Selected topics on Cosmic Magnetic fields Huirong Yan KIAA-PKU

Astrophysical magnetic fields are ubiquitous Cluster of galaxies Accretion disk galaxyQSO Solar wind Also in Molecular clouds, outflows, supernovae remnants, planetary nebulae…

There is no universal magnetic diagnostics in diffuse medium Zeeman splitting (B || ):  Zeeman splitting (B || ): Dense and cold clouds, with molecules or even denser with masers Dense and cold clouds, with molecules or even denser with masers Faraday rotation (B || ):  Faraday rotation (B || ): Uncertainty with the electron density distribution along los Uncertainty with the electron density distribution along los Light polarization by dust (B  ):  Light polarization by dust (B  ): Star light polarization Grain IR emission Most common ways of magnetic field studies:

4 Zeeman Effect       - ( Δm l = -1 ) (Δml=0 )(Δml=0 )  + ( Δm l = +1 )

5 Zeeman Effect 5

6 Faraday Rotation

7 Magnetic realignment Goldreich - Kylafis effect (upper state) polarization in radio and FIR emission due to anisotropic optical depth applicable to molecular clouds Ground state realignment unique mechanism to induce polarization in absorption lines due to anisotropic radiation ranging from submilimeter, IR, optical to UV general diffuse medium 7

Toy model:s Atomic alignment is differential occupation of the sublevels of the ground (or metastable) state. Atomic species on the ground state can be aligned by anisotropic radiation radiation Classical Analogy −−−−  Thermal system Radiatively pumped system Induced  1 transition followed by isotropic emission.

anisotropic radiation (unpolarized light is sufficient) there are at least 3 sublevels on the ground state. Alignment by unpolarized light requires >2 sublevels Incoming lighttransmitted light medium History: Laboratory studies on masers (experimental) (Brossel et al. 1952; Hawkins 1955; Kastler 1957) Suggested by Varshalovich (1971) for B studies in diffuse medium

Magnetic field induces precession and realigns atoms M=2 M=1 M=0 M=-1 M=-2 z BJ(F) rrrr Long lived ground state means sensitivity to weak B. Alignment is either parallel or perpendicular to B due to fast magnetic precession.B

Magnetic field induces precession and realigns atoms M=2 M=1 M=0 M=-1 M=-2 z BJ(F) rrrr Long lived ground state means sensitivity to weak B. Alignment is either parallel or perpendicular to B due to fast magnetic precession.B

Atomic realignment is sensitive to weak magnetic fields  L –Larmor frequency A-Einstein coefficient, ν ΔJ – E ΔJ /h  L (s -1 ) Δ v(cm/s) ΒGΒGΒGΒG   Hanleeffect LowerlevelHanleeffect Level interferences occur Atomicrealignment Zeeman effect  ΔJΔJΔJΔJ     Α             R-1R-1R-1R-1 For a wide range of field (>~ G, ~ ~ G, ~<1G) in diffuse medium, atomic alignment happens.            

Realignment depends on the angle between the B and light Alignment is determined by  r. The polarization degree depends on  and  r. For the degenerate case,  r and  coincide. r r

How to observe it? - I. Absorption           ∆  （   ）

Polarization of absorption is either || or  to the magnetic field Polarization changes direction at Van Vleck angle  r =  54.7 o,  180 o o. Yan & Lazarian (2006)

16 How to observe it? - II. Emission           ）    

Our results: many observed absorption lines have appreciable polarization IonC IC IISi ISi IIO IS I Wavl (A) P max 18%15%20%7%29%22% IonS IITi IICr ICr IINIS III Wavl (A) P max 12%7%5%21%5.5%24.5% Calculated Examples: Fe I, Fe II, Fe III, Fe III, Mn II, Ti III, C lI, N II, Cr I … Many more lines:

18 Magnetic field changes optical depth and line intensity!   One has to take into account the effect of magnetic field when analyzing spectrum.

Effect of nuclear spin 2. Reduce alignment for atoms with fine structures.  Enables alignment for Alkalii atoms. Similar structure   rrrr rrrr polarization of NI absorption Yan & Lazarian (2007) polarization of SII absorption NI and SII have same electron configurations (4S 3/2 4P 3/2,3/2  1 ). The difference in their polarizations shows the effect of hyperfine structure.

