2013ApJ...775...84A

In the interstellar medium (ISM), molecular hydrogen is expected to form almost exclusively on the surfaces of dust grains. Due to that molecule's large formation energy (-4.5 eV), several dynamical effects are likely associated with the process, including the alignment of asymmetric dust grains with the ambient magnetic field. Such aligned dust grains are, in turn, believed to cause the broadband optical/infrared polarization observed in the ISM. Here, we present the first observational evidence for grain alignment driven by H₂formation, by showing that the polarization of the light from stars behind the reflection nebula IC 63 appears to correlate with the intensity of H₂fluorescence. While our results strongly suggest a role for "Purcell rockets" in grain alignment, additional observations are needed to conclusively confirm their role. By showing a direct connection between H₂formation and a probe of the dust characteristics, these results also provide one of the first direct confirmations of the grain-surface formation of H₂. We compare our observations to ab initio modeling based on Radiative Torque Alignment (RAT) theory.