The role of olivine grains on thermal desorption of astrophysical relevant ice mixtures of acetaldehyde and acetonitrile

Introduction:  Millimeter and centimeter observations are discovering an increasing number of interstellar complex organic molecules (iCOMs) in a large variety of star-forming sites from the earliest stages of star formation to protoplanetary disks.

To correctly interpret the distribution and abundances of iCOMs observed along the formation process of a Sun-like star (i.e. in pre-stellar dense cores^1,2, hot corinos around protostars^3,4, in the associated jets and outflows^5,6, and protoplanetary disks⁷), we need to first comprehend their formation processes and the mechanism responsible for their release in gas-phase. In this context, it is pivotal to understand the influence of the solid phase interactions between iCOMs and grain surface in the thermal desorption process and the subsequent presence of molecular species in the gas phase. Thermal desorption can be characterized through laboratory experiments using interstellar ice analogs deposited on grains similar to interstellar ones deriving important parameters such as the desorption temperatures and energies. Up to now, temperature-programmed desorption (TPD) experiments have been carried out mainly from graphite and amorphous water ice surfaces^8,9,10,11. As far as we know, TPD experiments from grain surfaces are lacking in the literature, although mineral matrices can selectively adsorb, protect, and allow the iCOMs concentration on their surface. Molecules can diffuse inside the grains and so, the presence of grains can influence the desorption and release of the iCOMs in the gas phase. 

Laboratory measurements:  We report our new recently published laboratory results¹² on TPD experiments of astrophysical relevant ice mixtures of water, acetonitrile (CH₃CN), and acetaldehyde (CH₃COH) from micrometric grains of silicate olivine ((Mg,Fe)₂SiO₄) used as interstellar dust analog on which the icy mixtures were condensed at 17 K inside the ultra-high vacuum (UHV) chamber (P~6.68 × 10^-10 mbar). The ice mixtures condensed on olivine dust were heated at a constant rate, so the molecules diffused and desorbed from the dusty sample, and in situ TPD analysis was made.

Results: We found that in the presence of grains, only a fraction of acetaldehyde and acetonitrile desorbs at about 100 K and 120 K respectively, while 40% of the molecules are retained by fluffy grains of the order of 100 μm up to temperatures of 200 K.  In contrast with the typical assumption that all molecules are desorbed in regions with temperatures higher than 100 K, this result implies that about 40% of the molecules can survive on the grains enabling the delivery of volatiles towards regions with temperatures as high as 200 K. This may be important in protoplanetary disks where the submicrometric interstellar grains begin to agglomerate into fluffy grains of hundreds of microns. The diffusion of molecules on the silicate surface is a valuable process enabling the permanence of the ices in the inner part of the disk. This implies that O-rich and N-rich volatiles ice can survive up to ∼ 200 K, broadening the snowlines of O- and N-bearing molecules, such as CH₃CN and CH₃COH. The presence of dust grains in the protoplanetary disk can therefore determine the approach of the snowlines towards the star and allow the presence of water and volatile species in Earth-like planets forming region.

These studies offer a necessary support to interpret observational data and may help our understanding of iCOMs formation providing an estimate of the fraction of molecules released at various temperatures.

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