Magnetic Fields in Star Formation
Alyssa A. Goodman
Harvard-Smithsonian Center for Astrophysics
Tyler Bourke
Smithsonian Astrophysical Observatory/SMA
Figure 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
figure courtesy NASA
Question 2:
How, Exactly,
Do Magnetic
Fields Matter
in the
Disk/Outflow
System?
figure from Ostriker & Shu 1998
Quic kTime™ and a TIFF (Uncompress ed) dec ompres sor are needed to s ee this pic ture.
B-Observers Toolkit
Neutral ISM
Polarimetry
Background Starlight
Thermal Emission
Zeeman
Thermal
Emission
Absorption
Masers
Polarized Spectral Lines
Ionized ISM
Polarized continuum
B direction
Faraday Rotation
B=RM/DM
Recombination Line
Masers
Which Polarimetry Where
"Cores" and
Outflows
Background Starlight
but not inside cold, dark clouds
Large
Molecular
Clouds
Thermal Emission
Jets and
Disks
Solar System
Formation
nothing yet...
Thermal Emission
& Scattered Light
Which Zeeman Where
"Cores" and
Outflows
H I, including self-absorption, OH
Large
Molecular
Clouds
OH and CN
in Cores
Jets and
Disks
Solar System
Formation
nothing yet...
H2O and OH
Maser Emission
Polarized (Thermal) Spectral Lines
"Cores" and
Outflows
nothing yet…
Large
Molecular
Clouds
Jets and
Disks
CO detected
at BIMA
& JCMT
Solar System
Formation
nothing yet...
nothing yet…
B-Analysis Toolkit
Analytic Predictions
Numerical Simulations
Chandrasekhar-Fermi Method
A Truly Theoretical Set of Polarization Maps
Dark Cloud,
Theory #1
Naïveté or
the Simplest
Analytic
Models:
The way
we once thought
polarization
maps might
look…
(or)
Dark Cloud,
Theory #2
Core
Disk
+ Star
Magnetohydrodynamic Models
Synthetic Polarization Maps from Ostriker, Stone & Gammie 2001;
see also Heitsch et al. 2001; Padoan et al. 2003
Strong Field
b=0.01, M=7
Weak Field
b=1, M=7
The Chandrasekhar-Fermi Method
~modeling field strength from polarization map messiness
messyweak field
orderedstrong field
v
B 

4 
N corr
Spectral-line
maps
Extinction,
dust emission,
or spectral-line
maps
velocity dispersion
 numerical factor 
 density
polarization dispersion
Simulations often
imply Ncorr~4 in
“dark clouds”
Polarization
Maps
see Myers & Goodman 1991; Sandstrom & Goodman 2003 for details
B-Observers Toolkit
Neutral ISM
Polarimetry
Background Starlight
Thermal Emission
Zeeman
Thermal
Emission
Absorption
Masers
Polarized Spectral Lines
The Galaxy
Serkowski, Mathewson & Ford, et al.
Note: Background starlight polarization is parallel to l.o.s. field
Dark Cloud Complexes: 1-10 pc scales
Dark Cloud Complexes: 1-10 pc scales
Polarization of
Background
Starlight in Taurus
Dark Cloud Complexes: 1-10 pc scales
Magnetic Fields
Background Starlight Polarimetry “Fails” at
AV>1.3 mag in Dark Clouds
0
1
2
3
4
3.0
PR [%]
Arce et al. 1998
2.5
2.0
“Bad Grains” in Cold Cloud Interiors
1.5
1.0
0.5
0.0
0
1
AV
2
[mag]
3
4
Background to Cold Dark Cloud
Background to General ISM
cf. Goodman et al. 1992; 1995
Thermal Emission Polarimetry
Wavelength [cm]
10
far-IR:
KAO
SOFIA
-1
Bn [erg sec cm Hz ster ]
10
-10
10
1
0.1
0.01
0.001
Emissivity-Weighted,
normalized, blackbodies
-1
-12
10
-14
-1
JCMT, CSO
SMA
-2
sub-mm:
10
100
-8
mm:
OVRO, BIMA,
CARMA
ALMA
10
10
-16
-18
100 K
10
30 K
-20
10 K
10
8
10
9
10
10
10
11
Frequency [Hz]
10
12
10
13
10
14
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”
“Polarization
Holes”
W51 Polarization from BIMA: Lai et al. 2001
C1
3-D simulation
•super-sonic
•super-Alfvénic
•self-gravitating
C2
Model A:
Uniform grainalignment
efficiency
C3
Padoan, Goodman, Draine, Juvela,Nordlund, Rögnvaldsson 2001
How to Interpret Maps with “Holes”?
C1
3-D simulation
•super-sonic
•super-Alfvénic
•self-gravitating
C2
Model B:
Poor Alignment at
AV≥3 mag
C3
AV,0 =3 mag
Padoan, Goodman, Draine, Juvela,Nordlund, Rögnvaldsson 2001
How to Interpret Maps with “Holes”?
Core C1
Core C2
Core C3
Core C1; A V,0=3 mag
Core C2; A V,0=3 mag
Core C3; A V,0=3 mag
Padoan, Goodman, Draine, Juvela,Nordlund, Rögnvaldsson 2001
SCUBA-like Cores with Holes
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
Detections hard to
come by
In general, B less than
or “close” to
equipartition
see Bourke et al. 2001; Crutcher
1999 and references therein
The Chandrasekhar-Fermi Method
with10 correction factors
suggested by
simulations, agrees well
with10 Zeeman data, but
is MUCH easier to use
-2
-3
Sandstrom & Goodman 2003
B (Gauss)
-4
10
-5
10
-6
C-F Method B region
Zeeman Data
10
Shown here for optical polarization, in dark clouds,
-7
but seems10to
work2 (compare
well with
measured
3
4 5 6 7 89
2
3
4 5 6 7 89
21
22
23
10
10
Zeeman) for10emission polarization
as well.-2
NH2 (cm )
2
3
4
5 6 7 8 9
24
10
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
CO Polarization
Dust Polarization
(in white)
Girart et al. 1999
A Truly Theoretical Set of Polarization Maps
Dark Cloud,
Theory #1
(or)
Dark Cloud,
Theory #2
“Not ,
Exactly”
Core
Disk
+ Star
B-Analysis “Challenges”
Line of sight averaging of vector
quantity=complex radiative transfer
Decline of grain alignment efficiency in highdensity regions (how to interpret data
w/holes?)
Multiple velocity components in spectral lines
(particularly bad in Zeeman case)
Ambiguities in interpreting polarized spectralline emission (depends on t, 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 singledish beams
Zeeman 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
Analytical
Detailed 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)