EVIDENCE FOR H₂FORMATION DRIVEN DUST GRAIN ALIGNMENT IN IC 63

Abstract

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 H 2 formation, by showing that the polarization of the light from stars behind the reflection nebula IC 63 appears to correlate with the intensity of H 2 fluorescence. 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 H 2 formation and a probe of the dust characteristics, these results also provide one of the first direct confirmations of the grain-surface formation of H 2. We compare our observations to ab initio modeling based on Radiative Torque Alignment (RAT) theory.

A Unified Model of Grain Alignment: Radiative Alignment of Interstellar Grains With Magnetic Inclusions

“…First of all, the existence of the high-J attractor point depends on the q max parameter of grains, but its distribution is uncertain for irregular interstellar grains. Moreover, in the absence of high-J attractor points, the degree of alignment still can be significantly increased if grains are also subject to suprathermal torques and the evidence of this effect has been claimed in observations (Andersson et al 2013). The MRAT scenario elaborated in this work provides an alternative explanation.…”

“…First of all, it was found that the alignment can be enhanced in the presence of H 2 torques (Andersson et al 2013;Hoang et al 2015). This does not contradict to MRAT process, as the effect of H 2 torques on grain alignment is expected to be still applicable because the small size cut-off will shift to a smaller size due to H 2 torques, allowing more grains to be aligned and thus increasing the total polarization.…”

Are the carriers of diffuse interstellar bands and extended red emission the same?

“…This led to a colour excess E B−V = 1.23 and a magnitude of V = 18.24. The discrepancy with the earlier measurements is consistent with an inappropriate sky background value used in the SDSS photometry pipeline employed by Andersson et al (2013).…”

Interstellar Dust Grain Alignment

“…To probe the effect of pinwheel torques, Andersson et al (2013) investigated the polarization properties toward stars behind the reflection nebula IC 63 ( Jansen et al 1994( Jansen et al , 1995Luhman et al 1997;France et al 2005;Andersson et al 2013). Eighty-five percent of electronic photoexcitations that initiate photodestruction of the molecule relax to the ground state.…”