Impulsive Spot Heating and Thermal Explosion of Interstellar Grains Revisited

Abstract

The problem of impulsive heating of dust grains in cold, dense interstellar clouds is revisited theoretically, with the aim to better understand leading mechanisms of the explosive desorption of icy mantles. It is rigorously shown that if the heating of a reactive medium occurs within a sufficiently localized spot (e.g., heating of mantles by cosmic rays), then the subsequent thermal evolution is characterized by a single dimensionless number λ. This number identifies a bifurcation between two distinct regimes: When λ exceeds a critical value (threshold), the heat equation exhibits the explosive solution, i.e., the thermal (chemical) explosion is triggered. Otherwise, thermal diffusion causes the deposited heat to spread over the entire grain -this regime is commonly known as the whole-grain heating. The theory allows us to find a critical combination of the physical parameters that govern the explosion of icy mantles due to impulsive spot heating. In particular, the calculations suggest that heavy cosmic ray species (e.g., iron ions) colliding with dust are able to trigger the explosion. Based on the recently calculated local cosmic-ray spectra, the expected rate of the explosive desorption is estimated. The efficiency of the desorption, which affects all solid species independent of their binding energy, is shown to be comparable with other cosmic-ray desorption mechanisms typically considered in the literature. Also, the theory allows us to estimate maximum abundances of reactive species that may be stored in the mantles, which provides important constraints on available astrochemical models.

Methylacetylene (CH3CCH) and propene (C3H6) formation in cold dense clouds: A case of dust grain chemistry

“…This efficiency is highly dependent on the binding energy as well as the semi-empirical parameter ε of (Minissale et al 2016) which corresponds to the fraction of kinetic energy retained by the product colliding with the surface. It should be noted that other non-thermal desorption mechanisms, such as cosmic-ray explosive desorption (Ivlev et al 2015) may play a role. More accurate predictions of species formed on interstellar grains can only be provided by more detailed models of these surfaces, as well as through precise measurements (or calculations) of the efficiency of the chemical desorption mechanism in interstellar ice analogs.…”

Grain Surface Models and Data for Astrochemistry

“…This process had fallen out of favor as it became increasingly evident that photodesorption was an efficient (and well understood) mechanism for releasing molecules from the ice mantle at low temperatures. Recent revisitations of this process suggest that the overall effect is of the same order of magnitude as other processes considered to influence the gas-grain balance in dark clouds such as photodesorption (e.g., Ivlev et al 2015).…”

How chemistry influences cloud structure, star formation, and the IMF

“…The process that is dubbed CR desorption, which is not included in this work, typically indicates the desorption following the dust heating upon direct collision with a CR. This can proceed via 'whole-grain heating' (Leger, Jura & Omont 1985;Hasegawa & Herbst 1993) or through explosive desorption due to 'impulsive spot heating' (Ivlev et al 2015b). …”