3.2 Dust in the ISM Flashcards
How does dust interact with radiation?
It effectively reprocesses it
It can scatter, absorb and emit it
What is dust made up of?
Mainly small (0.1-1)μm grains of Si, C, O
What are the two sources of dust origin?
Stellar winds and supernova remnants
Describe the origin of dust from stellar winds
Very young stars/evolved stars (not MS) can expel the metal rich outer layers of their atmosphere
Describe the origin of dust from supernova remnants
Metal rich material cooling down after explosions
What percent of mass and light does dust contribute in the MW?
It is 0.1% of the mass, but reprocesses 30% of the light
Define extinction
The change in magnitude between the magnitude measured in the presence of dust and the magnitude measured without the presence of dust
How does distance affect distance indicators?
It increases the apparent magnitude which increases the DM
- Objects appear further away
What are the three regimes of scattering and its wavelength dependence?
Small grains (r << λ) Intermediate grains ( r ~ λ) Big grains (r > λ)
Describe what happens in the small grain regime of scattering (r «_space;λ)
r - radius of dust
Called Rayleigh scattering
- Causes bluer photons to scatter more effectively than redder photons
σ ~ λ^-4
Describe what happens in the intermediate grain regime of scattering (r ~ λ)
Mie scattering σ ~ λ^-1
Describe what happens in the big grain regime of scattering (r»_space; λ)
No dependence on wavelength anymore, σ is constant
What is fragmentation, and what is a consequence of it?
Dust grains can collide and fragment, so small grains always out number big grains so Rayleigh scattering is important
Observe in IR
How can we measure relative extinction?
Measure extinction at two different wavelengths and call it reddening
reddening = E(B-V) = A_B - A_V
where B and V are the blue and visible bands
What relationship does the extinction law provide?
A relation linking reddening to a parameter called K_λ