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Magnetic fields in Orion’s Veil

This study explores the magnetic fields in Orion's Veil and their importance in the interstellar medium. It discusses the effects of flux freezing and the strength of the magnetic field in various regions of high mass star formation. Zeeman effect measurements and aperture synthesis studies are used to map the magnetic field structure in the veil.

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Magnetic fields in Orion’s Veil

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  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.

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