Complex organic molecules

Link: New review article in Chemical Reviews

Stars form within interstellar clouds, which are made up of dust and molecular gas. In simplistic terms, the star-formation process begins with the formation of a dense core; the gradual collapse and inflow of surrounding material onto this core results in a protostar with a surrounding envelope. As the star-formation process advances, the density and temperature of the core and envelope increase. Stages in this progression may be traced by the infrared emission characteristics of different objects.

Observations of some star-forming cores, at centimeter, millimeter, and sub-mm wavelengths, reveal rich molecular emission spectra, with rotational (excitation) temperatures >100 K. Such sources are known as hot cores, in the case of high-mass star formation, or hot corinos, in the case of low-mass protostars. The molecules from which the emission derives range from simple species like CO, CS, and HCN, up to highly complex organics such as ethyl formate (HCOOC2H5; fig. left) and n-propyl cyanide (C3H7CN; fig. right), which were detected toward the Galactic Center source Sagittarius B2(N) (Belloche et al., 2009). It is currently unclear to what degree this chemically-complex material may survive through to the protoplanetary phase and beyond. However, the chemical composition of the gas phase in hot cores is remarkably similar to the composition of solar-system comets.

Until recently, gas-phase processes – mostly ion-molecule reactions – were thought to be responsible for the formation of the most complex molecules in star-forming regions. However, experimental and computational studies have shown that many such reactions are prohibitively inefficient, including those thought to be responsible for species like methyl formate (HCOOCH3), which is abundant and ubiquitous in hot-core spectra.

Hot-core chemistry simulations

In recent years, I and collaborators have developed chemical models to explain the formation of complex organic molecules (COMs) in star-forming regions (Garrod & Herbst 2006; Garrod, Widicus Weaver & Herbst 2008; Belloche et al. 2009; Garrod 2013). In these models, COMs are formed on the surfaces or within the ice mantles of dust grains, as a result of the thermal and UV processing of the organic ice mantles formed at earlier times. Ultraviolet rays resulting from collisions of cosmic rays with gas-phase particles can photodissociate molecules in the ices, producing reactive radicals that react with each other to form new species.

Methanol (CH3OH) is found to be especially important to the formation of more complex molecular structures, as confirmed by laboratory study (Öberg et al. 2009). Methanol is formed on the dust grains during the early, cold phase of cloud development, resulting from the addition of atomic hydrogen (H) and CO on the grains, both of which species are derived from the gas phase. Methanol has been observed to comprise up to ~30% of the molecules in protostellar dust-grain ices (Boogert et al. 2008).

UV photolysis of methanol (or hydrogenation of CO) can produce radicals such as HCO, CH3, CH3O, and CH2OH on the ice surface or within the bulk ice, which may react together (as shown in figure, right) to produce complex organic molecules that have been detected in hot cores. The gradual warm-up of the dust during the star-formation process allows these radicals to become mobile, while at yet higher temperatures the COMs formed as a result can evaporate into the gas phase, where they may be detected by their rotational emission spectra.

The figure, left, shows the formation and evolution of a selection of typical complex organic molecules as the core warm-up occurs. The time axis is plotted logarithmically; temperature follows a quadratic time-dependence. These results were obtained using the new three-phase (i.e. gas/surface/bulk-ice-mantle) chemical kinetic model MAGICKAL (Model for Astrophysical Gas and Ice Chemical Kinetics And Layering) developed by Garrod (2013).

Solid lines denote gas-phase species; dotted lines of the same colour denote the same species on the grains. Abundances are shown as a fraction of the gas density (107 cm-3). The formation on the grains of many commonly-detected COMs may be seen to occur at temperatures of ~20 – 40 K, at which point the HCO and CH3 radicals become mobile on the grains or within the ice mantles, finding reaction partners (CH3O/CH2OH) to form species such as methyl formate (HCOOCH3).

At temperatures greater than ~100 K, water (H2O), methanol (CH3OH), and many COMs that are tightly-bound to the grain surface/ice-mantle sublimate into the gas phase, producing abundances in good agreement with observational values for COMs (typically ~10-9 – 10-7 of the total gas content).

The importance of water

The new Garrod (2013) model also included a fairly complete set of hydrogen-abstraction reactions by OH, NH2, and several other radicals within the ice mantles, using best estimates of activation energies. The abstraction of hydrogen atoms by OH radicals from molecules in the ices was found to be generally more important than their direct UV photo-dissociation. Activation barriers to H-abstraction by OH are typically very low (compared to OH diffusion barriers), while OH is produced easily by the UV photo-dissociation of the abundant water in the ices.

Thus, the production of reactive radicals in the ices is caused mainly by photo-dissociation of water molecules. The presence of water itself in the ices may therefore be of critical importance to the conversion of simple organics into complex molecules.

Interstellar glycine & spectroscopic simulations