Understanding dust grain alignment

Molecular hydrogen (H2) is the most dominant component of the dense interstellar medium and giant molecular clouds.  Molecular hydrogen cannot form from the collision of two hydrogen atoms because the binding energy of H2 (−4.5 eV) is too high to be radiated by the molecule, and so a third body is needed. In 1948, Henrik van der Hulst proposed a model where H2 forms on the surface of dust grains.  This model has been supported by theory and laboratory experiments, but observational evidence remains limited.

The formation of H2 is also thought to contribute to dust grain alignment, which causes starlight passing through the dusty interstellar medium (ISM) to become polarized. Interstellar polarization has been observed since 1949, and is thought to be caused by asymmetric spinning dust grains being aligned with  magnetic fields in the ISM.  Exactly how the dust grains interact with hydrogen, become aligned and are spun up has been a mystery.  A complete theory of spinning dust grain alignment would help astronomers better understand magnetic fields in the ISM (which help regular star formation and hence are crucial in galaxy evolution) and provide a better characterisation of interstellar dust (which changes the way we see starlight in our own galaxy and in distance galaxies).

In 1979, Edwin Purcell looked at H2 formation on the surface of grains, and predicted that if the formation sites were localised over time, then as the newly formed H2 molecules were ejected from the grain, the reaction force would produce a net torque that would spin up the grains, like little rockets known as  “Purcell Rockets”. Spinning grains will interact with the magnetic field of the ISM and become aligned with their spin axes parallel to the magnetic field direction.  More recently in 2007, Alexandre Lazarian and graduate student Thiem Hoang proposed a new theory – Radiative Alignment Torque (RAT) – which predicts how irregular dust grains are spun up like propellers by photons and have their orientation modified by the magnetic field.

Molecular hydrogen is formed in the ISM when two hydrogen atoms on the surface of a dust grain migrate to an "active site" and combine. As the newly formed H2 molecule is ejected, the reaction force on the dust grain causes it to spin rapidly. The spinning grain then aligns with the magnetic field. (Credit: B-G Andersson, et. al)

Molecular hydrogen is formed in the ISM when two hydrogen atoms on the surface of a dust grain migrate to an “active site” and combine. As the newly formed H2 molecule is ejected, the reaction force on the dust grain causes it to spin rapidly. The spinning grain then aligns with the magnetic field. (Credit: B-G Andersson, et. al)

The results from observations using optical and near infrared polarimetry, optical spectroscopy and photometry, and imaging of a near infrared emission line of H2 have helped piece together the connection between H2 formation, spinning grains, grain alignment and ISM magnetic fields.  Observations of the reflection nebula IC 63 in Cassiopeia indicate that as starlight passes through dust clouds in the ISM, polarization is increased due to the intense formation of hydrogen molecules.

Intense molecular hydrogen formation seen in the near infrared   (false colour) in the reflection nebula IC 63. The white lines represent the polarization strength and orientation, with the largest polarization coincident with the most intense emission. (Credit: B-G. Andersson, et. al./USRA)

Intense molecular hydrogen formation seen in the near infrared (false colour) in the reflection nebula IC 63. The white lines represent the polarization strength and orientation, with the largest polarization coincident with the most intense emission. (Credit: B-G. Andersson, et. al./USRA)

The recent research  confirms some of the observational prediction of RAT theory, which include (i) that there will be an anisotropic radiation field with λ < 2a where a is the grain size, (ii) that the efficiency of the dust grain alignment will vary with the intensity of the radiation field,  (iii) the size distribution of the grains will also be varied with the radiation field, and (iv) that H2 formation can enhance the grain alignment, and the alignment of the grains will be dependent on the angle that is between the magnetic and radiation field anisotropy.

The results offer new potential for astronomers to use polarized visible and near infrared light to determine the strength and structure of magnetic fields in the ISM.

For more information, see

[Sarah Maddison & Sheridan Lacey]

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