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?

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)

RESULTS

Source

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

13

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

15

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 - PERSEUS

NAME CLASS Tbol (K) Lbol (L⊙) Menv(L⊙)

HH 211 0 24 1.5 3.0

NGC 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 3.2

NGC 1333-IRAS 6

I 304 6.2 1.9

SOURCE LIST - OPHIUCHUS

NAME 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 0.22

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?

October 8, 2008October 8, 2008 ALMA Band 1 WorkshopALMA Band 1 Workshop 2424

– 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, ???

– 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 Restoring ALMA Capabilities