Magnetic Fields in Star Formation

Alyssa A. GoodmanHarvard-Smithsonian Center for

Astrophysics

Tyler BourkeSmithsonian Astrophysical

Observatory/SMAFigure credit: Heitsch et al. 2001 simulation

Question 1:How Much

Do Magnetic Fields Matter in Molecular

Clouds?

see Bourke et al. 2001; Crutcher 1999

and references therein

Question 2:How, Exactly, Do Magnetic Fields Matter

in the Disk/Outflow

System?

figure

fro

m O

stri

ker

& S

hu 1

99

8

QuickTime™ and aTIFF (Uncompressed) decompressorare needed to see this picture.

figure

court

esy

NA

SA

B-Observers Toolkit

Neutral ISMPolarimetry

Background Starlight

Thermal Emission

ZeemanThermal

EmissionAbsorption

Masers

Polarized Spectral Lines

Ionized ISMPolarized continuum

B directionFaraday Rotation

B=RM/DM

Recombination Line Masers

Large Molecular

Clouds

Jets and Disks

"Cores" and Outflows

Solar System Formation

Which Polarimetry Where

Background Starlight

nothing yet...

Thermal Emission

Thermal Emission& Scattered Light

but not inside cold, dark clouds

Large Molecular

Clouds

Jets and Disks

"Cores" and Outflows

Solar System Formation

Which Zeeman Where

H I, including self-absorption, OH

nothing yet...

OH and CNin Cores

H2O and OHMaser Emission

Large Molecular

Clouds

Jets and Disks

"Cores" and Outflows

Solar System Formation

Polarized (Thermal) Spectral Lines

CO detectedat BIMA & JCMT

nothing yet…

nothing yet... nothing yet…

NEW!

B-Analysis Toolkit

Analytic Predictions

Numerical Simulations

Chandrasekhar-Fermi Method

Naïveté or the Simplest Analytic Models:The waywe once thoughtpolarization maps might look…

(or)

Disk + Star

Core

Dark Cloud, Theory #2

Dark Cloud, Theory #1

A Truly Theoretical Set of Polarization Maps

Magnetohydrodynamic Models

Strong Field=0.01, M=7

Weak Field=1, M=7

Synthetic Polarization Maps from Ostriker, Stone & Gammie 2001; see also Heitsch et al. 2001; Padoan et al. 2003

The Chandrasekhar-Fermi Method

€

Bo =σv

σθ

4πρNcorr

= numerical factor× velocity dispersion

polarization dispersion× density

see Myers & Goodman 1991; Sandstrom & Goodman 2003 for details

~modeling field strength from polarization map messiness

messyweak fieldorderedstrong field

Simulations oftenimply Ncorr~4 in “dark clouds”

Polarization Maps

Spectral-linemaps

Extinction,dust emission,or spectral-line

maps

B-Observers Toolkit

Neutral ISMPolarimetry

Background Starlight

Thermal Emission

ZeemanThermal

EmissionAbsorption

Masers

Polarized Spectral Lines

The GalaxyThe GalaxySerkowski, Mathewson & Ford, et al.

Note: Background starlight polarization is parallel to l.o.s. field

Dark C

loud

Com

ple

xes: 1

-10 p

c sca

les

Dark C

loud

Com

ple

xes: 1

-10 p

c sca

les

Polarization ofBackground Starlight in Taurus

Magnetic Fields

Dark C

loud

Com

ple

xes: 1

-10 p

c sca

les

Background Starlight Polarimetry “Fails” at AV>1.3 mag in Dark Clouds

3.0

2.5

2.0

1.5

1.0

0.5

0.0

PR

[%]

43210

43210AV [mag]

Background to Cold Dark Cloud

Arce

et a

l. 1998

Background to General ISMcf. Goodman et al. 1992; 1995

“Bad Grains” in Cold Cloud Interiors

Thermal Emission Polarimetry

10-20

10-18

10-16

10-14

10-12

10-10

10-8

B [

erg

sec-1

cm

-2 H

z-1 s

ter-1

]

0.0010.010.1110100

Wavelength [cm]

108

109

1010

1011

1012

1013

1014

Frequency [Hz]

Emissivity-Weighted, normalized, blackbodies

10 K

30 K

100 K

sub-mm:sub-mm:JCMT, CSOJCMT, CSOSMASMA

far-IR:far-IR:KAOKAOSOFIASOFIA

mm:mm:OVRO, BIMA, OVRO, BIMA, CARMACARMAALMAALMA

Thermal Emission Results Summary

>pc-scales: No earthbound instrument sensitive enough, no space instrument capable (a shame!)

~pc-scales: KAO/STOKES, CSO/HERTZ, JCMT/SCUBA have all had success, and all see “polarization holes” at high density (see Brenda Matthews’ talk!)

