Shock Waves inShock Waves inInterstellar SpaceInterstellar Space

Prof. Wayne G. RobergeProf. Wayne G. RobergeA Passion for PhysicsA Passion for Physics

Oct. 2, 2008Oct. 2, 2008

Outline

• A Famous Example

• Anatomy of a Shock Wave

• Shock Waves on Earth

• Shock Waves in Space

• What I do

Nuclear Explosion

shock front

What’s going on?

Disturbance(supersonic)

Swept-Up Gas(hot)

(high pressure)

Undisturbed Gas(cold)

(low pressure)

ShockFront

F/A 18 in Transonic Flight

Prandtl-Glauert Condensation(makes shock visible)

Rifle Bullet in Air

spherical shockcone-shaped shock

Supersonic Object in Air

object

“bow shock”

shock front

Battleship Firing Big Guns

Shock Wave in Trafficv vv

shockfront

undisturbed fluid disturbance

• Initially: uniform speed v and separation, L.• Next: driver on right touches brakes.• Next: a traffic pileup (“shock”) ensues.

v

L L

WHY?

Velocity of Information

L• Left driver sees light and reacts.• News travels left with speed c

relative to traffic.

c = LΔt

Δt ≅ human reaction time

Signal Velocity Rel. to Ground

*Marc Green, ”How Long Does it Take to Stop?”, Transportation Human Factors, vol. 2, p.195.

u = v-c• Shock (pileup) if u>0 or v>c.• Now put in some numbers!

V = 70 mph = 100 ft/sL = (N+1)hh = 15 ft (one car length)Δt = 1.5 s*

FOR HOW MANY CAR LENGTHS (N)does v=c?

Earth’s Bow Shock

solar wind

Protostars Have Jets!

vv

HH47: 3 trillion Mile Jet

A Theoretical Model

HH30: Disk Detected!

Georg Friedrich Bernhard Riemann1826 - 1866

The Riemann Problem(fluid dynamics)

LEFT GAS RIGHT GAS

VR = VELOCITYNR = DENSITYPR = PRESSURE

x

VL = VELOCITYNL = DENSITYPL = PRESSURE

THE RIEMANN PROBLEM(MULTIFLUID MHD)

B0B0

*MNRAS, IN PRESS (ARXIV:0708.4260).

MOLECULAR JET CLOUD

VL = 20 KM/SNL = 4 X 104 CM-3

TL= 10 KBL= 50 µG

x

VR = 0NR = 4 X 104 CM-3

TR= 10 KBR= 50 µG

Summary

• Shock waves = hydrodynamical surprises.

• Caused by supersonic disturbances.

• Found almost everywhere in space.

• Light up and heat cold interstellar gas.

• Allow us to see, study star-forming material.