magnetic field morphologies in ngc1333 iras4a: evidence for hour glass structure r. rao (cfa) j. m....
Post on 21-Dec-2015
229 Views
Preview:
TRANSCRIPT
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)
top related