Download - Magnetic fields in Orion’s Veil
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Magnetic fields in Orion’s Veil
T. TrolandPhysics & Astronomy Department
University of Kentucky
Microstructures in the Interstellar MediumApril 22, 2007
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Collaborators
C. M. Brogan NRAO R. M. Crutcher Illinois W. M. Goss NRAO D. A. Roberts Northwestern & Adler
Back off, I’m a scientist!
B = ? ...about -50 G
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A brief history of magnetic field studies
B = ?
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Hiltner & Hall’s discovery - 1948
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Verschuur’s discovery - 1968
I swear it’s true!
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A good review of magnetic field observations and their implications
Heiles & Crutcher, astro-ph/0501550 (2005)
In Cosmic Magnetic Fields
Check it out!
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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.
Shu, The Physical Universe (1982)
B
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Why is IS magnetic field important?
Effects of flux freezing – Gravitational contraction leads to increase in gas density & field strength.
Shu, The Physical Universe (1982)
B
B n
= 0 - 1
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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
22
2
1
8 NTvB
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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)
25.2
NT
eq v
Bn 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.
Stokes V dI/dV
losz Bdv
dIv
dv
dIV
2
1cos
2
1 Line-of-sight
component of B
I = LH + RH
V = LH - RH
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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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optical CO, J=1-0
6. Orion region
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Orion Region
Plume et al. 2000
13CO, J=1-0
“integral sign”
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Orion Region
2MASS, JHK
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Orion Region
2MASS JHK image + 13CO, J=1-0
2MASS + 13CO, J=1-0
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Orion Region
Lis et al. 1998
BN-KL
Orion S
350 dust
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7. Orion Nebula & foreground veil
I snapped this shot!
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Orion Nebula Optical
HST (O’Dell & Wong)
Dark Bay
Trapezium stars
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Orion Nebula - optical extinction
optical 20 cm radio continuum
O’Dell and Yousef-Zadeh 2000
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Orion Nebula - optical extinction
O’Dell & Yusef-Zadeh, 2000, contours at Av = 1, 2
Optical extinction derived from ratio of radio continuum to H
Dark Bay
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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 (O’Dell et al. 1992).
Orion Nebula – Extinction in veil
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A model of the nebula region
O’Dell & Wen, 1992
Veil (site of Av & 21cm HI absorption)
H+
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7. Aperture synthesis studies of Orion
UKIRT (WFCAM)
M43
VLA observations of Zeeman effect in 21 cm HI & 18 cm OH absorption lines toward Orion A (M42) & M43
Absorption arises in veil
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Orion veil - 21cm HI absorption*
*toward Trapezium stars
Component AComponent B
VLSR
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Orion veil - 21cm HI optical depth (HI)*
*toward Trapezium stars
HI N(H0) / Tex
VLSR
Component BComponent A
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Orion veil - 21cm HI optical depth
Colors – HI scaled to N(H0)/Tex 1018 cm-2 K-1
(HI N(H0) / Tex)
Contours - 21 cm continuum
M43Line saturation
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Orion veil – 18 cm* OH optical depth
Colors – OH scaled toNOH/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
Blos = -52 4.4 G
Blos = -47 3.6 G
Stokes I
Stokes V
V dI/dV
A B
*toward Trapezium stars
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A
Orion veil – Blos from HI Zeeman effect
Component A
Colors – Blos
Contours – 21 cm radio continuum
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A
Orion veil – Blos from HI Zeeman effect
Component A
Colors – Blos
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B
Orion veil – Blos from HI Zeeman effect
Component B
Colors – Blos
Contours – 21 cm radio continuum
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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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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
Magnetic fields in veil from HI Zeeman effect
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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)
Abel et al. 2004
H2 H0 H0
Veil components A & B
D
H+
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Physical conditions in veil
Abel et al. (2006) used HST STIS spectra in UV to model veil components A & B separately.
Optical D
epth
0.1
0.2
0.3
0.4
0.5
VLSR (km/ s)
-10 -5 0 5 10
Optical D
epth
0.0
0.1
0.2
0.3
0.4
0.5
Kr I
Optica
l D
epth
1
2
3
4
5
6
VLSR (km/s)
Optical D
epth
0.2
0.4
0.6
0.8 O I
VLSR (km/ s)
-10 -5 0 5 10
Optica
l D
epth
0.0
0.1
0.2
0.3
0.4
0.5
AB
HB2B v=0-3 P(3)
C I
H I
21cm
uv
uv
uv
uv
Optical depth profiles
B A
VLSR
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Physical conditions in veil - Results
N(H)
cm-2
n(H)
cm-3
thickness
(pc)TK
Component A 1.6 1021 102.5
(102.1-3.5)
1.3 50
Component B
Compared to A
3.2 1021 103.4
(102.3-3.5)
denser
0.5
thinner
80
hotter
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Physical conditions in veil
Recall
25.2
)(
NT
eq v
BHn
Blos
(G)
n(H)/neq*
Component A -45 0.03*
Component B -55 1*
*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.
*
Denser Thinner Hotter More turbulent Blueshifted 4 km s-1
A BH+B
**
*
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
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.
I waited 70 years to find this out!