The binding energies of atoms on amorphous silicate dust: A computational study

Context. We investigate the binding energies of atoms to interstellar dust particles, which play a key role in their growth and evolution as well as the chemical reactions on their surfaces. Aims. We aim to compute the binding energies of abundant elements in the interstellar medium (C, N, O, Mg, Al, Si, S, Ca, Fe, and Ni) to silicate dust. Methods. We used the Geometries, Frequencies, and Non-covalent Interactions Tight Binding (GFN1-xTB) method to compute the binding energies. An FeMgSiO 4 periodic surface model was amorphized using a molecular dynamics simulation. We then calculated the binding energies of each element to 81 local minima on the resulting surface. Results. A range of binding energies was found for each element. The mean of the binding energies follows the order Si (15.3 eV) > Ca (13.5 eV) > Al (12.8 eV) > C (9.2 eV) > O (8.1 eV) > N (6.4 eV) > Fe (5.9 eV) > S (5.2 eV) > Mg (2.6 eV). The probability distribution of binding energies for each element except Ca is statistically consistent with a log-normal distribution. Conclusions. In general, Si, Ca, and Al atoms have large binding energies, while the binding energies of the other atoms (C, N, O, Mg, S, Fe and Ni) are weaker. However, even the weakest computed binding energies for these elements are still far stronger than the energies associated with dust temperatures typical of the ambient interstellar medium, suggesting that silicate grains are generally stable against sublimation. We estimate sublimation temperatures for silicate grains to range from 1600 K to 3000 K depending on assumed grain size and lifetime. These binding energies on silicate dust grains, estimated from first principles for the first time, provide invaluable input to models of dust evolution and dust-catalyzed chemical reactions in the interstellar medium and grain dynamics in circumstellar environments such as asymptotic giant branch stars and protoplanetary disks.