Magnetic Fields in Star Formation Alyssa A. Goodman Harvard-Smithsonian Center for Astrophysics Tyler Bourke Smithsonian Astrophysical Observatory/SMA Figure credit: Heitsch et al 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 from Ostriker & Shu 1998 figure courtesy NASA

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

Large Molecular Clouds Jets and Disks "Cores" and Outflows Solar System Formation Which Polarimetry Where Background Starlight nothing yet... 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 CN in Cores H 2 O and OH Maser Emission

Large Molecular Clouds Jets and Disks "Cores" and Outflows Solar System Formation Polarized (Thermal) Spectral Lines CO detected at 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 way we once thought polarization maps might look…

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 see Myers & Goodman 1991; Sandstrom & Goodman 2003 for details ~modeling field strength from polarization map messiness messy  weak field ordered  strong field Simulations often imply N corr ~4 in “dark clouds” Polarization Maps Spectral-line maps Extinction, dust emission, or spectral-line maps

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

Polarization of Background Starlight in Taurus

Magnetic Fields Dark Cloud Complexes: 1-10 pc scales

Background Starlight Polarimetry “Fails” at A V >1.3 mag in Dark Clouds P R [%] A V [mag] Background to Cold Dark Cloud Arce et al Background to General ISM cf. Goodman et al. 1992; 1995 “Bad Grains” in Cold Cloud Interiors

Thermal Emission Polarimetry B [erg sec cm -2 Hz ster ] Wavelength [cm] Frequency [Hz] Emissivity-Weighted, normalized, blackbodies 10 K30 K 100 K sub-mm: JCMT, CSO SMA far-IR:KAOSOFIA mm: OVRO, BIMA, CARMA ALMA

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 “Polarization Hole”

W51 Polarization from BIMA: Lai et al “Polarization Holes”

How to Interpret Maps with “Holes”? Padoan, Goodman, Draine, Juvela,Nordlund, Rögnvaldsson D simulation super-sonic super-Alfvénic self-gravitating Model A: Uniform grain- alignment efficiency

Padoan, Goodman, Draine, Juvela,Nordlund, Rögnvaldsson D simulation super-sonic super-Alfvénic self-gravitating Model B: Poor Alignment at A V ≥3 mag How to Interpret Maps with “Holes”?

SCUBA-like Cores with Holes Padoan, Goodman, Draine, Juvela,Nordlund, Rögnvaldsson 2001

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 Sandstrom & Goodman 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, st detection in a star-forming region (NGC 1333): Girart et al (BIMA) Subsequent detection with JCMT/SCUBA (in NGC2024): Greaves et al Still very difficult to interpret (polarization can be parallel or perpendicular to B!--need context)

NGC 1333 IRAS 4A Girart et al CO Polarization Dust Polarization (in white)

“Not, Exactly”

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 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) D 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)

Polarization vs. Intensity Padoan, Goodman, Draine, Juvela,Nordlund, Rögnvaldsson 2001

Effects of Viewing Angle x y z Uniform Alignment No Alignment A V >3

The Way Numerical Models say it Is…

Polarization of Background Starlight

Polarization of Thermal Dust Emission

(Sample) SCUBA Polarimetry of Dense Cores & Globules Does polarization map give true field structure? Polarization drops with sub- mm flux (similar to p decreasing with A V ) Plots and data from Henning, Wolf, Launhart & Waters 2001

2" resolution; Lai et al. 2002