Physical Processes in Astrophysics

Transcription

1 Physical Processes in Astrophysics Huirong Yan Uni Potsdam & Desy 1

2 Reference Books: Plasma Physics for Astrophysics, Russell M. Kulsrud (2005) The Physics of Astrophysics, Frank H. Shu (1991) Physical Processes in the Interstellar Medium, Lyman Spitzer (1978) The Physics of Fluids and Plasmas, Arnab Raichoudhuri, (1998) Plasma Physics, Peter A. Sturrock (1994) 2

3 Outline 1. Interstellar medium: components, phases, interconnection 2. Particle motions 3. Basic MHD 4. Nonlinear Phenomena: Turbulence in magnetized fluids 5. Interaction of high energy particles with turbulent magnetic field 6. Origin of high energy particles 7. Magnetic reconnection 8. Particle acceleration processes in astrophysics 9. Galactic cosmic rays and supernova remnants 10. Magnetohydrodynamic (MHD) processes in star formation 11. Relation to intergalactic media, ϒ ray burst 12. Confronting theory with observations, future perspective 3

4 Lecture I: Interstellar medium: components, phases, interconnection 4

5 90% of the visible matter in the Universe is in plasma state (dilute gas of ions, electrons, atoms, and molecules). 5

6 Coronal gas H II region Interstellar warm medium: cool region components, H 2 phases, interconnection Idealized phases: Observed: X ray emission, UV absorption HII region, f~0.1, 157μm) T~10^4K, ncm -3 Heated and ionized by photons Optical, HI 21cm HI 21cm, Observed: optical, radio, UV absorption HI, f~0.5, (thermal) warm, Optical T~6000K, & UV n~0.3 cool H I region Diffuse H 2 Dense f T (K) >3x Stellar outflows n (cm -3 ) ~ (M/ 10-6 M yr) Corona gas, f ~ 0.4, T, n~0.003cm -3, shock heated (10km s -1 / V wind ) cooling observed Expansion, X ray emission X ray emission, UV absorption lines Optical lines Radio continuum, UV absorption lines FIR ([CII] absorption lines FIR emission [C II] CO 2.6mm, Optical & UV absorption lines FIR emission CO 2.6mmm emission, Dust FIR Radio (HI & CO) Dust FIR emission, optical absorption 6

7 Magnetic field { Typical interstellar value ~ 3x10-6 G, comparable to other form of energies, thermal, turbulence, cosmic rays, etc. Origin: dynamo (?) Interacts with: cosmic rays, plasma, partially ionized gas Functions: 1. Glue the components together 2. Influence propagation of polarized radiation 3. Accelerates and scatters cosmic rays 4. Supports clouds against collapse 5. Redistributes angular momentum when a rotating cloud collapses Relevant to star formation 7

8 Cosmic rays Accelerated charged particles (10 11 ev ev) Coming isotropically Halo of cosmic rays + magnetic field Galaxy A number of acceleration mechanisms exist. Most efficient- First order Fermi acceleration in strong shocks and magnetic reconnections. 8

9 Matter balance Intergalactic matter Infall~1M yr -1 Galactic wind? Energy balance photons ISM 5x10 9 M Self gravity Star formation 3-10M yr -1 ~ 1M yr -1 Stellar ejecta radiation stars Extragalactic background Cosmic rays ISM outflows Radiative cooling Cold sky Stars Complex structure of the ISM stems from these energy flows 9

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18 de Broglie wavelength = h p ' h p mkb T, Ehrenfest s Theorem hn 1/3 p mkb T >> 1 Quantum memchanical << 1 classical Plasma (Levels 0 and 1 same as above) Level Description of State Dynamical equations 2: Distribution Function 2.5: Two-fluid model 3: One-fluid model Neutral fluids Level Description of State Dynamical equations 0: N quantum particles 1: N classical particles 2: Distribution Function 3: Continuum (of fluid cells) ψ(x 1,, x N ) (x 1, x N, v 1,, v N ) f(x, v, t) ρ(x), T(x), v(x) f(x, v, t) ρ(x), T(x), v(x), B(x) Schrödinger eqn. Newton s Laws Boltzmann eqn. Hydrodynamic eqns. Vlasov eqn. MHD eqn. Why can E be ignored? 18

19 Basic properties of plasma Saha equation for ionization: x 2 x 1 = (2 m e) 3/2 (k B T ) 5/2 h 3 p gas exp( k B T ) only applies in thermodynamic equilibrium! H II region, e.g., is completely ionized by UV photons! s k B T Debye length D = 4 nq 2 Derive it (r) = Q r exp( r/ D) 19

20 Different Plasma systems Plasma parameter g 1 n 3 D = (4 )3/2 n 1/2 e 3 (k B T ) 3/2 < 1 for plasma 20

21 Lecture III: Particle motions 21

22 Motion in uniform B field Motion of individual particles is primarily controlled by magnetic field Important concept: pitch angle, guiding center Ions motion about B is clockwise electrons motion about B is anticlockwise 22

23 Motion in a nonuniform B field Gradient drift v D L v th Magnetic moment (right hand grip rule) For Larmor motion, µ ~µ IS ~ = 1 2 qr v ~µ W? B ˆb Only depends on energy, no q or m dependence Plasma is diagmagnetic! 23

24 Potential energy Force U ~µ B F = ru = r(~µ B) To balance this force with qv D B/c, again v D L v th is needed! Similarly, external forces also produce the drift in the same fashion! General expression v D = c F B qb 2 24

25 Curvature drift Take B constant; radius of curvature R e. To 1st order the particle just spirals along the field. Figure 2.5: Curvature and Centrifugal Force In the frame of the guiding center a force appears because the plasma is rotating about the center of curvature. This centrifugal force is F cf as a vector F cf = m v 2 R c pointing outward (2.38) F cf = m v 2 R c R c 2 (2.39) [There is also a coriolis force 2m(ω v) but this averages to zero over a gyroperiod.] Use the previous formula for a force v d = 1 F cf B q B 2 = m v 2 R c B (2.40) q B 2 R 2 c This is the "Curvature Drift". 25

26 1 st adiabatic invariant magnetic momentμ (assignment: please use the force exerted by the rb k to prove) Requirement: T 1, R c rb B 1 Application: magnetic mirror, sin 2 θ C =B/B max 26

27 Formation of the Van Allen belt 27

28 Cool solar corona 28

29 2 nd adiabatic invariant & Fermi acceleration Magnetic clouds Requirement: Fermi (1949) T Much more stringent than the condition for the 1 st adiabatic invariant!