SLIDE 1
Ay 102 Physics of the Interstellar Medium
supplemental material Hillenbrand – Winter Term 2019-2020
The Dynamic ISM
Ay 102 Physics of the Interstellar Medium supplemental material - - PowerPoint PPT Presentation
Ay 102 Physics of the Interstellar Medium supplemental material - - PowerPoint PPT Presentation
Ay 102 Physics of the Interstellar Medium supplemental material Hillenbrand Winter Term 2019-2020 The Dynamic ISM The Dynamic ISM What happens when flow velocities exceed the local pattern speed?
SLIDE 2
SLIDE 3
https://www.youtube.com/watch?v=Suugn-p5C1M&NR=1&feature=fvwp
The Dynamic ISM
What happens when flow velocities exceed the local “pattern speed”?
https://www.youtube.com/watch?v=Q78Kb4uLAdA
SLIDE 4
The Dynamic ISM
Earth B-field vs Solar wind Solar wind vs local ISM NASA/IBEX
Shocks are everywhere, occurring
- n all astrophysical scales.
SLIDE 5
The Dynamic ISM
Jets from Young Stars
Russell CromanAstrophotography
Stellar Winds
SLIDE 6
Red = x-ray Green = forbidden optical line
The Dynamic ISM
Supernova Remnant Guitar Nebula: pulsar moving @2000 km/s!
charge exchange
High velocity “black widow” pulsar
SLIDE 7
The Dynamic ISM
Cloud-Cloud Collisions HII Regions
SLIDE 8
Galactic Spiral Density Waves
The Dynamic ISM
SLIDE 9
Collision of gas leads to shocks - > star formation
The Dynamic ISM
The Antennae Galaxies
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Dynamics Nomenclature
Mach number, M = v / cs
- v >> cs è strong shock
- v ≥ cs è weak/mild shock
- v < cs è no shock
SLIDE 11
Dynamics – Getting to the Math
Maoz
Adopt a frame in which the shock is stationary. Cold ”pre-shock” / “upstream” gas moves into the shock at high velocity. Hot “post-shock” / “downstream” gas moves away with |v2 | < |v1|. Consider shock to be plane-parallel, such that properties of the fluid depend only
- n the linear distance, x, and all v’s are vx.
Neglect viscosity except in the shock transition zone, Δx, where large dv/dx means kinetic energy transformed into heat (viscous dissipation). Δx è 0 is a discontinuity or “jump”.
Δx
SLIDE 12
Dynamics – Getting to the Math
Arce “downstream” “upstream” Radiative shock: cools by emitting radiation - more efficiently than via adiabatic cooling v ~ 10’s to 100’s of km/s n ~ 104 – 105 cm-3 Non-radiative shock: cools adiabatically, by expansion - more efficiently than by emitting radiation v ~ 1000 - 104 km/s n ~ 103 – 104 cm-3 (note: opposite
- rientation from
previous slide)
SLIDE 13
Dynamics – The Math
“downstream” “upstream”
Conserve mass Conserve momentum Conserve Energy
SLIDE 14
density pressure velocity
ρ1 P1 u1 u2 ρ2 P2
Non-Radiative Shocks - Do not Cool Efficiently
Shu
✗ ✗
“continuous” density pressure velocity
u2 ρ2 P2 ρ1 P1 u1
“jump”
- case of Δx ~ few mean free paths, i.e a
transition zone è continuous shock
- case of è jump shock
SLIDE 15
Non-Radiative Shocks – Do not Cool Efficiently
Shu/ Goodman (to the right, pay attention to the axis notation - velocity decreases after the shock, but is plotted as u1/u2 instead of u2/u1 like the other quantities.)
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Note:
- Shocks are irreversible processes.
- Thus, entropy is not in fact conserved.
- Hence, the truly adiabatic, non-radiative case is fictitious.
- A more appropriate term is “viscous shock” with viscosity ν ~ l * vshock
- These are usually M1 >> 1 circumstances with high vshock and low ρ.
Non-Radiative Shocks – Do not Cool Efficiently
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Radiative Shocks – Cool Efficiently
Shu L = L (ρ, T) = Λ – Γ “net cooling function”
SLIDE 18
Radiative Shocks – Special Case of “Isothermal”
??? via Goodman NOTE: notation here uses only 1 è 2 whereas previous slides had 1 è2 compression/heating and then cooling è 3 misnomer, since duringthe passage of shock, T does increase!
SLIDE 19
Radiative Shocks - Special Case of “Isothermal”
Shu/ Goodman Cs= Cs 2
Cs
(to the right, pay attention to the axis notation - velocity decreases after the shock, but is plotted as u1/u3 instead of u3/u1 like the other quantities.) misnomer, since duringthe passage of shock, T does increase!
SLIDE 20
Shock Nomenclature
- J-shock (“jump” in conditions across shock boundary)
- C-shock (“continuous” change)
- Radiative è can be considered J-shock especially if
Δx size scale over which the Δu deceleration occurs is very small.
- Non-radiative è can be J-shock or C-shock.
- MHD shock è always C-shock.
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(MHD case)
More Realistic (Non-Cartoon) Models
SLIDE 22
Dopita & Sutherland
Radiation from Shocked Ho Hot Gas
Different emission lines are seen as a function of position along the shock direction, depending on the density and temperature of the gas. “downstream” “upstream”
SLIDE 23
Dopita & Sutherland
Radiation from Shocked Ho Hot Gas
Note the high ionization species near the shock front. “upstream” “downstream”
SLIDE 24
Radiative Processes for Shocked Co Cold Gas
“upstream” “downstream” Note that the status of the dust must be considered, in addition to the gas, for overall cooling function.