Magnetic fields in Orion’s Veil
T. TrolandPhysics & Astronomy Department
University of Kentucky
Microstructures in the Interstellar MediumApril 22, 2007
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
A brief history of magnetic field studies
B = ?
Hiltner & Hall’s discovery - 1948
Verschuur’s discovery - 1968
I swear it’s true!
A good review of magnetic field observations and their implications
Heiles & Crutcher, astro-ph/0501550 (2005)
In Cosmic Magnetic Fields
Check it out!
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)
Why is IS magnetic field important?
Effects of flux freezing – Interstellar cloud dynamically coupled to external medium.
Shu, The Physical Universe (1982)
B
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
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
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
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
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
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)
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 = ?
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)
6. Orion region
optical IRAS
optical CO, J=1-0
6. Orion region
Orion Region
Plume et al. 2000
13CO, J=1-0
“integral sign”
Orion Region
2MASS, JHK
Orion Region
2MASS JHK image + 13CO, J=1-0
2MASS + 13CO, J=1-0
Orion Region
Lis et al. 1998
BN-KL
Orion S
350 dust
7. Orion Nebula & foreground veil
I snapped this shot!
Orion Nebula Optical
HST (O’Dell & Wong)
Dark Bay
Trapezium stars
Orion Nebula - optical extinction
optical 20 cm radio continuum
O’Dell and Yousef-Zadeh 2000
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
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
A model of the nebula region
O’Dell & Wen, 1992
Veil (site of Av & 21cm HI absorption)
H+
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
Orion veil - 21cm HI absorption*
*toward Trapezium stars
Component AComponent B
VLSR
Orion veil - 21cm HI optical depth (HI)*
*toward Trapezium stars
HI N(H0) / Tex
VLSR
Component BComponent A
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
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
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
A
Orion veil – Blos from HI Zeeman effect
Component A
Colors – Blos
Contours – 21 cm radio continuum
A
Orion veil – Blos from HI Zeeman effect
Component A
Colors – Blos
B
Orion veil – Blos from HI Zeeman effect
Component B
Colors – Blos
Contours – 21 cm radio continuum
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
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
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!
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+
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
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
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.
Physical conditions in veil
Component A dominated by magnetic energy, far from magnetic equipartition!
Component B in approximate equipartition.Dominated!
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.)
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
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)
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!