Hidden nano-diamonds in the interstellar medium

Nano-diamonds remain an enigmatic component of cosmic dust. A significant fraction of the pre-solar nano-diamonds extracted from primitive meteorites were associated with distant exploding stars (supernovae). While they have been found in many Solar System bodies, they have been detected in only two proto-planetery disks and one evolved star, and they have never been observed in the interstellar medium. Newly-derived nano-diamond optical constants generated with the THEMIS dust model developed at IAS explain why it is hard to observe nano-diamonds in space.

In the diffuse ISM about 50% of carbon is in dust and if 3-5% were in nano-diamonds (which correspond to the pre-solar, meteoritic abundance) they would be in undetectable in thermal emission and extinction. The new computations show indeed that their infrared features will be in fact drowned out by other carbonaceous dust emission features (see the figure below). Thus, up to a few percent of the available carbon budget could be hidden in the form of nano-diamonds in the diffuse ISM.

For the few circumstellar regions where nano-diamonds emission were detected (two proto-planatery disks and one evolved star), observations with the MATISSE instrument at the VLTI will permit to constrain their exact location at astronomical unit scale. This is an attempt to derive critical information about their sizes, compositions, and evolution using the recently-derived set of optical constants calculated by THEMIS. This could allow us to better understand the properties (e.g., mean sizes on the order of 2–3 nm) of the pre-solar meteoritic nano-diamonds. The derived optical constants calculated by THEMIS model provide a means to explore the existence and viability of nano-diamonds in a wide range of astronomical sources.

An article in press for A&A presents theses results (preprint)

Contacts at IAS: Anthony Jones, Nathalie Ysard, Émilie Habart
 

Schematic representation of the nano-diamonds surfaces with CH and CH2 groups on facets.


Emission of the diffuse interstellar dust component including hydrogenated nano-diamonds in blue.
 


Size distribution of the diffuse interstellar dust component including hydrogenated nano-diamonds (in blue) with an abundance close to that of the pre-solar meteoritic abundance.