Section 8 Flashcards
What happens when a cloud core is only supported by thermal pressure?
kinetic energy = thermal energy
Why are there emissions over a range of velocities?
Due to doppler shifting
What is the shape of the emission line?
Gaussian shaped with a dispersion on the order of 0.1km s^-1
Full width half maximum is about 2.3 times the dispersion
How are molecular sizes in clouds mapped out?
By their element components
What happens when the size of the cloud is increased?
The measured velocity dispersion increases almost linearly in log log space
Larger width measured
What do the smallest clouds approach and what does this mean?
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
What do non-thermal velocities observe dominate over?
Thermal component
What is Larson’s law?
An empirical relationship between line width and cloud (core ) size
(holds for single molecular cloud complexes)
What is the timescale for large changes in the structure of a molecular cloud?
The crossing time and it around 2 times the free-fall time
The crossing time disturbs star formation due to dispersion
What is the timescale of observed internal motion?
Time taken for pressure waves to propagate through cloud
What occurs during the timescale?
Dissipation of turbulent motions and gravitational collapse and star formation in some parts of the cloud
What occurs in the crossing time?
The cloud is partially or completely dispersed or restructured by effects of stellar winds or HII regions
Large structural changes take place
What does Larson’s law show?
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
Why do larger clouds live longer?
It takes longer for pressure wave to travel through them
What does combining Jeans mass and the free-fall time of a typical cloud show?
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
What does the low rate of actual star formation and timescale of dispersion show?
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
What is another potential source of cloud support?
Magnetic fields and turbulence
Where does rotation source of support come from?
from Galactic rotation or cloud-cloud collisions
small compared to gravitational energies
Are magnetic fields a source of support?
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
What serves as evidence for supersonic turbulence?
Supersonic line widths which are used to determine properties such as lifetime, morphology and star formation rate
What is turbulance?
When kinetic energy cascades from large scales to small scales
What is the issue with turbulance?
It decays very quickly which effects formation and lifetime of GMCs
