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

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

IR Astronomy

IR Astronomy

Heiles Compilation IR Astronomy

Synchrotron Polarization from Sukumar and Allen IR Astronomy

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

IR Astronomy efficient alignment + turbulence ?? IR Astronomy

Whittet et al. 2008 IR Astronomy

Whittet et al. 2008 IR Astronomy

Ward-Thompson et al. preprint IR Astronomy

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

IR Astronomy

IR Astronomy

IR Astronomy

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

IR Astronomy

IR Astronomy

Grain Size Distribution IR Astronomy

Radiative Alignment Theory Hoang and Lazarian, 2009 IR Astronomy

IR Astronomy

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

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