Magnetic field morphologies in ngc1333 iras4a evidence for hour glass structure
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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?.

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

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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 via

  • Zeeman effect -- strength and direction of B_los

  • 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)


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


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

Control computer

Waveplate


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)


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