The Evolution of Magnetic Field Structure in Star-forming Cores
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Transcript of The Evolution of Magnetic Field Structure in Star-forming Cores
THE EVOLUTION OF MAGNETIC FIELD STRUCTURE IN STAR-FORMING 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!
PBp or P‖Bp
Polarization of Dust Emission: PBp Absorption: P‖Bp
DUST POLARIZATION OBSERVATIONS
EXAMPLE: NGC1333 IRAS4A
(Girart, Rao, & Crutcher 1999)
EXAMPLE: CN IN DR21(OH)
Crutcher et al. 1999Lai et al. 2003
PREVIOUS RESULT – NGC 1333 IRAS 4A
Polarization map (left) and magnetic field map (right) fromGirart, Rao, & Marrone (2006), Science, 313, 812
MASSIVE STAR FORMING CORES
Girart et al (in prep)
G5.89 G31.41
Tang, Lai et al. (2009, submitted)
IRAS16293 (CLASS 0) : B MAP
Rao, Lai et al., in prepPmax~5%
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)
RESULTSSourc
e Type
Peak I (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
L1551IRS 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
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ALMA is The Future
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 - PERSEUSNAME CLASS Tbol (K) Lbol (L⊙) Menv(L⊙)
HH 211 0 24 1.5 3.0NGC 1333-IRAS 4A
0 51 4.2 7.8
NGC 1333-IRAS 2A
0 69 19.0 2.8
SVS 13A I 188 32.5 3.2NGC 1333-IRAS 6
I 304 6.2 1.9
SOURCE LIST - OPHIUCHUSNAME CLASS Tbol (K) Lbol (L⊙) Menv(L⊙)
IRAS 16253-2429
0 35 0.25 0.51
IRAS 16293-2422B
0 54 6.9 1.16
VLA 1623 0 57 0.41 0.24
GSS 30 I 133 18.3 0.22EL 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?
October 8, 2008 ALMA Band 1 Workshop 24
– 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, ???
Restoring ALMA Capabilities