Non-thermal desorption from interstellar dust grains via exothermic surface reactions

By R. Garrod, V. Wakelam, E. Herbst. 2007

387 citations recorded by the source corpus.

Aims. The gas-phase abundance of methanol in dark quiescent cores in the interstellar medium cannot be explained by gas-phase chemistry. In fact, the only possible synthesis of this species appears to be production on the surfaces of dust grains followed by desorption into the gas. Yet, evaporation is inefficient for heavy molecules such as methanol at the typical temperature of 10 K. It is necessary then to consider non-thermal mechanisms for desorption. But, if such mechanisms are considered for the production of methanol, they must be considered for all surface species. Methods. Our gas-grain network of reactions has been altered by the inclusion of a non-thermal desorption mechanism in which the exothermicity of surface addition reactions is utilized to break the bond between the product species and the surface. Our estimated rate for this process derives from a simple version of classical unimolecular rate theory with a variable parameter only loosely constrained by theoretical work. Results. Our results show that the chemistry of dark clouds is altered slightly at times up to 10 6 yr, mainly by the enhancement in the gas-phase abundances of hydrogen-rich species such as methanol that are formed on grain surfaces. At later times, however, there is a rather strong change. Instead of the continuing accretion of most gas-phase species onto dust particles, a steady-state is reached for both gas-phase and grain-surface species, with significant abundances for the former. Nevertheless, most of the carbon is contained in an undetermined assortment of heavy surface hydrocarbons. Conclusions. The desorption mechanism discussed here will be better constrained by observational data on pre-stellar cores, where a significant accretion of species such as CO has already occurred.

Subfields: Astronomy and Astrophysics, Materials Chemistry, Physical and Theoretical Chemistry

View Paper: https://www.semanticscholar.org/paper/e9d1fa96609307981c05f29d3cc85d295f62bb3d

How researchers see this paper

The researchers whose trusted networks rank it highest, strongest advocate first. Every researcher gets a different order.

