Magnetic Field Morphologies in NGC1333 IRAS4A: Evidence for Hour Glass Structure

1. Magnetic Field Morphologies in NGC1333 IRAS4A:Evidence for Hour Glass Structure R. Rao (CfA) J. M. Girart (CSIC/IEEC) D. P. Marrone (CfA)

2. Prologue • Alyssa Goodman’s talk c. 1992 • Workshop on Polarimetry with the SMA c. 1998

3. Why observe polarization? • Magnetic fields are believed to play an important role in the star formation process - support against collapse, ambipolar diffusion • Polarization is the characteristic signature of magnetic fields • Detect magnetic fields via • Zeeman effect -- strength and direction of B_los • Linear polarization of aligned dust grains (absorption and emission) -- only direction of B_sky

4. Polarized Dust Emission • Grains that are polarized in absorption must be polarized in emission as well • Advantages - 1) No need for background object 2) No contamination from extinction and scattering • Single dish: JCMT SCUBA and CSO Hertz polarimeters • Mm-wave arrays: OVRO and BIMA

5. OMC1 Observations • Observations with the Hertz polarimeter at the CSO of showed that the magnetic field was indeed pinched in OMC1 (Schleuning 1998) • Furthermore, there was a decrease in the fractional polarization toward the center Schleuning (1998)

6. OMC1 Observations contd. • BIMA observations at higher angular resolution showed that there is considerable small scale structure near IRc2 (Rao et al. 1998) Rao et al. 1998

7. NGC 1333 IRAS 4A • Low mass Class 0 protostar • Distance uncertain (220 or 350 pc) • Strong dust continuum emission • Resolved into binary (multiple) components (Lay et al. 1995; Looney et al. 1997) • Components 4A1 and 4A2 at a separation of 2” with total mass ~ 1 M_sun • Large scale CO outflow (Blake et al. 1995) • Kinematic studies reveal signatures of infall, outflow, rotation and turbulence (diFrancesco et al. 2001) • Age of 10^4 years from accretion rate

8. NGC 1333 IRAS 4A is an ideal target for the SMA Hayashi et al. 1995 Lai 2001 Akeson & Carlstrom 1997

9. Challenges in Polarimetric Observations • Requires very high signal to noise as the polarization fraction is low • Requires very accurate calibration of the instrumental polarization • Special issues while doing interferometric mm and submm polarization

10. Implementation of Orthogonal CP System at SMA • Design Frequency is 345 GHz • The feed-horns are intrinsically linearly polarized • Circular polarization is produced by inserting a QWP made of a dielectric material • The response is frequency dependent • Fast Walsh function switching in order to simulate simultaneous dual polarization • Nasmyth vs. traditional Cass focus -> effect on parallactic angle

11. SMA Polarization Hardware Control computer Waveplate

12. SMA Observations • Dates: December 4th and 5th, 2004 • Array Configuration: compact (5/6 antennas) • Weather: Excellent; tau ~ 0.04 - 0.06 • Frequency: CO 3-2 at 345.8 GHz • Continuum Bandwidth: 2 GHz in each sideband • Instrumental polarization: 1% in USB; 3 % in LSB.

13. NGC 1333 IRAS4A - Dust Continuum • Beamsize 1.6 x 1.0 arcsec • Resolve into 4A1 and 4A2 • Peak intensity 1.9 Jy/bm

14. NGC 1333 IRAS4A - E vectors • Contours - I • Pixel - polarized flux density sqrt(Q^2+U^2) • RMS = 3 mJy/bm • Peak pol = 9 % at PA 153 degrees • At the peak of Stokes I - pol = 1% • Averaged pol = 4.7% @ 145 degrees

15. NGC 1333 IRAS4A - B vectors • Polarization hole • Polarization peak is offset • Hour glass shape of the magnetic field structure in the circumbinary envelope • The large scale field is well aligned with the minor axis • We will need some higher angular resolution observations to map the structure of the field between the two cores

16. Conclusions/Future Work • Successful mapping of B field structure at high resolution • We can clearly see the expected hour glass shape of the magnetic field structure • In collaboration with theorists, we can try to understand the effects of B-field • Future higher resolution observations and other frequencies (690 GHz)