Discovery of Elementary Composite Binary and Grain Alignment Locked in Dust Growth

Planets are known to grow out of a star-encircling disk of the gas and dust inherited from an interstellar cloud; their formation is thought to begin with coagulation of submicron dust grains into aggregates, the first foundational stage of planet formation. However, with small solids unobservable directly in the interstellar medium (ISM) and protoplanetary disks, exactly how dust grains grow is unclear, as are the morphology and structure of interstellar grains and the whereabouts and form of “missing iron.”

Primitive interplanetary dust particles (IDPs), or those exhibiting properties consistent with cometary origin, are remnants of the unprocessed protoplanetary dust aggregates of ultrafine grains that preserve a record of fundamental dust growth processes and physical conditions in the ISM and protoplanetary disk. A typical 10-micron-sized IDP is an aggregate of ~ 10⁴-10⁵ grains and pores intricately connected in 3D space, a viable way to untangle growth processes is to noninvasively unlock the physical or morphological structures of whole aggregate particles in 3D nanoscale detail.

Here we show an elementary composite binary (termed an astrobinary) and its structure in 3D sub-10 nm detail–and the alignments of its two subunits and a population of elongated composite grains locked in a primitive IDP—noninvasively uncovered with phase-contrast X-ray nanotomography (Hu & Winarski, ApJL 923: L4 (8pp) 2021). The astrobinary with a long-to-short axis ratio of about 3.3 comprises a pair of nearly equal-sized, oblate, quasi-spheroidal grains whose alignment and shape meet the astronomical constraints on polarizing interstellar grains. Each member of the pair contains a high-density core of octahedral nanocrystals whose shape and twin relationship are consistent with the magnetite’s diagnostic properties at low temperatures, with a lower-density mantle exhibiting nanoscale heterogeneities, rounded edges, and pitted surfaces. The revealed core–mantle structure and density gradients from core to surface are in general agreement with interstellar processes and astronomical evidence for differential depletion. That the elongated grains with the high-density cores are highly aligned is unexpected from Brownian motion-driven dust growth. The findings suggest that the ISM is threaded with dust grains containing preferentially oriented iron-rich magnetic nanocrystals that may hold answers to astronomical problems from dust evolution, grain alignment, and the structure of magnetic fields to planetesimal growth.