Lecture 14 - Molecular clouds Flashcards
What are the densities of dark clouds
100 to 10000 cm^-3
What are the densities of giant molecular clouds?
100 to 1000 cm^-3
Surveys of molecular gas are biased toward…
large and warm clouds (with thick lines)
How quickly does the sun move around the center of the Milky Way? (What’s the orthogonal velocity?)
220 m/s
When is H+H+X important as a reaction to create H2?
When the densities are much higher than in the interstellar medium
How are H2 created in the interstellar medium?
H sticks on the surface of dust grains and react there on.
Why are there more atomic hydrogen near the edges of molecular clouds?
Because the photodissociation is caused by UV-emission which can’t penetrate all the way in to the center
If you see warmer, broad lines coming from a cloud, what type of cloud is it probably?
Giant Molecular Cloud
Dark clouds have ___ and ___ lines
cold, narrow
Why are our telescopes restricted to large (or close) molecular clouds?
Because the 12CO J=1->0 transition is the strongest emission from molecular clouds and its wavelength is 2.6 mm. The angular resolution is therefore limited
What problem arises on the locations when kinematic distances are obtained from assuming circular orbits?
The distance ambiguity and uncertainty of about 1 kpc due to non-circular motions
In the mass spectrum is has exponent alpha larger magnitude for giant molecular clouds or for stars and what are the implications?
The stars have a larger value for alpha. It is over one so most of the stellar mass is in small stars. Giant molecular clouds have most mass in the largest clouds since alpha is less than one (0.6 more precisely)
How do radiative association and electron detachment look for hydrogen?
Radiative association: H + e- –> H- + h*nu
Electron detachment: H- + H –> H2 + e-
On what levels is isotopic shifts relevant for the emission lines?
Rotational and vibrational, but not electronic
