Magnetic Fields in Star Formation Alyssa A Goodman

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 figure from Ostriker & Shu 1998 Question 2: How, Exactly, Do Magnetic Fields Matter in the Disk/Outflow System?

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. . . H 2 O and OH Maser Emission

Polarized (Thermal) Spectral Lines "Cores" and Outflows nothing yet… Large Molecular Clouds ! W E N 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

Naïveté or the Simplest Analytic Models: The way we once thought polarization maps might look…

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 Spectral-line maps Simulations often imply Ncorr~4 in “dark clouds” Extinction, dust emission, or spectral-line maps 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: -10 10 -12 10 -14 10 -16 10 -18 1 0. 01 0. 001 Emissivity-Weighted, normalized, blackbodies -1 JCMT, CSO SMA -2 sub-mm: 10 10 -1 -1 Bn [erg sec cm Hz ster ] KAO SOFIA 100 -8 mm: OVRO, BIMA, CARMA ALMA 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” W 51 Polarization from BIMA: Lai et al. 2001

3 -D simulation • super-sonic • super-Alfvénic • self-gravitating Model A: Uniform grainalignment efficiency Padoan, Goodman, Draine, Juvela, Nordlund, Rögnvaldsson 2001 How to Interpret Maps with “Holes”?

3 -D simulation • super-sonic • super-Alfvénic • self-gravitating Model B: Poor Alignment at AV≥ 3 mag Padoan, Goodman, Draine, Juvela, Nordlund, Rögnvaldsson 2001 How to Interpret Maps with “Holes”?

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 3 D 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 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, 1981 1 st detection in a star-forming region (NGC 1333): Girart et al. 1999 (BIMA) Subsequent detection with JCMT/SCUBA (in NGC 2024): Greaves et al. 2001 Still very difficult to interpret (polarization can be parallel or perpendicular to B!--need context)

NGC 1333 IRAS 4 A CO Polarization Dust Polarization (in white) Girart et al. 1999

“Not , Exactly”

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 3 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 3 D 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)