1. University of Minnesota
Grain Alignment in Starless Cores
Terry Jay Jones
University of Minnesota
Megan Krejny
Kathleen DeWahl
Pierre Bastien
B.G. Andersson
IR Astronomy

2. Goal: Measure the magnetic field geometry in the interstellar medium in the Milky Way and other galaxies.
Why? We want to learn what role magnetic fields play in physical processes in the ISM. In large volumes we find:
Does the magnetic field control phenomena such as star formation, bulk gas motion, coupling of turbulence, confinement, formation of giant molecular clouds etc??
IR Astronomy

3. IR Astronomy

4. IR Astronomy

5. Heiles Compilation
IR Astronomy

6. Synchrotron Polarization from Sukumar and Allen
IR Astronomy

7. Mantra
We can NOT claim to use interstellar polarization to measure the magnetic field geometry without a solid understanding of the grain alignment mechanism.
This is critical in dense, star forming regions, where we do not have the corroborating evidence we see in the very diffuse ISM.
IR Astronomy

8. IR Astronomy efficient alignment + turbulence??
IR Astronomy

9. Whittet et al. 2008
IR Astronomy

10. Whittet et al. 2008
IR Astronomy

11. Ward-Thompson et al. preprint
IR Astronomy

12. What about the Near IR? IR Astronomy J band K band
JHK imaging by Pierre Bastien
IR Astronomy

13. IR Astronomy

14. IR Astronomy

15. IR Astronomy

16. What about polarization in EMISSION?
L183 – 850um (Ward-Thompson et al.)
So called Starless Core’
M17 West – 100um (Dotson et al.)
Warm dense core with Star Formation
IR Astronomy

17. IR Astronomy

18. IR Astronomy

19. Grain Size Distribution
IR Astronomy

20. Radiative Alignment Theory
Hoang and Lazarian, 2009
IR Astronomy

21. IR Astronomy

22. This power law slope steepens beyond AV ~ 10 to about -0.7.
Without a central light source, the polarizing efficiency of dust grains in molecular clouds decreases with optical depth with a power law slope:
This power law slope steepens beyond AV ~ 10 to about -0.7.
By AV ~ 40, Sub-MM observations suggest the slope steepens to -1, which indicates there is NO grain alignment at these high optical depths.
IR Astronomy

23. In the diffuse ISM and in molecular clouds with a luminous, embedded YSO i.e., where there is a strong, asymmetric radiation field, grains are very efficiently aligned.
In molecular cloud regions without an external or internal radiation field, grain alignment is inefficient, and becomes absent at high optical depths.
We must conclude that the presence of a radiation field is an essential component to the grain alignment mechanism.
IR Astronomy