Документ взят из кэша поисковой машины. Адрес оригинального документа : http://www.atnf.csiro.au/research/Astro2010/talks/harvey-smith.pdf Дата изменения: Wed Jun 16 07:50:22 2010 Дата индексирования: Tue Oct 2 16:54:07 2012 Кодировка: Поисковые слова: molecular cloud

Using the Faraday Effect to Probe Magnetic Fields in HII regions
Lisa Harvey-Smith

Cosmic Magnetism - From Stellar to Intergalactic Scales Southern Cross Astrophysics Conference Series, Kiama, NSW

Wednesday, 16 June 2010


The Big Issues
>
>
Star formation relies on the accumulation of very diffuse material from large volumes. What is the role of magnetism in star-formation?

Diffuse ISM

Molecular Clouds

Bn в

B n

Crutcher (2008)
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Wednesday, 16 June 2010


The Big Issues
>
>
Star formation relies on the accumulation of very diffuse material from large volumes. What is the role of magnetism in star-formation?

Diffuse ISM

Molecular Clouds

Bn в

B n

HII regions

Crutcher (2008)
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Wednesday, 16 June 2010


Background
Questions:

> What is the magnetic field strength in HII regions? > How does magnetic field strength scale with density in the diffuse ISM? > What is the scale of magnetic field reversals in HII regions?
Previous Work:

> RRLs and HI/OH Zeeman effect can yield B|| in much denser material close to periphery of HII
regions, (e.g. Bloemhof et al. 1992; Roshi et al. 2007). (Heiles et al. 1980, 1981) in the 1-10 cm material. regions.

> Only a handful of measurements been made of magnetic field strengths within HII regions -3 > Recent improvements in H and radio polarization data allow an in-depth study of B|| in HII
Methodology:

> We use measurements of the Faraday effect on linearly polarized radio waves from distant
galaxies to estimate the magnetic field strength in 5 Galactic HII regions.

> Examine B vs. n relation for 5 HII regions over a factor of 20 change in density.

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The Faraday Effect
> Faraday effect: Change in polarization angle in a magneto-ionic medium due to circular birefringence (LCP and RCP having different speeds).
I'll wager ё20 that

= RM2

Astrophysical Measurement of the Faraday Effect Distant galaxies
Linearly polarized radiation

HII region

obs

Telescope

n e , B||

RM = 0.81 n e B|| dl
src

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A Rotation Measure Image of the Sky
Image: NVSS rotation measure catalogue, (Taylor, Stil & Sunstrum, 2009)
obs

RM = 0.81 n e B|| dl
src

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H Image of the Sky
Image: H

all sky map, Finkbeiner (2003)

I

H

Emission Measure, EM =



obs



n e dl

2

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Calculating n0 and B
Model: > If an HII region is clumpy, with clumps ne = n0 and elsewhere ne = 0, then:

||

n0 =

EM fL

B|| =

RM 0.81n 0 fL

- f = filling factor, L = path length through HII region - Other considerations: optical extinction, RM and EM due to back/foreground. - Given a model for the HII region and filling factor, can determine n0 and B|| at each position where we have an RM. - Use a back/foreground correction to isolate the in situ magnetic field. - Choose regions outside Galactic plane (dust extinction).

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Sharpless 2-27

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Sharpless 2-27

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Sharpless 2-27

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The HII Regions

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The HII Regions

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Method
1. Define boundary of HII region 2. Inside boundary: For each RM position, calculate the EM from I
H

3. Correct each EM for extinction by interstellar dust (assume dust in front) 4. Outside boundary: Calculate the RM and EM not due to the HII region and _subtract from data within the boundary. 5. Calculate B|| and n0 for each sightline. 6. Plot B|| vs. n0 for each HII region.
RM 0.81n 0 fL

B|| =

n0 =

EM fL
All-sky EB-V map (Schlegel, Finkbeiner & Davis 1998)
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Results

Sh 2-27
Correlation implies in situ magnetic field

Sh 2-264

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Results: B|| vs. n
Filled symbols: B > 0 Open symbols: B < 0

0

10
P
mag

>P P
mag

B = 16 n0 kT
therm

Pmagnetic Pthermal
B|| = 1 Btotal 6


therm

|B||| [ G]

Order of magnitude increase in n0, very little increase in B. Enhanced RM in HII regions due to increased electron density.
f = 0.04

1

f = 0.10 f = 0.25

Uncertainties: Filling factor, f of each sightline unknown. Dust correction assumes all dust is in front of HII region.

1 n0 [cm ]

10
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Results: Derived Parameters

HII Region
Sh 2-27 Sh 2-264 Sh 2-220 Sivan 3 Sh 2-171

R

max

(pc)

n0 (cm-3)
10.7 (2.7) 9.8 (2.3) 9.9 (3.1) 1.5 (0.4) 17.9 (10.4)

B|| (G)
- 6.1 (2.6) + 2.8 (1.8) - 6.9 (2.4) - 2.9 (1.5) - 2.3 (1.3)

15 25 20 40 30

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Magnetic Field in the Local Galaxy

o xo o o

Magnetic field structure of Milky Way derived from pulsars. Han & Zhang (2008)
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Summary
> HII regions have magnetic fields with B|| 5 µG (diffuse ISM). > Characteristic electron densities range between 1 < n0 < 30 cm-3 > Magnetic fields within an HII region range from 1 < B|| < 10 µG. > B has a uniform line-of-sight orientation on scales of 15 - 40 pc. > There is little or no change in B in the diffuse ISM between 1 -10 cm-3. >P P mag thermal > The parallel component of the magnetic field in HII regions is consistent with
the Galactic magnetic field structure derived by Han & Zhang (2008).
.

> Future studies: RM structure functions to investigate scaling of turbulence. > Talk: "The role of magnetic fields in controlling the structure of HII
regions" (Gary Ferland, Thursday afternoon)

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