Astrochemistry

Where the elements came from

In the first few minutes after the Big Bang, protons and neutrons fused into deuterium, helium and a trace of lithium. The universe cooled too quickly to go further, and no stable nucleus has a mass number of 5 or 8 to act as a stepping stone, so ordinary matter began as roughly three-quarters hydrogen and one quarter helium by mass.

Everything heavier was made in stars. Hydrogen fuses to helium in stellar cores; in hotter, older cores three helium-4 nuclei combine to give carbon-12 (the triple-alpha process), and further fusion builds oxygen, neon, silicon and so on up to iron. Iron-56 sits near the peak of binding energy per nucleon, so fusing beyond it absorbs energy instead of releasing it. Heavier elements form by neutron capture followed by \beta^- decay: slowly in ageing giant stars (the s-process) or rapidly in violent events such as neutron-star mergers (the r-process), the main source of gold and platinum.

Reading spectra

Each element absorbs and emits light at characteristic frequencies, because its electrons can only move between fixed energy levels (\Delta E = h\nu). Dark lines in starlight act as a barcode for the elements in a star's atmosphere; helium was identified in the Sun's spectrum decades before it was found on Earth.

Molecules also have rotational energy levels, far more closely spaced, so their transitions fall in the microwave and radio region. Only molecules with a permanent dipole moment show these lines. That is why H_2, by far the most abundant molecule, is almost invisible to radio telescopes while CO shines brightly; astronomers map CO and scale up to estimate how much hydrogen is present.

Why ions rule cold chemistry

At 10 K, RT is under 0.1 kJ mol^{-1}, so any reaction with a real activation barrier is frozen. Ion-molecule reactions avoid this. The charge induces a dipole in the neutral partner and the long-range attraction draws them together with no barrier. Langevin capture theory gives rate coefficients of about 10^{-9} cm^3 s^{-1}, independent of temperature and roughly ten times the collision rate of two neutrals.

The chain starts with cosmic rays, which ionise H_2 deep inside clouds that ultraviolet light cannot penetrate:

H_2 has a low proton affinity, so H_3^+ hands its proton to almost anything:

OH^+ gains hydrogen in successive reactions with H_2 to give H_3O^+, and dissociative recombination with an electron releases H_2O and OH. Networks of a few thousand such steps (from databases such as UMIST and KIDA) reproduce many observed abundances surprisingly well.

Dust grains as catalysts

Two H atoms meeting in the gas form an H_2 molecule with too much energy and no partner to take it away, so it falls apart. On a dust grain the surface absorbs the excess, which is where interstellar H_2 forms. In dense clouds, molecules freeze onto grains as water-dominated ice mantles. Hydrogen atoms move across the ice, partly by tunnelling, and hydrogenate CO stepwise:

Gas-phase routes to methanol are far too slow, so this grain chemistry is its accepted source.

Rotational spectra

For a rigid linear rotor E_J = hBJ(J+1), so the J+1 \rightarrow J line appears at \nu = 2B(J+1). For ^{12}CO, J = 1 \rightarrow 0 lies at 115.27 GHz and J = 2 \rightarrow 1 near 230.5 GHz. Line ratios give excitation temperatures, and the rarer ^{13}CO corrects for ^{12}CO lines that are optically thick.

Deuterium fractionation

\mathrm{H_3^+ + HD \rightleftharpoons H_2D^+ + H_2} is exothermic by about 230 K (in temperature units) because of zero-point energy differences. At 10 K the reverse reaction stalls, so H_2D^+ and the molecules it protonates become far richer in deuterium than the cosmic D/H ratio of about 10^{-5}.

Detection has moved from single small molecules to whole families. Deep line surveys with the Green Bank Telescope identified individual polycyclic aromatic hydrocarbons by their rotational lines, beginning with the cyanonaphthalenes in the cold cloud TMC-1 in 2021; before that, PAHs were known only as a class from broad infrared bands. ALMA resolves chemistry inside protoplanetary discs and warm protostellar cores rich in complex organic molecules (COMs) such as methyl methanoate and glycolaldehyde. JWST records ice absorption spectra towards background stars, measuring CO, CO_2, CH_3OH and NH_3 in grain mantles directly.

The central open question is where COMs form. Early models made them by radical recombination on grains warmed by a young star, but COMs also turn up in dark clouds at 10 K, where radicals should be immobile. Candidate explanations include non-thermal desorption driven by cosmic rays or reaction heat, gas-phase radiative association, and non-diffusive surface mechanisms. None has settled the matter.

Laboratory astrophysics carries much of the load. Every detection needs an accurately measured rest frequency, usually from microwave or millimetre-wave spectroscopy of a molecule made in a discharge. Networks lean on rate coefficients that have often never been measured below 50 K, so ion traps, CRESU flows (supersonic expansions cooling gas to a few kelvin) and ultra-high-vacuum ice experiments fill the gaps. Sensitivity analyses show that a handful of poorly known rates can move predicted abundances by an order of magnitude.

Other live threads: assigning the diffuse interstellar bands (C_{60}^+ explains a few, hundreds remain unassigned), the cosmological lithium problem, and isotope ratios in meteoritic organics that tie Solar System material to cold cloud chemistry.