1. Magnetic fields in Orion’s Veil T. Troland Physics & Astronomy Department University of Kentucky Microstructures in the Interstellar Medium April 22, 2007

2. Collaborators u C. M. BroganNRAO u R. M. CrutcherIllinois u W. M. GossNRAO u D. A. RobertsNorthwestern & Adler Back off, I’m a scientist! B = ?...abou t -50  G

3. A brief history of magnetic field studies B = ?

4. Hiltner & Hall’s discovery - 1948

5. Verschuur’s discovery - 1968 I swear it’s true!

6. A good review of magnetic field observations and their implications u Heiles & Crutcher, astro- ph/0501550 (2005) u In Cosmic Magnetic Fields Check it out!

7. 1. Why is IS magnetic field important? u Magnetic fields B are coupled to interstellar gas (flux freezing), but how? u Ions in gas coupled to B via Lorentz force, neutrals coupled to ions via ion-neutral collisions*. *Coupling breaks down at very low fractional ionization (in dense molecular cores)

8. Why is IS magnetic field important? u Effects of flux freezing – Interstellar cloud dynamically coupled to external medium. Shu, The Physical Universe (1982) B

9. Why is IS magnetic field important? u Effects of flux freezing – Gravitational contraction leads to increase in gas density & field strength. Shu, The Physical Universe (1982) B B  n   = 0 - 1

10. 2. How strong must the magnetic field be? u Magnetic equipartition occurs if magnetic energy density = turbulent energy density, that is: u  v NT = 1-D line broadening from turbulent (non- thermal) motions

11. Magnetic equipartition density (n eq ) u In observational units where n = n(H o ) + 2n(H 2 ) u If n / n eq > 1 – Turbulent energy dominates turbulence is super-Alfvenic) u If n / n eq < 1 - Magnetic energy dominates (turbulence is sub-Alfvenic) cm -3

12. 3. Magnetic fields the via Zeeman effect u Zeeman effect detected as frequency offset  v z between LH & RH circular polarizations in spectral line. Stokes V  dI/dV Line-of-sight component of B I = LH + RH V = LH - RH

13. Magnetic fields via the Zeeman effect u B los measured via Zeeman effect in radio frequency spectral lines from selected species* HI ( 21cm) OH ( 18 cm, 1665, 1667 MHz) CN ( 2.6mm) I am unpaired! *species with un-paired electron

14. 4. Magnetic equipartiton (n/n eq  1) u Magnetic equipartition appears to apply widely in the ISM: u Diffuse ISM (CNM) – HI Zeeman observations (Heiles & Troland 2003 - 2005, Arecibo Millennium Survey) u Self-gravitating clouds – Zeeman effect observations in molecular clouds (see Crutcher 1999)

15. 5. Aperture synthesis studies of Zeeman effect u Makes use of 21 cm HI and 18 cm OH absorption lines against bright radio continuum of H + regions. u Allows mapping of B los in atomic & molecular regions of high-mass star formation. B = ?

16. Aperture synthesis studies of Zeeman effect Sources observed to date: u Cas A u Orion A (M42) u W3 main u Sgr A, Sgr B2 u Orion B (NGC 2024) u S106 u DR21 u M17 u NGC 6334 u W49 Map of B los in HI for W3 main (Roberts et al. in preparation)

17. 6. Orion region opticalIRAS

18. opticalCO, J=1-0 6. Orion region

19. Orion Region Plume et al. 2000 13 CO, J=1-0 “integral sign”

20. Orion Region 2MASS, JHK

21. Orion Region 2MASS JHK image + 13 CO, J=1-0 2MASS + 13 CO, J=1-0

22. Orion Region Lis et al. 1998 BN-KL Orion S 350  dust

23. 7. Orion Nebula & foreground veil I snapped this shot!

24. Orion Nebula Optical HST (O’Dell & Wong) Dark Bay Trapezium stars

25. Orion Nebula - optical extinction optical 20 cm radio continuum O’Dell and Yousef-Zadeh 2000

26. Orion Nebula - optical extinction O’Dell & Yusef-Zadeh, 2000, contours at A v = 1, 2 u Optical extinction derived from ratio of radio continuum to H  Dark Bay

27. u A v correlated with 21 cm HI optical depth across nebula (latter from VLA data of van der Werf & Goss 1989). u Correlation suggests most of A v arises in a neutral foreground “veil” where HI absorption also arises (O’Dell et al. 1992). Orion Nebula – Extinction in veil

