Magnetic fields in Orion’s Veil

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 Back off, I’m a scientist! • C. M. Brogan NRAO • R. M. Crutcher Illinois • W. M. Goss NRAO • D. A. Roberts Northwestern & Adler ...about -50 G B = ?

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 • Heiles & Crutcher, astro-ph/0501550 (2005) • In Cosmic Magnetic Fields Check it out!

7. 1. Why is IS magnetic field important? • Magnetic fields B are coupled to interstellar gas (flux freezing), but how? • 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? • Effects of flux freezing – Interstellar cloud dynamically coupled to external medium. B Shu, The Physical Universe (1982)

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

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

11. Magnetic equipartition density (neq) • In observational units where n = n(Ho) + 2n(H2) • If n /neq> 1 – Turbulent energy dominates turbulence is super-Alfvenic) • If n /neq< 1- Magnetic energy dominates (turbulence is sub-Alfvenic) cm-3

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

13. Magnetic fields via the Zeeman effect • Blos 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/neq 1) • Magnetic equipartition appears to apply widely in the ISM: • Diffuse ISM (CNM) – HI Zeeman observations (Heiles & Troland 2003 - 2005, Arecibo Millennium Survey) • Self-gravitating clouds – Zeeman effect observations in molecular clouds (see Crutcher 1999)

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

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

17. 6. Orion region optical IRAS

18. 6. Orion region optical CO, J=1-0

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

20. Orion Region 2MASS, JHK

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

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

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

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

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

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

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

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

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

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

31. Orion veil - 21cm HI optical depth (HI)* HI  N(H0) / Tex Component A Component B *toward Trapezium stars VLSR

32. Orion veil - 21cm HI optical depth M43 Line saturation Colors – HI scaled to N(H0)/Tex  1018 cm-2 K-1 (HI  N(H0) / Tex) Contours - 21 cm continuum

33. Orion veil – 18 cm* OH optical depth Colors – OH scaled to NOH/Tex  1014 cm-2 K-1 (OH  NOH / Tex) Contours - 18 cm continuum *1667 MHz

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

35. Orion veil – Blos from HI Zeeman effect ComponentA • Colors – Blos • Contours – 21 cm radio continuum A

36. Orion veil – Blos from HI Zeeman effect ComponentA • Colors – Blos A

37. Orion veil – Blos from HI Zeeman effect ComponentB • Colors – Blos • Contours – 21 cm radio continuum B

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

39. Magnetic fields in veil from HI Zeeman effect • High values of Blos* 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

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

41. Physical conditions in veil - Results • n(H) = 103.1 0.2averaged over components A & B • D = 1018.8 0.1( 2 pc) Veil components A & B D H2 H0 H0 H+ Abel et al. 2004

42. B A H I O I C I Kr I HB2Bv=0-3 P(3) B A Physical conditions in veil 21cm • Abel et al. (2006) used HST STIS spectra in UV to model veil components A & B separately. uv uv uv uv Optical depth profiles VLSR

43. Physical conditions in veil - Results

44. Physical conditions in veil • Recall *Assuming B = Blos, however, B Blos.

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

46. HI Magnetic fields in veil • Similarity of Blos 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). • (If B nearly along los, then measured Blos  Btot in veil components.)

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

48. HI Magnetic fields in veil • Possible scenario – Veil in pressure equilibrium with stellar radiation field (like M17, Pellegrini et al. 2007) • Prad(stars)  PB implies B2  Q(H0)/R2 • So B  30 G Q(H0) is number of ionizing photons /sec (1049.3 for 1C Ori) R is distance of veil from stars (2 pc)

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