Thermal Desorption of Astrophysically Relevant Ice Mixtures of Acetaldehyde and Acetonitrile from Olivine Dust*

Abstract

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 and in comets. In this context it is pivotal to understand how the solid-phase interactions between iCOMs and grain surfaces influence the thermal desorption process and, therefore, the presence of molecular species in the gas phase. In the laboratory, it is possible to simulate the thermal desorption process, deriving important parameters such as the desorption temperatures and energies. We report new laboratory results on temperature-programmed desorption from olivine dust of astrophysical relevant ice mixtures of water, acetonitrile, and acetaldehyde. We found that in the presence of grains, only a fraction of acetaldehyde and acetonitrile desorb 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 190–210 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 toward regions with temperatures as high as 200 K and shifting inwards the position of the snow lines in protoplanetary disks. These studies offer a necessary support to interpret observational data and may help our understanding of iCOM formation, providing an estimate of the fraction of molecules released at various temperatures.

Formation of Complex Organic and Prebiotic Molecules in H2O:NH3:CO2 Ices at Temperatures Relevant to Hot Cores, Protostellar Envelopes, and Planet-Forming Disks

“…In Figure , we present TPD curves for mass 46 for 13 CH 3 13 CHO and CH 3 CHO (as a reference curve). The peak at ∼100 K corresponds to the desorption of acetaldehyde . The TPDs are shown up to 200 K as there are misleading peaks above 200 K, because of the desorption of 13 C-bearing formamide ,, and, possibly, H 13 COO – …”

Acetaldehyde binding energies: a coupled experimental and theoretical study

“…We will first compare the case of the pure acetaldehyde ice. Here, experiments provide a BE of 4650 K, while calculations dealing with the ideal model of a molecule above the (010) acetaldehyde surface gives a BE of 2650 K. Given the robustness of the measurement (confirmed by Corazzi et al 2021), one would conclude that what is measured does not correspond to what is calculated. In reality, when a molecular film is heated, it will constantly reorganise itself, and the situation of a molecule on top of the surface is not the most energetically stable configuration.…”

“…Comparison with previous experimental estimates: Now we have a tool to compare BE provided by different values of pre-exponential factors. Corazzi et al (2021) recently reported an experimental value of BE=3100 K of acetaldehyde co-deposited with water on a substrate made of micrometer-sized olivine grains. It obviously differs from the measured values here and it is lesser than any calculated value.…”

“…To our best knowledge, Wakelam et al (2017) is the first theoretical study, but it only considered one water molecule to simulate the interstellar water ice. These authors found a BE equal to ∼5400 K. Recently, Corazzi et al (2021) reported an experimental study of the BE on olivine surfaces covered by an ice of pure acetaldehyde or mixed with water. They found that the acetaldehyde BE on a mixture of iced water-acetaldehyde is 3079 K, substantially different from the Wakelam et al's estimate.…”

Desorption of organic molecules from interstellar ices, combining experiments and computer simulations: Acetaldehyde as a case study

“…Chaabouni and coworkers (2018) provided desorption energies of methylamine and formamide from amorphous solid water. In recent work, Corazzi et al (2021) studied the sublimation of acetaldehyde and acetonitrile from olivine dust. Each of the different iCOMs presented different desorption profiles, which depend on the desorption substrates.…”