28. A model of the nebula region O’Dell & Wen, 1992 Veil (site of A v & 21cm HI absorption) H+H+

29. 7. Aperture synthesis studies of Orion UKIRT (WFCAM) M43 u VLA observations of Zeeman effect in 21 cm HI & 18 cm OH absorption lines toward Orion A (M42) & M43 u Absorption arises in veil

30. Orion veil - 21 cm HI absorption* *toward Trapezium stars Component A Component B V LSR

31. Orion veil - 21 cm HI optical depth (  HI )* *toward Trapezium stars  HI  N(H 0 ) / T ex V LSR Component B Component A

32. Orion veil - 21 cm HI optical depth Colors –  HI scaled to N(H 0 )/T ex  10 18 cm -2 K -1 (  HI  N(H 0 ) / T ex ) Contours - 21 cm continuum M43 Line saturation

33. Orion veil – 18 cm* OH optical depth Colors –  OH scaled to N OH /T ex  10 14 cm -2 K -1 (  OH  N OH / T ex ) Contours - 18 cm continuum *1667 MHz

34. Orion veil – B los from HI Zeeman effect B los = -52  4.4  G B los = -47  3.6  G Stokes I Stokes V V  dI/dV AB *toward Trapezium stars

35. A Orion veil – B los from HI Zeeman effect Component A u Colors – B los u Contours – 21 cm radio continuum

36. A Orion veil – B los from HI Zeeman effect Component A u Colors – B los

37. B Orion veil – B los from HI Zeeman effect Component B u Colors – B los u Contours – 21 cm radio continuum

38. Magnetic fields in veil from HI Zeeman effect u All B los values negative (B los toward observer) u B los similar in components A & B u Over most of veil, B los  -40 to -80  G u In Dark Bay, B los  -100 to -300  G

39. u High values of B los * imply veil directly associated with high-mass star forming region. (Such high field strengths never detected elsewhere.) *relative to average IS value B  5  G Magnetic fields in veil from HI Zeeman effect

40. 8. Physical conditions in veil u Abel et al. (2004, 2006) modeled physical conditions to determine n(H) in veil & distance D of veil from Trapezium. u They used 21 cm HI absorption lines and UV absorption lines toward Trapezium (IUE data). u Results apply to Trapezium los only!

41. Physical conditions in veil - Results u n(H) = 10 3.1  0.2 averaged over components A & B u D = 10 18.8  0.1 (  2 pc) Abel et al. 2004 H2H2 H0H0 H0H0 Veil components A & B D H+H+

42. Physical conditions in veil u Abel et al. (2006) used HST STIS spectra in UV to model veil components A & B separately. Kr I O I AB H B 2 B v=0-3 P(3) C I H I 21cm uv Optical depth profiles BA V LSR

43. Physical conditions in veil - Results N(H) cm -2 n(H) cm -3 thickness (pc) TKTK Component A 1.6  10 21 10 2.5 (10 2.1-3.5 ) 1.350 Component B Compared to A 3.2  10 21 10 3.4 (10 2.3-3.5 ) denser 0.5 thinner 80 hotter

44. Physical conditions in veil u Recall B los (  G) n(H)/n eq * Component A -45  0.03* Component B -55  1* *Assuming B = B los, however, B  B los.

45. Physical conditions in veil u Component A dominated by magnetic energy, far from magnetic equipartition! u Component B in approximate equipartition. Dominated!

46. HI Magnetic fields in veil u Similarity of B los in veil components A & B suggests B nearly along los. If so, veil gas can be compressed along los, increasing n but not B (B  n  with   0). u (If B nearly along los, then measured B los  B tot in veil components.)

47. HI Magnetic fields in veil u Possible scenario – Component B closer to Trapezium, this component accelerated & compressed along B by momentum of UV radiation field and/or pressure of hot gas near Orion H + region. * Denser Thinner Hotter More turbulent Blueshifted 4 km s -1 AB H+H+ B * * * See, also, van der Werf & Goss 1989

48. HI Magnetic fields in veil u Possible scenario – Veil in pressure equilibrium with stellar radiation field (like M17, Pellegrini et al. 2007) u P rad (stars)  P B implies B 2  Q(H 0 )/R 2 u So B  30  G Q(H 0 ) is number of ionizing photons /sec (10 49.3 for  1 C Ori) R is distance of veil from stars (2 pc)

49. Some Conclusions r.e. Orion veil u Orion veil a (rare) locale where magnetic field (B los ) can be mapped accurately over a significant area. u Veil reveals magnetic fields associated with massive star formation (B los  -50 to -300  G). u One velocity component of veil appears very magnetically dominated. u B in veil may be in pressure equilibrium with stellar uv radiation field, as for M17. I waited 70 years to find this out!