Interstellar CO2 ice

In interstellar clouds of sufficient opacity, dust grains may build up ice mantles, through the accretion and surface reaction of gas-phase atoms and simple molecules. The majority of this ice is composed of water, but CO and CO2 are also present in large quantities (up to ~25% of water). Other molecules such as methane (CH4), ammonia (NH3), and formaldehyde (H2CO) are also present in quantities on the order of a few percent or less, while methanol (CH3OH) may take abundances up to 30% in some star-forming regions.

A major challenge in astrochemical modeling over recent years has been the reproduction of observed solid-phase CO2 abundances, and its behavior at low extinctions. Solid interstellar carbon dioxide is thought to be formed from CO that accretes onto the grains from the gas phase. However — puzzlingly — observational lines of sight indicate that CO becomes abundant later than CO2; The threshold visual extinction at which CO is observed is around 6.7, while water and CO2 are observed at around 3.2 and 4.3, respectively (Whittet et al. 2007). Toward dark interstellar clouds, two spectral features are identified for CO2 — one that is polar, i.e. mixed with water, and another that is apolar, and which is mixed with only CO.

Furthermore, chemical simulations have been unable to reproduce appropriate quantities of CO2 on interstellar dust grains at the canonical temperature of 10 K.

Garrod & Pauly addressed these problems with a chemical modeling study (G&P, 2011, ApJ, 735, 15). They found firstly that, using an a more advanced treatment for the behavior of the chemical reaction CO + OH → CO2 + H, carbon dioxide could be formed in reasonable abundance at low temperatures, by considering the formation of the OH radical on top of a CO surface, by the addition of atomic H and O from the gas phase. This allowed the reagents for CO2 formation to be brought together without the need for CO or OH themselves to move (which is extremely slow at 10 K).

In addition to this, the consideration of a dust temperature that is dependent on visual extinction, through the balance of external heating and thermal cooling, was found to result in the efficient conversion of CO to CO2 at low visual extinctions, producing a CO2 well mixed with H2O, in keeping with the observed polar component.

The slow contraction/collapse of a dense interstellar core, with its associated fall in dust temperature, was found to produce a CO2-rich ice, followed by a CO:CO2 mixture in good agreement with observed values. The data also agree well with the extinction thresholds for each of the molecules H2O, CO and CO2.

The figure shows the production and composition of each layer of the ice formed on the surface of an interstellar dust grain, during the collapse of a dense core.

The first fifty layers of the ice are rich in CO2. After this point, as the temperature drops below around 12 K, CO is no longer very mobile on the grain surface, so that it is unable to find OH radicals on the surface before those radicals are hydrogenated to become water molecules. At this point, the dominant CO2-formation mechanism becomes the formation of OH radicals while in contact with a CO molecule, allowing the CO2-forming reaction to proceed immediately. This results in a CO-dominated ice, with modest amounts of CO2, as per observations.

We hypothesize that the absence of water in the observed CO:CO2 mixtures that correspond with the upper layers (>50) of the model ice is caused by preferential binding of CO and CO2 with each other, due to residual mobility of CO molecules on the surface, even at low temperatures.

For more information, please see the publication:

“On the formation of CO2 and other interstellar ices.”
Garrod, R. T. & Pauly, T. A.
2011, ApJ, 735, 15