Transcript SMA Observations of Dust Polarization in VLA1623 and L1551

THE EVOLUTION OF MAGNETIC
FIELD STRUCTURE IN STARFORMING CORES
Shih-Ping Lai
賴詩萍
(National Tsing-Hua University, Taiwan)
清華大學, 台灣
INTRODUCTION - THEORIES
Magnetic fields are believed to
control the formation and
evolution of the molecular
clouds and core.
 Questions:
 B Geometry vs. Age
 How much magnetic flux will
be
diffused
along
the
evolutionary stages?
Image taked from Crutcher (2006), Science

B FIELD VS TURBULENCE?
HOW TO MEASURE B?


Zeeman Effect – Very Difficult!!

The only way to measure magnetic field
strength Blos
Polarized Molecular Line Emission
(the Goldreich-Kylafis Effect) – Very Difficult too!
PBp or P‖Bp

Polarization of Dust Emission: PBp
Absorption: P‖Bp
DUST POLARIZATION OBSERVATIONS
EXAMPLE: NGC1333 IRAS4A
(Girart, Rao, & Crutcher 1999)
EXAMPLE: CN IN DR21(OH)
Lai et al. 2003
Crutcher et al. 1999
PREVIOUS RESULT – NGC 1333 IRAS 4A
Polarization map (left) and magnetic field map (right) from
Girart, Rao, & Marrone (2006), Science, 313, 812
MASSIVE STAR FORMING CORES
G31.41
Girart et al (in prep)
G5.89
Tang, Lai et al. (2009, submitted)
IRAS16293 (CLASS 0) : B MAP
Pmax~5%
Rao, Lai et al., in prep
EVOLUTION OF B FIELDS IN LOW MASS
CORES?
VLA1623
(Class 0)
L1551 IRS1
(Class I)
HL Tau
(Class II)
Lai et al. (2009, in preparation)
RESULTS
Source Peak I
Type (Jy/beam)
P% at
peak
Polarization
Angle at
peak(deg)
Total I
(Jy)
Total P
(%)
Total PA
(deg)
NGC1333
IRAS 4A
Class 0
1.91
(0.04)
0.77
(0.17)
-35.8
(6.2)
6.7
13.1
22.5
VLA 1623
Class 0
0.94
(0.04)
1.06
(0.35)
-43.7
(9.5)
1.7
0.63
-11
L1551
IRS 5
Class I
2.41
(0.04)
0.51
(0.21)
22
(11)
3.7
0.46
-44
HL Tau
Class II
0.31
(0.04)
0.78
(0.32)
63
(12)
0.3
--
--
POSSIBLE EXPLANATIONS




3 targets are observe pole-on…. (very unlikely)
Growth of dust size reduce the magnetic alignment
efficiency
Magnetic field evolve from uniform to random during
Class 0 stage, and remain random after Class 0 stage
Magnetic field diffuse quickly during Class 0 stage


Ambipolar diffusion time scale ~ 105 yrs
Statistical life time for class 0 ~ 105 yrs
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CONCLUSIONS



We have sensitive polarization observations for VLA
1623, L1551 IRS5, and HL Tau (uncertainty in
polarization is ~ 0.2-0.3%)
Compared to the previous NGC 1333 IRAS4A
observation, our results show significant weaker
polarization.
Our results suggest that the polarization is weaker in
the later stage of the evolution
Magnetic flux decay quickly after Class 0 stage
ALMA is required to measure more cores to support our conclusion
ALMA is The Future
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ALMA DESIGN REFERENCE SCIENCE PLAN 2.1
MY PROPOSAL

We can do more!
 Using
ACA+12m to increase sensitivity
 7-pointing small mosaic to preserve extended flux
 1 hr in each individual field will give σ=0.17 mJy for
the mosaic image, enough to detect 1%
polarization with S/N=6 for a 0.1 Jy source
 10 source * 1hr * 7 pointings = 70 hr
SENSITIVITY
NGC 1333 IRAS 4A
(Lai 2001, BIMA data)
*
BAND 7 IS THE BEST
MY PROPOSAL

We can do more!
Using ACA+12m to increase sensitivity
 7-pointing small mosaic to preserve extended flux
 1 hr in each individual field will give σ=0.17 mJy for the
mosaic image, enough to detect 1% polarization with
S/N=6 for a 0.1 Jy source
 10 source * 1hr * 7 pointings = 70 hr


Choose 5 objects in Oph and 5 in Perseus
Both have abundant YSOs at different stages
 YSO content is well study by Enoch et al. (2009) using
Spitzer + Bolocam data

SOURCE LIST - PERSEUS
NAME
CLASS
Tbol (K)
Lbol (L⊙)
Menv(L⊙)
HH 211
0
24
1.5
3.0
NGC 1333IRAS 4A
0
51
4.2
7.8
NGC 1333IRAS 2A
0
69
19.0
2.8
SVS 13A
I
188
32.5
3.2
NGC 1333IRAS 6
I
304
6.2
1.9
SOURCE LIST - OPHIUCHUS
NAME
CLASS
Tbol (K)
Lbol (L⊙)
Menv(L⊙)
IRAS 162532429
0
35
0.25
0.51
IRAS 162932422B
0
54
6.9
1.16
VLA 1623
0
57
0.41
0.24
GSS 30
I
133
18.3
0.22
EL 29/GY214
I
257
17.9
0.07
PROBLEM



Dual Polarization – probably ready
Wave plates – still needed!
Calibration : “An ongoing program to better understand the
polarization performance of the array, and potential routes to
modify receiver optics to increase polarization accuracy and
sensitivity to further this goal is desirable.”
PREPARATION

What should we see from different models?
Restoring ALMA Capabilities
– WVR upgrades: phase correction is better understood
a next generation WVR sytem may be desirable
– polarimetry: developing a dedicated deployable
polarimetry system with rotating waveplates
– correlator data retention: correlator has very high time
resolution but raw data can’t presently be kept
– software upgrades: improve data taking efficiency,
new algorithms and heuristics, ???
October 8, 2008
ALMA Band 1 Workshop
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