<<pc scales: BIMA & OVRO have had success, and also see “polarization holes” at high density

Honestly: Results from all scales suggestive, but not yet “conclusive,” on field’s role at large or small scales. CF method promising.

Vallé et al. 2003

“Polarization Hole”

W51 Polarization from BIMA: Lai et al. 2001

“Polarization Holes”

How to Interpret Maps with “Holes”?

C2

C3

C1

Padoan, G

oodm

an, D

rain

e, Ju

vela

,Nord

lund, R

ögnvald

sson 2

00

1

3-D simulation•super-sonic•super-Alfvénic•self-gravitating

Model A:Uniform grain-alignment efficiency

Padoan, G

oodm

an, D

rain

e, Ju

vela

,Nord

lund, R

ögnvald

sson 2

00

1

3-D simulation•super-sonic•super-Alfvénic•self-gravitating

Model B:Poor Alignment at AV≥3 mag

C2

C3

C1

AV,0 =3 mag

How to Interpret Maps with “Holes”?

SCUBA-like Cores with Holes

Core C1

Core C2

Core C3

Core C1; AV,0=3 mag

Core C2; AV,0=3 mag

Core C3; AV,0=3 mag

Padoan, G

oodm

an, D

rain

e, Ju

vela

,Nord

lund, R

ögnvald

sson 2

00

1

It seems nearly all polarization maps show decrease in polarizing efficiency with density.

Derived models of 3D field (for comparisons) need to

take this into account.

Zeeman Results Summary

see Bourke et al. 2001; Crutcher 1999 and references

therein

Detections hard to come by

In general, B less than or “close” to equipartition

The Chandrasekhar-Fermi Method

with correction factors suggested by simulations, agrees well with Zeeman data, but is MUCH easier to use S

andstro

m &

Goodm

an

2003

Shown here for optical polarization, in dark clouds,but seems to work (compare well with measured Zeeman) for emission polarization as well.

Polarized Spectral-Line Summary

Effect predicted by Goldreich & Kylafis, 1981

1st detection in a star-forming region (NGC 1333): Girart et al. 1999 (BIMA)

Subsequent detection with JCMT/SCUBA (in NGC2024): Greaves et al. 2001

Still very difficult to interpret (polarization can be parallel or perpendicular to B!--need context)

NGC 1333 IRAS 4A

Girart et al. 1999

CO Polarization

Dust Polarization(in white)

“Not ,Exactly”

(or)

Disk + Star

Core

Dark Cloud, Theory #2

Dark Cloud, Theory #1

A Truly Theoretical Set of Polarization Maps

B-Analysis “Challenges”

Line of sight averaging of vector quantity=complex radiative transfer

Decline of grain alignment efficiency in high-density regions (how to interpret data w/holes?)

Multiple velocity components in spectral lines (particularly bad in Zeeman case)

Ambiguities in interpreting polarized spectral-line emission (depends on , etc.)

Question 1:How Much

Do Magnetic Fields Matter in Molecular Clouds?

Question 2:How, Exactly, Do Magnetic

Fields Matter in the Disk/Outflow System?

The High-Resolution Future: Observations

SMA, CARMA, ALMA (~Question 2)Resolve field in circumstellar disks & flows near

YSOs Dust continuum polarimetry (see Matthews)

mm spectral-line polarimetry (see Greaves/Crutcher who’s there?)

Square Kilometer Array (~Question 1)Understand field-tangling/structure within big

single-dish beamsZeeman observations (see Bourke)

RM/DM & synchrotron observations (see Gaensler)

Connect our views of the field in neutral & ionized ISM??

Remember…1 arcsec = 100 A.U. at 100 pc

The High-Resolution Future: Theory & Simulation

AnalyticalDetailed predictions of the (about-to-be-observed)

interface between the stellar and disk/outflow (e.g. “X-wind”) field structure (Question 2)

Numerical(near-term) Models of synthetic polarization and

Zeeman observations at ~100 A.U. scales (Question 2)

(longer-term) High-resolution MHD simulation all the way from pc to A.U. scales (Questions 1& 2)(Current limits ~10 pc to 0.1 pc)

109 3D pixels gives resolution of ~10 A.U. over a volume of 0.1 pc

The Unconventional Future

Incorporating neutral/ion line width ratios to get 3D field (see Houdé et al. 2002)

Anisotropy in velocity centroid maps as a diagnostic of the mean magnetic field strength in cores (see Vestuto, Ostriker & Stone 2003)

Interpretation of microwave polarization (e.g. from WMAP) as due to rapidly spinning (magnetically aligned?) grains (see Finkbeiner 2003 and Hildebrand & Kirby 2003 & references therein)