magnetic fields in orion’s veil

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)*


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


A


Component A

Colors – Blos

Contours – 21 cm radio continuum

A


Component A

Colors – Blos

B


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


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.


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.)


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


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!

magnetic fields in orion’s veil

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