Efficiency of non-thermal desorptions in cold-core conditions

Abstract

Context. Under cold conditions in dense cores, gas-phase molecules and atoms are depleted from the gas-phase to the surface of interstellar grains. Considering the time scales and physical conditions within these cores, a portion of these molecules has to be brought back into the gas-phase to explain their observation by milimeter telescopes. Aims. We tested the respective efficiencies of the different mechanisms commonly included in the models (photo-desorption, chemical desorption, and cosmic-ray-induced whole-grain heating). We also tested the addition of sputtering of ice grain mantles via a collision with cosmic rays in the electronic stopping power regime, leading to a localized thermal spike desorption that was measured in the laboratory. Methods. The ice sputtering induced by cosmic rays has been added to the Nautilus gas-grain model while the other processes were already present. Each of these processes were tested on a 1D physical structure determined by observations in TMC1 cold cores. We focused the discussion on the main ice components, simple molecules usually observed in cold cores (CO, CN, CS, SO, HCN, HC3N, and HCO+), and complex organic molecules (COMs such as CH3OH, CH3CHO, CH3OCH3, and HCOOCH3). The resulting 1D chemical structure was also compared to methanol gas-phase abundances observed in these cores. Results. We found that all species are not sensitive in the same way to the non-thermal desorption mechanisms, and the sensitivity also depends on the physical conditions. Thus, it is mandatory to include all of them. Chemical desorption seems to be essential in reproducing the observations for H densities smaller than 4 × 104 cm−3, whereas sputtering is essential above this density. The models are, however, systematically below the observed methanol abundances. A more efficient chemical desorption and a more efficient sputtering could better reproduce the observations. Conclusions. In conclusion, the sputtering of ices by cosmic-rays collisions may be the most efficient desorption mechanism at high density (a few 104 cm−3 under the conditions studied here) in cold cores, whereas chemical desorption is still required at smaller densities. Additional works are needed on both mechanisms to assess their efficiency with respect to the main ice composition.

ALMA Observations of Molecular Complexity in the Large Magellanic Cloud: The N 105 Star-forming Region

“…Several desorption mechanisms have been proposed and include photodesorption, explosion of UV-irradiated ices, grain heating following cosmic-ray impact (Leger et al 1985), cosmic-ray sputtering (Wakelam et al 2021), as well as "reactive desorption" in which the energy released in exothermic grain-surface reactions is sufficient to overcome the physisorption binding energy (Minissale et al 2016;Chuang et al 2018). Alternatively, a localized kinematic origin has been proposed for the origin of methanol and other putative ice-mantle molecules in Galactic dark clouds.…”

The KOSMA-τPDR model

“…where z = log 10 ζ CR . Energy deposition by CRs directly into the ice mantle (Wakelam et al 2021) is not considered but will be implemented in a future code update.…”

“…Unfortunately, no laboratory data seem to be available for the desorption rate in case of more than one reaction product so that η i cannot be determined from a fit to the experimental data. Wakelam et al (2021) used an equal split between all atoms within the reaction products, assigning equal energy fractions to all atoms within the molecules. We propose a different approach based on the nature of the chemical reaction that produces the exothermicity.…”

Unusual Chemical Processes in Interstellar Chemistry: Past and Present

“…Cosmic rays are mainly highly ionized ions ranging in mass from protons to heavy nuclei. The laboratory study of radiolysis using high-energy protons or electrons but occasionally high-energy nuclei (Wakelam et al, 2021) is well advanced with many experiments in many laboratories (Bennett et al, 2005;Hudson and Moore, 2018;Christoffersen et al, 2020;Ioppolo et al, 2021) and some corresponding theory (see Figure 6). It was not until a few years ago that a theoretical approach to the chemistry was developed by astrochemists , tested successfully against simple cold interstellar ices in the laboratory such as O 2 and amorphous water, and applied to interstellar icy mantles Shingledecker et al, 2020).…”