Transcript Polarization with the Submillimeter Array (SMA)

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