Cosmic dust is an elusive component of the interstellar medium (ISM) because it is difficult to be observed. Despite the dust is a key ingredient in the formation and evolution of planets to whole galaxies, little is known about its formation, evolution, size distribution and composition. The amount of metals incorporated into cosmic dust can be derived indirectly, via the comparison of the ISM-gas-phase abundances and a reference standard. Using our proposed cosmic abundance standard (CAS) derived from a representative sample of early B-type stars in the solar neighbourhood (Nieva & Przybilla 2006-2012) we find a silicate/oxide-rich and relatively carbon-poor composition of the local ISM dust-phase. Moreover, a comparison of the CAS with gas-phase abundances in the Orion nebula implies that the HII region is devoid of carbonaceous dust. Our results imply that amorphous carbon dust grains are either efficiently destroyed inside the ionized region, or they were a minority species initially as well. The combined evidence from abundances in the ISM and the Orion HII region indicates that dust models considering silicates, PAHs, organic refractory material and possible amorphous carbon, but not graphite, should be investigated more closely.
Recently, Altobelli et al. (2016) using the dust analyzer on the Cassini probe detected 36 interstellar dust grains lacking carbon-bearing compounds and homogenized in the interstellar medium into silicates with iron inclusions: Mg/Si, Fe/Si, Mg/Fe, and Ca/Fe ratios are on average CI chondritic, which agrees with the composition inferred by us. This first direct measurement of the chemical composition of interstellar dust grains confirms thus the robustness of our results.
Ongoing efforts are invested to assess ~220 sightlines in the solar neighbourhood for which the (massive) background stars and diffuse-gas (with emphasis on new neutral hydrogen column densities ) are being analyzed for the first time self-consistently. This novel analysis will allow us to accurately constrain the dust chemical composition and dust-to-gas ratio in interstellar environments with different levels of extinction, and will motivate new developments for dust formation and evolution models, which might affect several fields in astrophysics.