20 Is there any circular polarization? 20

Our results: polarization of resonant lines scattered from aligned atoms ionHI (Bal) Na IK IN VO IP VS IIAl IITi II Wavl P max 25%21%20%22%2.3%27%31%20%7.3% Many more lines: N I, N II, N III, P III, Al I, Al III, Fe I, Fe II, Fe III, Fe III, Mn II, Ti III, C lI, N II, Cr I … Calculated Examples:

Using several lines, it’s possible to get 3D B 3D information: from degree of polarization P (  2 0 (  r ),  ). P (  2 0 (  r ),  ). Two lines are enough P/  ΟΙ  rrrr   ΟΙ  SII % Optical depth  2 0 (  r ) ---Normalized density matrix coefficient from atomic physics

23 3rd possibilty: Forbidden (submm, IR) transitions within the aligned ground state Schematics of UV pumping of OI 63.2  m emission 3S  m 3P 0 3P 1 3P A 1302A 1304A (WF Effect) Schematics of UV pumping of OI 63.2  m emission 3S  m 3P 0 3P 1 3P A 1302A 1304A (WF Effect) Schematics of UV pumping of OI 63.2  m emission 3S  m 3P 0 3P 1 3P A 1302A 1304A (WF Effect) Schematics of UVpumping of CI 610  m emission   3P o 610  m 3P 1 3P 0 3P 2 3P 2 [CI] Emission  

Forbidden (submm, IR) transitions within the aligned ground state are polarized Calculated Examples: Many more lines:

Ex. I: alignment allows studies of magnetic field in a comet wake Resonance scattering of solar light by sodium tail from comet. MHD simulations of comet’s wake. P: -8%~14% for Na D2 Time variation of turbulent magnetic field can be studied.

Ex. II: Alignment allows tracing heliosphere B with comet Satellite based model of Heliospheric B ( Opher et al ) Earth Comet orbit Cost effective way of study of B in heliosphere! Sun synthetic measurements of comet B ∗

Ex. III: Polarization from circumstellar envelope O I ( ) emission Polarization vectors are spherically symmetric without magnetic realignment and the classical expression applies Spherical symmetry is broken With alignment

Β LOS z  x Polarization from Toroidal Disk

Ex. V: Atomic alignment influences the FIR line intensity ouudistortion to CMB This influences Distortion of CMB and our estimates for early universe metalicity. ΔT/T cmb  Schematics of UV pumping of OI 63.2  m emission 3S  m 3P 0 3P 1 3P A 1302A 1304A

Ex. VI: alignment allows study of magnetic field in the epoch of Reionization OI 63.2  m transmission  UV excitation rate/ CMB excitation rate (Yan & Lazarian 2008) From Cantalupo et al. (2008) Schematics of reionization 63.2  m 3S 1 3P 0 3P 1 3P A 1302A 1304A Schematics of UV pumping of OI 63.2  m emission

CSO Steward Observatory Sounding rocket flight is funded.(PI Nordsieck) Observations for IBEX. (w. Paul Smith) Studies of [C I] polarization are intended.(w.Houde) Ongoing projects open wide avenues for the technique VLT  VLT  VLT  Observations for SNRs. (w. F. Snick)

32 Grain alignment Renewed interest to grain alignment also arise from CMB polarization studies

33 Paramagnetic alignment and its modifications fail observational testing Grains get aligned with rotation axis parallel to B (grain 1) 33 B Davis-Greenstein mechanism 1 2 This is textbook solution

Rao et al. 98         

                    

      “  ”            “  ”  Draine & Weingrartner 97 L. Spitzer: Alignment torques are not universal. How can a universal alignment exist? Angle between B and light direction

   

a1a1 k e2e2 e3e3 e1e1 Θ  Lazarian & Hoang 07         a 1 is grain max inertia axis k is light direction               

 Lazarian & Hoang 07                    

J//a 1 B               ’        

is angle between magnetic field and radiation is angle between a and magnetic field k B a  No wobbling: “wrong alignment” “wrong” alignment for a narrow range of angles  “  ”  Suppression of “wrong alignment” by wobbling   

               

RATs increase with a eff         

       

RAT alignment explains the existing data. RAT testing constrains grain composition. RAT alignment is present beyond ISM    B Davis-Greenstein mechanism 1 2 

Atomic alignment is sensitive to smaller scale fluctuations of magnetic field. Combining the information from both, we can get 3D information of magnetic field. Provide independent test to grain alignment theory. Comparison of atomic and grain alignment:

47 Get magnetic strength from Chandrasekar-Fermi (CF) method Assuming equipartition between kinetic energy and perturbed magnetic energy 47   

48 How good is the assumption of equipartition? sub-Alfvenic turbulence super-Alfvenic turbulence, turbulence dynamo 48

49 Summary of interstellar medium polarimetry Magnetic field in ubiquitous in astrophysics and need to be diagnosed from multiple scales with the synergy of various polarimetry techniques. Ground state alignment is a unique mechanism to induce polarization in absorption lines and detect ISM magnetic field. It also influence emission polarimetry and forbidden line polarimetry Multiple wavebands from submillimeter, IR, optical to UV can be used to detect 3D small scale magnetic field. Combined with grain alignment, CF techniques, it can be used to study both spatial and temporal variations of magnetic field. 49