IR Astronomy In Search of Grain Alignment Pierre Bastien Megan Krejny Kathleen DeWahl Terry Jay Jones University of Minnesota

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 Discovery of Interstellar Polarization by Hiltner and Hall in Cause? Aligned aspherical dust grains. Davis and Greenstein suggest dissipation in the grain by rotation in a magnetic field. Davis and Greenstein then produce their famous 1951 paper on paramagnetic relaxation.

IR Astronomy

Heiles Compilation

IR Astronomy Synchrotron Polarization from Sukumar and Allen

IR Astronomy Problems with classical DG Relaxation time is very long for paramagnetic substances Thermal fluctuations in the grain try and align the grain the opposite way Requires very different spin and thermal grain temperatures But, we observe nearly maximum alignment and short alignment times in many phases of the ISM.

IR Astronomy 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. Mantra

IR Astronomy

Whittet et al. 2008

IR Astronomy Whittet et al. 2008

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

IR Astronomy Hoang and Lazarian, 2009 Grain Alignment by Radiative Torques

IR Astronomy

efficient alignment + turbulence ??

IR Astronomy Ward-Thompson et al. preprint

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

IR Astronomy

E(H-K)/  mm P K /E(H-K) Normal Low Normal All grains are aligned Just like the diffuse ISM Why low submm P? Big grains dominate NIR extinction low NIR Pol only from small Low Normal grains Small grains not aligned Big grains are aligned Small grains not aligned Big grains partially aligned Conclusion?

IR Astronomy