Astrochemistry is defined as the study of the abundance of molecules in the Interstellar Medium (ISM). It comprises the study of the composition of the interstellar gas, but also its evolution from the early universe to the chemically complex systems we observe in more advanced objects, such as planetary systems. More generally, it can also be understood as the study of the origin of chemical complexity and, therefore, the origin of life.
The study of chemical evolution in space is a fairly recent concept; before the last century, it was believed that reactions in space were not possible due to the harsh environments (very low densities and temperatures) and that the ISM was mainly composed by H2. The first direct observation of an interstellar molecule didn't arrive until 1940, when McKellar published their detection of CH and CN (McKellar, 1940). The detection of interstellar CO (Penzias, 1971, 1972) allowed the identification of structures never identified before, the so-called Giant Molecular Clouds (GMCs) evidencing the potential use of molecules as tracers of different conditions in space. Many other molecules were, and are, identified: HCN, CS, C2H, CH3OH, C2H5OH, and other organic molecules generally called Complex Organic Molecules (COMs), ions, such as \hco, and even isotopologues of many species, such as deuterium. With the improvement of radio astronomy techniques and the arrival of interferometric techniques, with facilities like ALMA, the reservoir of stellar molecules has kept growing year after year to show a vast chemical network operating in interstellar conditions.
Astrophysics and astrochemistry are intrinsically related: we need to know the physical conditions of different astrophysical environments to elucidate which chemistry can occur in them, but we also need a good chemical model to relate the molecules we observe to the conditions at which they are present. To be able to construct a good chemical model, it is important to understand that chemical evolution is necessarily determined by the different stages of the evolution of the universe. The ISM, and everything that inhabits it, is mainly made by gas (mostly H2) and dust particles. These two, evolve with the stellar cycle dictating the formation and death of stellar systems. Each interstellar region and stage has its own physical conditions (density, temperature, dust abundance, radiation fields, etc) which determine the chemistry that can occur and the molecules that can be formed The physical conditions of these objects are varied and spawn orders of magnitude in density, tenths of degrees in temperature and different degrees and sources of radiation as molecular clouds evolve to form protostellar objects and finally planetary systems, therefore, the chemistry at the different stages is different. However, there are four main mechanisms that can be invoked in these environments that can promote chemical evolution.
Collisional association
The simplest way of forming a new molecule is the collision between two atoms resulting in the formation of a new chemical bond. However, the new molecule still has the kinetic energy from the free colliding atoms so, unless the excess energy is removed, the two atoms will simply bounce off. A way of getting rid of the excess energy of the newly formed molecule consists on the simultaneous collisions of a third atom that can absorb that energy, in what are called three-body reactions:
A + B + M → AB + M
The reverse reaction can also happen, where a collision dissociatesan existing molecule, in what’s called collisional dissociation:
AB + M → A + B + M
While possible, this formation pathway requires a density large enough for the probability of three atoms colliding to be high, whichmight not be the case in most astrophysical environments.
Radiative association
Another solution to get rid of the excess energy is that the energy is emitted as radiation:
A + B → AB + hν
The reverse process can also occur, and is called photodissociation:
AB + hν → A + B
While this process can happen in the ISM, in order to radiate the newly formed molecule needs to have a dipole moment and allowedro-vibrational transitions of the same energy as the excess energy.
Exchange reactions
Neutral exchange reactions are also theoretically possible:
AB + D → BD + A
However, these types of reactions have activation barriers and, there-fore, they can not happen in the diffuse ISM or dense clouds, where temperatures are ≈ 100 K, and they can only occur where an additional source of energy exists, such as circumstellar environments or shock regions.
Ionisation and ionic reactions
An additional problem for neutral reactions to happen is that they generally present activation barriers that are hard to overcome in astrophysical environments with few sources of energy. Therefore, a viable route to interstellar chemistry is the presence of ions in the gas. Radiation is present in very different environments, primarily, stars emit UV light which is able to ionise the molecules on the gas around them. Massive stars also produce X-rays, which are able to drive additional ionised chemistry. The ionisation process is:
A + hν → A+ + e–
This chemistry is likely to occur in regions where UV radiation can become in contact with the gas, such as circumstellar gas, the outer envelope or dark clouds and dense cores and regions close to massive stars or supernovae. However, on the inner radius of dark clouds, dense cores and protostellar objects, which are embedded in large quantities of dust, the UV radiation is absorbed by this dust, loses energy and is unable to ionise the most inner regions of the cores. In these regions, ionisation occurs mainly through cosmic rays (CRs), which are highly energetic particles (mainly protons, electrons and He nuclei) which are capable of creating one electron and one proton in each ionisation. CRs are able both to excite and to ionise a molecule, depending on the energy they carry:
AB + CR → AB∗ + CR
AB + CR → AB+ + e− + CR
Where energy is conserved and, therefore, the CR in the right-side of the equation carry less energy than before the reaction. CRs can also dissociate a molecule, either by simple dissociation or by dissociative ionisation:
AB + CR → A + B + CR
AB + CR → A + B+ + e− + CR
CRs are abundant in the ISM, they are produced by the acceleration of particles in very different environments, such as shocks, magnetic fields and can be produced in the surface of forming protostars in accretion shocks, where they can ionise the most inner part of dark clouds and dense cores (Padovani work, Cabedo2022). Once a molecule is ionised, it can undergo both ion-molecule and charge transfer reactions:
AB + D+ → BD+ + A
AB + D+ → AB+ + D
Surface Chemistry
What has become clear to scientists, is that gas-phase chemical models at those conditions (T~10 K) are not enough to explain the appearance and the abundance of most observed molecules, which lead to the proposition of chemical reactions occurring on the surface of the dust grains, which serve as a support for gas-dust interactions and aid chemical reactions to happen. These dust grains are believed to have a core mainly composed by silicates and carbonates (ref) and, under most astrophysical conditions, they'll be covered in ices formed by the frozen volatiles present in the gas phase (\vicky{Figure of dust grains}). These ices act by agglomerating molecules together, that otherwise will be in a medium too diffuse to react, and providing a 'third-body effect' by absorbing left over energy and allowing reactions to happen. Moreover, the dust grains also contain metallic inclusions which might be exposed under certain conditions, and which can act by catalysing reactions otherwise impossible to occur.
