The Evolution of Magnetic Field Structure in Star-forming Cores

1. Shih-Ping Lai 賴詩萍 (National Tsing-Hua University, Taiwan) 清華大學, 台灣 The Evolution of Magnetic Field Structure in Star-forming Cores

2. Introduction - Theories • Magnetic fields are believed to control the formation and evolution of the molecular clouds and core. • Questions: • BGeometry vs. Age • How much magnetic flux will be diffused along the evolutionary stages? Image taked from Crutcher (2006), Science

3. B field vs Turbulence?

4. 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Bpor P‖Bp • Polarization of Dust Emission:PBp Absorption: P‖Bp

5. Dust Polarization Observations

6. Example: NGC1333 IRAS4A (Girart, Rao, & Crutcher 1999)

7. Example: CN in DR21(OH) Lai et al. 2003 Crutcher et al. 1999

8. Previous Result – NGC 1333 IRAS 4A Polarization map (left) and magnetic field map (right) from Girart, Rao, & Marrone (2006), Science, 313, 812

9. Massive Star Forming Cores G5.89 G31.41 Tang, Lai et al. (2009, submitted) Girart et al (in prep)

10. IRAS16293 (Class 0) : B map Pmax~5% Rao, Lai et al., in prep

11. 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)

12. Results

13. 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 Possible explanations

14. 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 Conclusions Magnetic flux decay quickly after Class 0 stage ALMA is required to measure more cores to support our conclusion

15. ALMA is The Future

16. ALMA Design Reference Science Plan 2.1

17. 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 My proposal

18. Sensitivity NGC 1333 IRAS 4A (Lai 2001, BIMA data) *

19. Band 7 is the best

20. 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 My proposal

21. Source List - Perseus

22. Source List - OpHiuchus

23. 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.” Problem Preparation • What should we see from different models?

24. 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, ??? ALMA Band 1 Workshop