Carbon rain, in stars & in the lab

When water emerges from the gas phase in cold air, it often emerges as either snowflakes or as frozen droplets known as hail. When carbon emerges from the gas phase, polycyclic aromatic hydrocarbon "snowflakes" have been observed in a number of astrophysical environments [1]. In warmer settings, just as with hail on earth, the vapor may also emerge as liquid droplets which freeze, but in carbon's case when large enough they may be difficult to detect in space from a distance.

Interstellar core-rim carbon onion named "Vivian"; The image-field width is about 2.5 microns.

Fortunately, meteorites contain unmelted interstellar dust particles which include carbon particles with a frozen-droplet core [2-4], i.e. they include tiny "carbon hail stones" (see figure above) which have been wrapped with graphite layers, something like the layers of an onion. These little carbon hail stones (only about the size of a microbe), however, carry atoms to us from the atmosphere of red giant stars, and have tales to tell [5,6] that predate our species' "discovery" of fire, and even the much earlier formation of our planet and our sun...

This is "storyboard" for a graphical abstract to our "experimental carbon rain" paper [4].

Footnotes

A. G. G. M. Tielens (2008) "Interstellar polycyclic aromatic hydrocarbon molecules", Annual Review of Astronomy and Astrophysics 46, 289-337, link.
P. Fraundorf and Martin Wackenhut, "The core structure of pre-solar graphite onions" (2002) Astrophysical Journal Letters 578(2) L153-156.
Croat, T. K., Bernatowicz, T. J., & Daulton, T. L. (2014) "Presolar graphitic carbon spherules: Rocks from stars", Elements, 10, 441.
P. Fraundorf, Tristan Hundley and Melanie Lipp (2019) "Synthesis of unlayered graphene from carbon droplets: In stars and in the lab", HAL-02238804, webnotes.
Timothy J. Volkert and P. Fraundorf (2006) "The travel diary of a micron-sized sphere, found in the Murchison meteorite" (2006 NASA/MO space grant project, final report) pdf.
Our now ancient webpage on this topic, some more NASA/MO space grant reports., and our 2020 mobile-friendly notes on solidifying carbon.

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