Magnetic Field Morphologies in NGC1333 IRAS4A:

Evidence for Hour Glass Structure

R. Rao (CfA)

J. M. Girart (CSIC/IEEC)

D. P. Marrone (CfA)

Prologue

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

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 via1. Zeeman effect -- strength and direction of B_los2. Linear polarization of aligned dust grains

(absorption and emission) -- only direction of B_sky

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

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)

Rao et al. 1998

OMC1 Observations contd.

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

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

NGC 1333 IRAS 4A is an ideal target for the SMA

Hayashi et al. 1995

Lai 2001

Akeson &Carlstrom 1997

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

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

SMA Polarization Hardware

Waveplate

Control computer

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.

NGC 1333 IRAS4A - Dust Continuum

• Beamsize 1.6 x 1.0 arcsec

• Resolve into 4A1 and 4A2

• Peak intensity 1.9 Jy/bm

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

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

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)