Magnetic Fields in Star Formation

1. 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

2. see Bourke et al. 2001; Crutcher 1999 and references therein Question 1:How Much Do Magnetic Fields Matter in Molecular Clouds?

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

4. Successful Interferometry! 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

5. "Cores" and Outflows Large Molecular Clouds Jets and Disks Solar System Formation Which Polarimetry Where Background Starlight but not inside cold, dark clouds Thermal Emission nothing yet... Thermal Emission & Scattered Light

6. "Cores" and Outflows Large Molecular Clouds Jets and Disks Solar System Formation Which Zeeman Where H I, including self-absorption, OH OH and CN in Cores nothing yet... H2O and OH Maser Emission

7. "Cores" and Outflows nothing yet… Large Molecular Clouds Jets and Disks Solar System Formation nothing yet... nothing yet… Polarized (Thermal) Spectral Lines NEW! CO detected at BIMA & JCMT

8. B-Analysis Toolkit Analytic Predictions Numerical Simulations Chandrasekhar-Fermi Method

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

10. 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

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

12. B-Observers Toolkit Neutral ISM Polarimetry Background Starlight Thermal Emission Zeeman Thermal Emission Absorption Masers Polarized Spectral Lines

13. The Galaxy Serkowski, Mathewson & Ford, et al. Note: Background starlight polarization is parallel to l.o.s. field

14. Dark Cloud Complexes: 1-10 pc scales

15. Dark Cloud Complexes: 1-10 pc scales Polarization of Background Starlight in Taurus

16. Dark Cloud Complexes: 1-10 pc scales Magnetic Fields

17. 0 1 2 3 4 3.0 2.5 2.0 [%] “Bad Grains” in Cold Cloud Interiors R P 1.5 1.0 0.5 0.0 0 1 2 3 4 A [mag] V Background to Cold Dark Cloud Background to General ISM cf. Goodman et al. 1992; 1995 Background Starlight Polarimetry “Fails” at AV>1.3 mag in Dark Clouds Arce et al. 1998

18. Wavelength [cm] far-IR: KAO SOFIA 100 10 1 0.1 0.01 0.001 -8 10 Emissivity-Weighted, normalized, blackbodies -10 10 ] -1 -12 sub-mm: JCMT, CSO SMA 10 ster -1 Hz -14 10 -2 cm -1 -16 10 [erg sec mm: OVRO, BIMA, CARMA ALMA n B -18 10 100 K 30 K -20 10 10 K 8 9 10 11 12 13 14 10 10 10 10 10 10 10 Frequency [Hz] Thermal Emission Polarimetry

19. 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.

20. Vallé et al. 2003 “Polarization Hole”

21. “Polarization Holes” W51 Polarization from BIMA: Lai et al. 2001

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

23. 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

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

25. 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.

26. Detections hard to come by In general, B less than or “close” to equipartition Zeeman Results Summary see Bourke et al. 2001; Crutcher 1999 and references therein

27. with correction factors suggested by simulations, agrees well with Zeeman data, but is MUCH easier to use The Chandrasekhar-Fermi Method 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.

28. 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)

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

30. “Not ,Exactly”

31. 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 t, etc.)

32. Question 1:How Much Do Magnetic Fields Matter in Molecular Clouds? Question 2:How, Exactly, Do Magnetic Fields Matter in the Disk/Outflow System?

33. 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

34. 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

35. 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)