(This article originally appeared in the Q4 1991 STAR newsletter.)
Chemistry of an Interstellar Cloud
by Mike Albers
Chemical reactions between hydrogen, carbon and other atoms in interstellar clouds form molecules. Moving in random paths, atoms must collide with other atoms before a chemical reaction takes place. The more atoms in a volume of space, the more collisions and chemical reactions. Thus, cloud density affects the reaction rate, giving dense cores in a GMC an advantage over the rest of interstellar space.
Photons can hit an atom or molecule and knock an electron loose. Actually, since the forces holding an electron to an atom are stronger than the forces holding a molecule together, it’s hard for molecules to exist in space. High energy photons quickly break them apart. In fact, most molecules can only exist inside a molecular cloud. There the ultraviolet and x─ray photon flux is low, having been absorbed by the outer layers of the cloud; the environment is gentle enough for molecules to exist. But even there, many heavy element molecules are ionized.
There is no magical attraction between atoms which draws them together. Chemical reactions occur as a result of random collisions, with the gas atoms following a random path. Unless atoms collide, there is no reaction. So, the more atoms in a volume of space, the more collisions. This is why gas in the ISM maintains it’s high temperature. Collisions cool it but they are very, very rare. As the clouds get denser, the collision rate increases. However, time between collisions still measures in minutes or hours; even at 1000 particles per cubic centimeter, a core is an excellent vacuum by Earth standards. In the air around you, many times denser than space, the time between collisions drops to thousandths and millionths of seconds, resulting in rapid reactions.
Chemical reactions fall into two types, those absorbing heat and those releasing heat. Reactions which absorb heat are called endothermic, while reactions which release heat are called exothermic. Only exothermic reactions are important in a cloud. Exothermic reactions may either emit a photon which carries off the excess energy or two molecules are formed with the excess energy being converted into kinetic energy. The kinetic energy can then be transferred to other atoms, heating the cloud. The emitted photon may be absorbed by other atoms, heating the cloud, or it may escape, cooling the cloud.
The atom’s kinetic energy provides another major factor controlling reactions. Gas temperature directly relates to the atom’s kinetic energy (its speed). Hot gas atoms move fast while cold gas atoms crawl along. This gets affects how close they can get. The electrons around the atoms try to repeal each other. A slow moving atom gets deflected before it moves close enough to react. A fast moving atom overcomes the electron’s repulsion; the atoms can get close enough to react.
But moving too fast is also bad. The kinetic energy of both atoms must be dissipated before the molecule forms. When two atoms crash together in a high speed collision, the energy is too much to dissipate. So, the atoms bounce off each other without reacting
How long the atoms are in contact provides another factor controlling chemical reactions. A chemical reaction doesn’t occur instantly. Energy must be transferred between the atoms and the electrons need to adjust their orbits to allow for the second nucleus. A normal collision lasts 10─13 seconds but to react, atoms must be in contact for 10─8 seconds. This drives the reaction rate down to 1 reaction for 100,000 collisions. On Earth, there’s no problem, an atom collides more than that each second. However, in a cloud, things go much slower. With the low collisions rates in a core, hydrogen atoms last 300 years before they react to form a hydrogen molecule. Still, a short time when you have thousands of years to work with. Thus, most of the core is molecular hydrogen; given enough time all the hydrogen reacts.
How the Atoms React
The reactions in a cloud can be pictures as fitting one of two chemical equations. The first reaction forms a single molecule and photon. The second forms two molecules. The first chemical equation can be written as:
A + B –> AB + photon
The A and B can stand for any atom or molecule, such as hydrogen, carbon, oxygen, or a molecule of ammonia. All the excess energy, both kinetic energy carried by the atoms and the exothermic energy released in the reaction, is carried off by the photon. This equation describes how hydrogen reacts to form a hydrogen molecule. This equation also describes hydrogen and oxygen reacting to form a water molecule. The second chemical equation can be written as:
A + B –> C + D
In this reaction, two atoms or molecules react to form two different atoms or molecules. Notice, no photons are emitted by this reaction. Instead, the excess energy gets converted into kinetic energy and carried off by C and D. In other words, they end up moving faster than A and B were moving. C and D collide with other particles in the cloud, giving up extra energy and heating the cloud. Because it’s easier to convert the excess energy to kinetic energy than to a photon, this reaction occurs more frequently in the cloud.
How Cloud Chemistry Differs from Earth Chemistry
A major difference is that hydrogen exists as free atoms, while on Earth only hydrogen molecules exists. With the cloud’s low density and rate of collision between atoms, the hydrogen atoms react at a much slower rate. Also, ultraviolet photons break up the hydrogen molecules.
Single hydrogen atoms aren’t the only thing which exists in space but not on Earth. Another large group are the radicals, like CH+, CN+, OH─, C2H+, and HCO+. Radicals are charged molecules. They form by a photon knocking an electron free or by the photon knocking an atom free. Since their charge comes from a dangling chemical bond, they are very reactive and grab the first available atom which comes along. Just like free hydrogen atoms, radicals exist in the cloud because of the low collision rates.
Reactions on the Dust Grains
Most pictures of chemical reactions show two atoms colliding and forming into a molecule. However, in a cloud there are few collisions between gas atoms that have the proper energies to react. But molecules are very abundant in a GMC, almost all the hydrogen is in the molecular state and heavy elements are all tied up in the dust. It’s the dust grains which make the difference; atoms stick to them, react and get ejected. On the grain’s surface, reaction rates are much higher than those of the gas phase.
