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”? https://www.youtube.com/watch?v=Suugn-p5C1M&NR=1&feature=fvwp https://www.youtube.com/watch?v=Q78Kb4uLAdA

The Dynamic ISM Earth B-field vs Solar wind Shocks are everywhere, occurring on all astrophysical scales. Solar wind vs local ISM NASA/IBEX

The Dynamic ISM Jets from Young Stars Stellar Winds Russell CromanAstrophotography

The Dynamic ISM Supernova Remnant Guitar Nebula: pulsar moving @2000 km/s! Red = x-ray Green = forbidden optical line High velocity “black widow” pulsar charge exchange

The Dynamic ISM Cloud-Cloud Collisions HII Regions

The Dynamic ISM Galactic Spiral Density Waves

The Dynamic ISM The Antennae Galaxies Collision of gas leads to shocks - > star formation

Dynamics Nomenclature Mach number, M = v / c s • v >> cs è strong shock • v ≥ cs è weak/mild shock • v < cs è no shock

Dynamics – Getting to the Math 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 |v 2 | < |v 1 | . Consider shock to be plane-parallel, such that properties of the fluid depend only on the linear distance, x, and all v’s are v x . Neglect viscosity except in the shock transition zone, Δ x, where large dv/dx means kinetic energy transformed into heat (viscous dissipation). Δ x Δ x è 0 is a discontinuity or “jump”. Maoz

Dynamics – Getting to the Math (note: opposite orientation from previous slide) Arce “upstream” “downstream” 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

Dynamics – The Math “upstream” “downstream” Conserve mass Conserve momentum Conserve Energy

Non-Radiative Shocks - Do not Cool Efficiently ✗ ✗ case of Δ x ~ few mean free paths, i.e a • transition zone è continuous shock P2 case of è jump shock “continuous” • u1 ρ 2 velocity P2 u1 velocity ρ 2 ρ 1 u2 density P1 “jump” pressure ρ 1 density u2 P1 pressure Shu

Non-Radiative Shocks – Do not Cool Efficiently (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.) Shu/ Goodman

Non-Radiative Shocks – Do not Cool Efficiently 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 * v shock • These are usually M1 >> 1 circumstances with high v shock and low ρ .

Radiative Shocks – Cool Efficiently L = L ( ρ , T) = Λ – Γ “net cooling function” Shu

Radiative Shocks – Special Case of “Isothermal” misnomer, since during the passage of shock, T does increase! NOTE: notation here uses only 1 è 2 whereas previous slides had 1 è 2 compression/heating and then cooling è 3 ??? via Goodman

Radiative Shocks - Special Case of “Isothermal” misnomer, since during the passage of shock, T does increase! Cs = 2 Cs C s (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.) Shu/ Goodman

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.

More Realistic (Non-Cartoon) Models (MHD case)

“upstream” “downstream” Radiation from Shocked Hot Gas Ho Different emission lines are seen as a function of position along the shock direction, depending on the density and temperature of the gas. Dopita & Sutherland

Radiation from Shocked Ho Hot Gas “upstream” “downstream” Note the high ionization species near the shock front. Dopita & Sutherland

Radiative Processes “downstream” “upstream” for Shocked Co Cold Gas Note that the status of the dust must be considered, in addition to the gas, for overall cooling function.