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 Workshop on Polarimetry with the SMA c. 1998

Outline Scientific objectives Radio interferometric techniques Instrument details and calibration Current status

Importance of B-fields in the star formation process Cloud support - magnetic Ambipolar diffusion - redistributes flux Magnetic braking - remove angular momentum Magnetohydrodynamic waves and turbulence - explain linewidths

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 1.Zeeman effect -- strength and direction of B_los 2.Linear polarization of aligned dust grains (absorption and emission) -- only direction of B_sky

Methods of Detection Zeeman effect Linear Polarization

Linear Polarization Measurements of Dust First observed by Hall and Hiltner (1949; 1951) - absorption of star light in optical Polarization is caused by aligned aspherical dust grains Polarization can be used to infer the direction of the aligning magnetic field Both in absorption and emission

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 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. 1995Lai 2001 Akeson & Carlstrom 1997

Stokes Parameters Attempt to detect Stokes Q and U Linearly Polarized Flux P=√(Q^2+U^2) exp(I 2  ) Position angle  =arctan(U/Q)

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

Radio Interferometric Polarimetry Best done by cross-correlating orthogonal circular polarizations Pioneering work of Morris et al. (1964); Changes to formalism introduced by Hamaker, Bregman and Sault (1996; HBS)

Schematic of Polarization Interferometer

Morris Equation for Correlator Output

Ideal Orthogonal Circular Feeds

Matrix Formalism of HBS Elegant and simplified approach when compared with Morris et al. Uses Jones matrices for each element in the interferometer path If there are different elements, the net result is just an outer product of each individual Jones matrix

Jones Matrix Computations

Output of SMA interferometer For one pair of SMA antennas, the output can be written as Where the observed Visibilities are =>

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

Calibration of Instrument Calibration of polarization consists of two steps - a) Instrumental polarization or leakage AND b) Polarization angle calibration Leakage terms are determined by observing a point source over a wide range of parallactic angle Instrumental polarization will be frequency dependent May need to consider off-axis effects

Simplified Calibration Equations

Calibration Process Illustrated

Imaging Time average under Walsh cycle to get artificial dual polarization observations Apply calibration corrections Form Stokes I,Q,U,V visibilities from LL,RR,LR,RL correlations Invert to make maps Combine Q,U to form polarized flux P and position angle maps Errors occur due to imperfect instrumental calibration

Important Issues Software changes need to be made to the system in order to undertake these observations Future dual polarization receivers will aid significantly in both sensitivity and simplification of observing - both hardware and software

Current Status Prototype system installed on 3 antennas in Summer of 2003 ; used to test and debug system to identify hardware and software issues Full system will be installed in Spring/Summer 2004 First target observations of Sgra A*

SMA Observations Dates: December 4th and 5th, 2004 Array Configuration: compact (5/6 antennas) Weather: Excellent; tau ~ Frequency: CO 3-2 at 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 = 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)

Results There is depolarization towards the center Polarization peak is offset from the position of peak continuum flux density We can clearly see the expected 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