnetic fields can support clouds if B is sufficiently high

Perturbations in a magnetic cloud can give rise to magnetohydrodynamic (MHD) waves called Alfven waves

Question 20

What serves as evidence for supersonic turbulence?

Answer 20

Supersonic line widths which are used to determine properties such as lifetime, morphology and star formation rate

Question 21

What is turbulance?

Answer 21

When kinetic energy cascades from large scales to small scales

Question 22

What is the issue with turbulance?

Answer 22

It decays very quickly which effects formation and lifetime of GMCs