A derivation of nano-diamond optical constants

Here be nano-diamonds

Université Paris-Saclay, CNRS, Institut d’Astrophysique Spatiale, 91405 Orsay, France
e-mail: anthony.jones@universite-paris-saclay.fr

Received: 14 July 2021
Accepted: 10 September 2021

Abstract

Context. Nano-diamonds are an enticing and enigmatic dust component yet their origin is still unclear. They have been unequivocally detected in only a few astronomical objects, yet they are the most abundant of the pre-solar grains, both in terms of mass and number.

Aims. Our goal is to derive a viable set of nano-diamond optical constants and optical properties to enable their modelling in any type of astrophysical object where, primarily, the local (inter)stellar radiation field is well-determined.

Methods. The complex indices of refraction, m(n, k), of nano-diamonds, constrained by available laboratory measurements, were calculated as a function of size, surface hydrogenation, and internal (dis)order, using the THEMIS a-C(:H) methodology optEC_(s)(a).

Results. To demonstrate the utility of the optical properties (the efficiency factors Q_ext, Q_sca, and Q_abs), calculated using the derived m(n, k) data, we show that nano-diamonds could be abundant in the interstellar medium (ISM) and yet remain undetectable there.

Conclusions. The derived optical constants provide a means to explore the existence and viability of nano-diamonds in a wide range of astronomical sources. Here we show that up to a few percent of the available carbon budget could be hidden in the form of nano-diamonds in the diffuse ISM, in abundances comparable to the pre-solar nano-diamond abundances in primitive meteorites.