Magnetic fields in Orions Veil

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Magnetic fields in Orions Veil

• T. Troland
• Physics Astronomy Department
• University of Kentucky
• Microstructures in the Interstellar Medium
• April 22, 2007

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Collaborators
Back off, Im a scientist!

• C. M. Brogan NRAO
• R. M. Crutcher Illinois
• W. M. Goss NRAO
• D. A. Roberts Northwestern Adler

...about -50 ?G
B ?
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A brief history of magnetic field studies
B ?
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Hiltner Halls discovery - 1948
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Verschuurs discovery - 1968
I swear its 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)
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Why is IS magnetic field important?

• Effects of flux freezing Interstellar cloud
  dynamically coupled to external medium.

B
Shu, The Physical Universe (1982)
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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)
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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

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Magnetic equipartition density (neq)

• In observational units
• where n n(Ho) 2n(H2)
• If n / neq gt 1 Turbulent energy dominates
  turbulence is super-Alfvenic)
• If n / neq lt 1 - Magnetic energy dominates
  (turbulence is sub-Alfvenic)

cm-3
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3. Magnetic fields the via Zeeman effect

• Zeeman effect detected as frequency offset ?vz
  between LH RH circular polarizations in
  spectral line.

Line-of-sight component of B
I LH RH V LH - RH
Stokes V ? dI/dV
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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
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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)

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5. Aperture synthesis studies of Zeeman effect

• Makes use of 21 cm HI 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 ?
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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)
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6. Orion region
optical
IRAS
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6. Orion region
optical
CO, J1-0
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Orion Region
13CO, J1-0 integral sign
Plume et al. 2000
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Orion Region
2MASS, JHK
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Orion Region
2MASS 13CO, J1-0
2MASS JHK image 13CO, J1-0
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Orion Region
350 ? dust
BN-KL
Orion S
Lis et al. 1998
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7. Orion Nebula foreground veil
I snapped this shot!
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Orion Nebula Optical
Dark Bay
Trapezium stars
HST (ODell Wong)
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Orion Nebula - optical extinction
optical
? 20 cm radio continuum
ODell and Yousef-Zadeh 2000
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Orion Nebula - optical extinction

• Optical extinction derived from ratio of radio
  continuum to H?

Dark Bay
ODell Yusef-Zadeh, 2000, contours at Av 1, 2
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Orion Nebula Extinction in veil

• Av correlated with 21 cm HI optical depth across
  nebula (latter from VLA data of van der Werf
  Goss 1989).
• Correlation suggests most of Av arises in a
  neutral foreground veil where HI absorption
  also arises (ODell et al. 1992).

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A model of the nebula region
H
Veil (site of Av 21cm HI absorption)
ODell Wen, 1992
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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)
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Orion veil - 21cm HI absorption
Component A
Component B
toward Trapezium stars
VLSR
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Orion veil - 21cm HI optical depth (?HI)
?HI ? N(H0) / Tex
Component A
Component B
toward Trapezium stars
VLSR
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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
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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
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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
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Orion veil Blos from HI Zeeman effect

• Component A
• Colors Blos
• Contours 21 cm radio continuum

A
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Orion veil Blos from HI Zeeman effect

• Component A
• Colors Blos

A
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Orion veil Blos from HI Zeeman effect

• Component B
• Colors Blos
• Contours 21 cm radio continuum

B
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Magnetic fields in veil from HI Zeeman effect

• All Blos values negative (Blos toward observer)
• Blos similar in components A B
• Over most of veil, Blos ? -40 to -80 ?G
• In Dark Bay, Blos ? -100 to -300 ?G

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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
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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!

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Physical conditions in veil - Results

• n(H) 103.1 ?0.2 averaged over components A B
• D 1018.8 ?0.1 (? 2 pc)

Veil components A B
D
H2
H0
H0
H
Abel et al. 2004
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Physical conditions in veil
B
A
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
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Physical conditions in veil - Results
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Physical conditions in veil

• Recall

Assuming B Blos, however, B ? Blos.
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Physical conditions in veil

• Component A dominated by magnetic energy, far
  from magnetic equipartition!
• Component B in approximate equipartition.

Dominated!
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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.)

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HI Magnetic fields in veil

• 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.

B
H

A
B



Denser Thinner Hotter
More turbulent Blueshifted 4 km s-1
See, also, van der Werf Goss 1989
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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)
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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.