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(This article originally appeared in the Q4 1991 STAR newsletter. -Ian)
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.
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