A laboratory study of ion-induced erosion of ice-covered carbon grains

Abstract

Context. It has been confirmed that solid carbon dioxide (CO 2 ) is abundantly present along the line of sight to quiescent clouds and star-forming regions via space IR observations with ISO-SWS and Spitzer Space Telescope. Since CO 2 has low abundance in the gas-phase, the assumption is that it is synthesized on grains after energetic processing of icy mantles and surface reactions. Aims. The role of solid carbon is investigated as a reservoir for molecule formation and structural modifications of the material with and without an ice layer upon ion bombardment. Methods. A gas-phase condensation technique was used to prepare a layer of 13 C amorphous grains. These grains were covered with H 2 O and O 2 ice and finally bombarded with 200 keV protons. The formation of new molecular species was analyzed using IR spectroscopy. The formation cross sections of solid 13 CO and 13 CO 2 were determined from the increase in the column density as a function of the fluence. In addition, bare carbon grains were bombarded with a comparable fluence of protons to study the processing of the grains without ice layer. Imaging techniques such as transmission electron microscopy were used to monitor the changes in the structure. Results. CO and CO 2 were formed efficiently at the interface between ice and solid carbon grains at the expense of solid carbon, leading to strong grain erosion. Given the initial thickness of our C-samples (about 120 nm), this resulted in an erosion of about 50% after 200 keV proton bombardment with 6.76 × 10 16 ions/cm 2 . The column density of CO and CO 2 follows an exponential trend as a function of the irradiation fluence. The asymptotic values obtained when O 2 ice is deposited on top of the carbon grains are about one order of magnitude higher than the values obtained when H 2 O ice is deposited on the solid carbon layer. The carbon grains were strongly graphitized upon ion bombardment in a surface layer. Less graphitization accompanied by the formation of fullerene molecules and structures from cage fragments present in the original material were observed beneath the graphitic layer. Conclusions. The formation of CO and CO 2 at the expense of solid carbon strongly restricts the lifetime of the solid carbon material and may influence the formation of more complex molecules in astrophysical environments. Graphitization of carbonaceous grains upon ion bombardment affect the spectral properties of the carbon grains in particular in the far-IR range.

Separate Silicate and Carbonaceous Solids Formed from Mixed Atomic and Molecular Species Diffusing in Neon Ice

“…Ice chem-istry is complex, however. Irradiation causes the chemical erosion of carbon covered with H 2 O ice, producing CO and CO 2 molecules (Fulvio et al 2012;Sabri et al 2015). Thus it hinders the growth and the formation of carbon grains.…”

The Imprint of Exoplanet Formation History on Observable Present-Day Spectra of Hot Jupiters

“…But it shows that it might not be impossible to form carbon rich planets even in disks with (partial) carbon depletion in their inner regions. Whether such regions exist, and over which orbital distance they extend will depend on the mechanism that destroys carbon in the inner disk like ion-induced erosion of solid carbon (Sabri et al 2015). In an alternative model of Lee et al (2010), hot atomic oxygen produced by photodissociation of O bearing species erode carbon grains and release the carbon into the gas in the upper layers of the disk.…”

“…The efficiency of this process decreases with orbital distance, and finally drops outside of the iceline, as the oxygen is there locked on grain surfaces as water ice. Whether this quantitatively leads to significant C rich regions inside the ice line must be investigated with detailed disk chemistry models including chemical-kinetic pathways, ion irradiation and photochemistry, as purely thermodynamic condensation models are insufficient (Jura & Young 2014;Sabri et al 2015).…”

Laboratory experiments on cosmic dust and ices

“…The formation of CO and CO 2 in water and O 2 ices on top of hydrogenated fullerenelike carbon grains by proton bombardment at a temperature of 16 K was studied by Sabri et al (2015). Hydrogenated fullerene-like carbon grains composed of 13 C atoms were prepared by gas-phase condensation.…”