In Astronomy and Astrophysics

1. R. Garrod
2. P. Caselli
3. E. Herbst
4. Cecilia Ceccarelli
5. V. Wakelam
6. K. Oberg
7. E. Dishoeck
8. E. Bergin

In Materials Chemistry

1. R. Garrod
2. E. Herbst
3. A. Vasyunin
4. V. Wakelam
5. M. Ruaud
6. J. Loison
7. T. Pauly
8. K. Oberg

In Physical and Theoretical Chemistry

1. R. Garrod
2. E. Herbst
3. M. Ruaud
4. V. Wakelam
5. J. Loison
6. A. Vasyunin
7. T. Pauly
8. Cecilia Ceccarelli

Citations in the corpus

• Formation of Complex Molecules in Prestellar Cores: A Multilayer Approach (2017, background): “In the early theoretical study by Garrod et al. (2007) based on Rice-Ramsperger-Kessel (RRK) theory, the authors proposed the efficiency of RD to be of about 1–3% and only slightly dependent on the type of desorbing molecule.”
• Complex Chemistry in Star-forming Regions: An Expanded Gas-Grain Warm-up Chemical Model (2008, methodology): “Such may be the one suggested by Garrod et al. (2006, 2007) for the formation of gas-phase methanol in cold clouds, whereby the energy of formation of a surface molecule may break the surface–molecule bond with a probability on the order of 1%.”
• Molecular Evolution and Star Formation: From Prestellar Cores to Protostellar Cores (2007, methodology): “Since the formation of CH3OH is inefficient in the gas phase (Garrod et al. 2006, 2007), it must be formed by the hydrogenation of CO on grain surfaces in the prestellar core stage, and then sublimated to the gas-phase as the core is heated by the protostar.”
• ON THE FORMATION OF CO2 AND OTHER INTERSTELLAR ICES (2011, methodology): “We include the reactive desorption mechanism given by Garrod et al. (2007), whereby the formation of a molecule on the grain surface can result in its desorption into the gas phase, with a probability of ¡1 %.”
• On the reservoir of sulphur in dark clouds : chemistry and elemental abundance reconciled (2017, methodology): “Moreover, as hydrogenation is the most effective process in grains chemistry, models often predict that when sulphur atoms stick on the grain, they will consequently form H2S (Garrod et al. 2007).”
• THE 2014 KIDA NETWORK FOR INTERSTELLAR CHEMISTRY (2015, methodology): “In addition to thermal des- orption, Nautilus includes cosmic-ray induced desorption, as described in Hasegawa & Herbst (1993) and the desorption by exothermicity of surface reactions following Garrod et al. (2007) with the parameter a set to 0.01.”
• Our astrochemical heritage (2012, background): “([171]) assumed that the energy released during the formation process could be at least partially used for the surface species to desorb upon formation, reconciling observations with theory for CH3OH and propylene (if the desorption of this species is efficient).”
• Grain Surface Models and Data for Astrochemistry (2017, background): “…not by photoprocessing), especially in the multilayer regime, astrochemical models typically still account for such a process by implementing the Garrod et al. (2007) prescription P = aΞ 1 + aΞ (9) with Ξ = [ 1 − Ebind Eexo ]s−1 (10) where Eexo is the exothermicity of the reaction, and s is the…”
• A COLD COMPLEX CHEMISTRY TOWARD THE LOW-MASS PROTOSTAR B1-b: EVIDENCE FOR COMPLEX MOLECULE PRODUCTION IN ICES (2010, background): “This abundance is too high to explain with gas-phase formation of CH3OH through any mechanism investigated so far, i.e. gas phase reactions in quiescent cores result in CH3OH abundances below 10 −13 (Garrod et al. 2007).”
• Experimental evidence for water formation on interstellar dust grains by hydrogen and oxygen atoms (2009, background): “The most volatile species may be released in the gas phase upon formation (Garrod et al. 2007), while the refractory ones remain on the grain surface, building up a so called “dirty icy mantle”, and at least partilly may be sputtered by the heavy component of cosmic rays (Johnson et al. 1991).”
• Formation of complex organic molecules in cold objects: the role of gas-phase reactions (2015, background): “Species accrete on the grain surfaces and are desorbed by cosmic ray-induced heating of grains (Hasegawa & Herbst 1993) and by a non-thermal process that injects into the gas phase 1% of the species formed on the surface, following a process called “chemical desorption” (Garrod et al. 2007).”
• Astrochemistry and compositions of planetary systems (2020, background): “In cold, UV-shielded regions, desorption through release of chemical energy is suspected to be the dominant desorption mechanism (Garrod et al. 2007).”
• REACTIVE DESORPTION AND RADIATIVE ASSOCIATION AS POSSIBLE DRIVERS OF COMPLEX MOLECULE FORMATION IN THE COLD INTERSTELLAR MEDIUM (2013, methodology): “To quantify the level of agreement, we calculated individual confidence parameters κi for each ith species versus time in models M1 and M10 according to the prescription in Garrod et al. (2007) and then took the mean of the individual confidence parameters as the criterion of overall agreement.”
• A KINETIC DATABASE FOR ASTROCHEMISTRY (KIDA) (2012, background): “Models including interactions with grains and grain surface processes (Garrod et al. 2007) are, however, based on many uncertain processes.”
• Water and astrobiology (2007, methodology): “Garrod et al. (2007) used a coupled gas-surface model to show that species such as dimethyl ether, HCOOCH3 and formic acid observed in hot cores are not made following sudden evaporation of methanol in the gas phase, but during a slow warming of ices coating grains.”
• Complex organic molecules in protoplanetary disks (2013, methodology): “Investigations into the efficacy of reactive desorption in dark cloud models by Garrod et al. (2007) constrain the value for the probability of desorption toPrd = 0.01 and we adopt this value in our work.”
• Detection of complex organic molecules in a prestellar core: a new challenge for astrochemical models (2012, background): “If COMs form on grains, the most likely mechanisms to bring radical mobility and lead to molecular desorption are thus secondary UVs, cosmic rays, or the release of chemical energy due to exothermic reactions on the grains (e.g. chemical explosions, Garrod et al. 2007).”
• Binding energies: New values and impact on the efficiency of chemical desorption (2017, methodology): “Although the mechanism has been included in astrochemical models (for instance Garrod et al. 2007; Vasyunin & Herbst 2013; Wakelam et al. 2014), its efficiency has been a free parameter.”
• Gas and grain chemical composition in cold cores as predicted by the Nautilus three-phase model (2016, background): “…desorption mechanisms are also included: (1) the cosmic-rays desorption mechanism (see Hasegawa & Herbst 1993a), (2) the chemical desorption mechanism (desorption due to the exothermicity of surface reactions, set to ∼ 1% efficiency) (see Garrod et al. 2007), and (3) the photo-desorption.”