ON THE FORMATION OF CO2 AND OTHER INTERSTELLAR ICES

By R. Garrod, T. Pauly. 2011

165 citations recorded by the source corpus.

We investigate the formation and evolution of interstellar dust-grain ices under dark-cloud conditions, with a particular emphasis on CO2. We use a three-phase model (gas/surface/mantle) to simulate the coupled gas–grain chemistry, allowing the distinction of the chemically active surface from the ice layers preserved in the mantle beneath. The model includes a treatment of the competition between barrier-mediated surface reactions and thermal-hopping processes. The results show excellent agreement with the observed behavior of CO2, CO, and water ice in the interstellar medium. The reaction of the OH radical with CO is found to be efficient enough to account for CO2 ice production in dark clouds. At low visual extinctions, with dust temperatures ≳12 K, CO2 is formed by direct diffusion and reaction of CO with OH; we associate the resultant CO2-rich ice with the observational polar CO2 signature. CH4 ice is well correlated with this component. At higher extinctions, with lower dust temperatures, CO is relatively immobile and thus abundant; however, the reaction of H and O atop a CO molecule allows OH and CO to meet rapidly enough to produce a CO:CO2 ratio in the range ∼2–4, which we associate with apolar signatures. We suggest that the observational apolar CO2/CO ice signatures in dark clouds result from a strongly segregated CO:H2O ice, in which CO2 resides almost exclusively within the CO component. Observed visual-extinction thresholds for CO2, CO, and H2O are well reproduced by depth-dependent models. Methanol formation is found to be strongly sensitive to dynamical timescales and dust temperatures.

Subfields: Astronomy and Astrophysics, Atomic and Molecular Physics, and Optics, Biomedical Engineering

View Paper: https://www.semanticscholar.org/paper/ba05d1affd6df2b4fd1481493792baac13fd18b5

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Citations in the corpus

Astrochemistry and compositions of planetary systems (2020, background): “The early-forming water-dominated ice layer should also be the main host of CH4 and NH3, which are expected to form mainly from atomic carbon and nitrogen before N and C are completely converted into CO and N2, respectively (Tielens and Hagen 1982; Garrod and Pauly 2011).”
H$_2$ formation on interstellar dust grains: the viewpoints of theory, experiments, models and observations (2017, background): “ example, H 2 + OH (Oba et al., 2012; Meisner et al., 2017). Recently, astrophysicists have modiﬁed gas-grain codes to take into account the competition between reaction and di usion on the surfaces (Garrod and Pauly, 2011; Ruaud et al., 2016). This di usion increases signiﬁcantly the eciency of rea”
Formation of Complex Molecules in Prestellar Cores: A Multilayer Approach (2017, methodology): “Hasegawa et al. (1992) proposed to calculate the transmission probability through the rectangular activation barrier of 1 Å width, while Garrod & Pauly (2011) proposed to use the width of the rectangular activation barrier of 2 Å.”
Complex organic molecules in protoplanetary disks (2013, background): “Historically, the above reaction has been included inchemical networks with a small reaction barrier of≈ 80 K based on the gas-phase reaction potential energy surface (see, e.g., Smith 1988; Ruffle & Herbst 2001; Garrod et al. 2008; Garrod & Pauly 2011).”
On the reservoir of sulphur in dark clouds : chemistry and elemental abundance reconciled (2017, methodology): “The model also takes into account the newly implemented competition between reaction, diffusion and evaporation as suggested by Chang et al. (2007) and Garrod & Pauly (2011).”
THE SPITZER ICE LEGACY: ICE EVOLUTION FROM CORES TO PROTOSTARS (2011, background): “CO2 may still form from OH+CO, but at later stages CO is more abundant than O, resulting in sparse H2O formation compared to CO and thus in a CO-rich CO2 ice (see Garrod & Pauly 2011, for a similar discussion from a theoretical point of view).”
The molecular composition of the planet-forming regions of protoplanetary disks across the luminosity regime (2015, methodology): “We also include reactiondiffusion competition in which the reaction probability is determined by the relative rates between the barrier-mediated reaction and thermal diffusion (see, e.g., Chang et al. 2007; Garrod & Pauly 2011).”
Grain Surface Models and Data for Astrochemistry (2017, background): “…surface species within the top (few) monolayer(s), and fully inert bulk species in the core of the ice mantle, with terms that allow surface material to be incorporated into the bulk (or vice versa), as the ice thickness increases or decreases (Hasegawa and Herbst 1993b; Garrod and Pauly 2011).”
REACTIVE DESORPTION AND RADIATIVE ASSOCIATION AS POSSIBLE DRIVERS OF COMPLEX MOLECULE FORMATION IN THE COLD INTERSTELLAR MEDIUM (2013, background): “As expected, our chemical model produces a typical ice composition which is similar to that calculated in a number of previous studies (Garrod & Pauly 2011; Vasyunin & Herbst 2013), and which is also generally similar to observed ice abundances toward protostars (Öberg et al. 2011).”
Our astrochemical heritage (2012, background): “ond most abundance molecule (after H 2) and the mantle starts to accumulate CO. CO 2 also starts to form, either via cosmic-ray bombardment ([220]) and/or via the CO+OH reaction ([319]; [221]; [317]; [169]). Here, the limited amount of CO freeze-out limits the degree of deuteration to levels of a fe”
A THREE-PHASE CHEMICAL MODEL OF HOT CORES: THE FORMATION OF GLYCINE (2013, methodology): “Garrod & Pauly (2011) found that the production of interstellar CO2 ice could be well reproduced by the adoption of ratiosEdif :Edes.”
Gas and grain chemical composition in cold cores as predicted by the Nautilus three-phase model (2016, methodology): “…and dnm(i)/dt|m→s are related to the net rate of change in total surface material, those terms do not represent a physical motion of material between surface and mantle but rather a way to treat the continuous renewal of the grain surface material (Hasegawa & Herbst 1993b; Garrod & Pauly 2011).”