Making Hydrogen Molecules
Hydrogen almost always reacts on the dust grains. The entire process takes four distinct steps: (1) accretion onto the grain’s surface, (2) diffusion across the surface, (3) reacting with other atoms, and (4) ejection from the grain back into the gas phase.
Accretion is the fancy name for the hydrogen atom colliding and sticking to the grain. As more and more hydrogen atoms collide with the grain, its surface becomes littered with hydrogen atoms.
Diffusion is the movement of hydrogen atoms across the grain’s surface. They don’t remain at the spot where they hit, instead they move across the grain and concentrate is depressions. This improves the chance of a reaction.
When the atom moves into a depression containing another hydrogen atom, the two atoms draw together and react, forming a hydrogen molecule. The reaction is exothermic, releasing energy. Part of the energy goes into heating the grain and the rest goes into ejecting the newly formed hydrogen molecule.
With its share of the heat liberated in the reaction, the hydrogen molecule flies off into the cloud. The molecule may be ejected at a greater speed than the speed of the original atoms, thus helping to heat the cloud. But all the energy doesn’t go into kinetic energy, some goes into vibrational and rotational energy. The atom eventually radiates away this energy, making the cloud visible in the microwave and infrared regions of the spectrum.
From here, the cycle starts over. The hydrogen molecule gets broken up by an ultraviolet photon, freeing two hydrogen atoms which finally collide with a dust grain. The cycle continues until the hydrogen gets drawn into a protostar.
Making Other Molecules
The process of accretion, diffusion, reaction and ejection describes how all elements react to form molecules on dust grains. Each reaction releases a different amount of energy. Whether the molecule gets ejected in the reaction depends on the released energy. When it is high enough, the molecule gets ejected. At low energy levels, the molecule remains on the grain, building up layer after layer of icy mantel.
The carbon, nitrogen and oxygen on the grain surface can react with the hydrogen atoms, forming simple radicals: CH+, NH+ and OH+. All three of these radicals remain on the grain after the reaction, giving it a positive charge. But most radicals are ejected, like: H3+, CH3+, HCO+, C2H2+, HS+, and N2H+.
In turn, these radicals and simple molecules react and form more complex molecules. Most of the heavy elements which are not part of the dust grains are in complex molecules. Current estimates have these complex molecules containing seventy percent of the heavy element atoms in the cloud. Only thirty percent are tied up in the radicals and simple molecules like water and methane.
Gas Phase Reactions
Although most chemical reactions occur on dust grains, some do occur in the gas phase. The random collision of atoms results in several different molecules forming; most of which involve ions.
Even deep inside a core there are ions. Although the outer layers of the cloud stop ultraviolet and x─ray photons, cosmic rays still penetrate to the center. The cosmic rays ionize helium atoms, hydrogen atoms and hydrogen molecules, producing He+, H+, or H2+.
Some Gas Phase Reactions
A couple of reactions which occur in the gas phase involve hydrogen molecules, and oxygen, nitrogen or CH+ ions. The reaction can produce two results, both of which produce a ion. The reaction involving oxygen is:
OH + H+ O+ + H2 –> OH+ + H
The other reactions are basically the same, just replace the oxygen ion with a nitrogen or CH ion.
A very important reaction is the one producing carbon monoxide. This molecule provides the main source of core cooling. It gets formed by one of two reactions:
C+ + OH –> CO + H+ CH+ + O –> CO + H+
The second reaction depends on oxygen abundance. It’s a fast reaction, quickly depleting any available free oxygen from the cloud. Once the CO molecule forms, it lasts a long time because of its high ionization potential.
Organic Chemistry
Seventy percent of the heavy elements were tied in complex molecules. Many of these molecules are organic molecules (containing carbon).
Carbon is important in cloud chemistry for two reasons. First, it reacts with oxygen to form carbon monoxide, the primary cloud coolant. The second is that carbon forms large molecules, tying up large numbers of hydrogen, nitrogen and oxygen atoms. During the cloud’s lifetime, almost half of the carbon, nitrogen, and oxygen will be converted into organic molecules.
Organic molecules form on dust grains, like hydrogen molecules. But, unlike hydrogen, the reaction is very temperature dependent. Grain temperature must be above 27 degrees. However, normal temperature in a dense core is between ten to twenty degrees. Organic radicals form and remain on colder grains, but no complex molecules form. When something heats the grain, such as approaching a star or drifting to the core’s edge, the radicals react in a massive chain reaction. The heat generated in the first reaction warms the grain, allowing higher temperature reactions to occur. Complex organic molecules are formed and expelled from the grain.
Molecule Destruction
An individual molecule doesn’t last forever, eventually it either reacts with another molecule to form a new molecule or it absorbs a photon and breaks apart. About one in ten hydrogen molecules break up after absorbing an ultraviolet photon. In clouds containing less than 1500 solar masses, the destruction rate exceeds the formation rate, meaning these clouds contain little molecular hydrogen.
The rate of molecular destruction decreases as you move deeper into the cloud. Since the outer layers stop ionizing photons, inner parts escape the bombardment and the molecules lifetime increases.
