ASTROCHEMISTRY - Part 1
Molecules Detected in Interstellar and Circumstellar Regions
Many of the following molecules have been detected with the major isotope
substituted by a minor isotope. The actual number of detected molecular
species is much larger than shown below.
Two Atoms
H2, OH, SO, SiO, SiS, NO, NS, HCl, PN, NH, CH+, CH, CN, CO, CS, C2, CO+,
SO+, SiN, NaCl, KCl, AlF.
Three Atoms
H2O, H2S, SO2, NH2, N2H+, HNO, HCN, HNC, C2H, HCO, HCO+,OCS, HCS+, C2S,
C2O, NaCN, SiC2 (cyclic), MgNC.
Four Atoms
NH3, H3O+, H2CO, HNCO, H2CS, HNCS, C3N, C3H (cyclic), C3H (linear),
C3S, C3O, C2H2, HOCO+, HCNH+.
Five Atoms
HC3N, C4H, CH2NH, CH2CO, NH2CN, HOCHO, C3H2 (cyclic), CH2CN, H2C3,
CH4, HC2NC, SiH4.
Six Atoms
CH3OH, CH3CN, CH3SH, NH2CHO, CH3NC,HC2CHO, H2C4, C2H4, H2C3N+.
Seven Atoms
HC5N, CH3CCH, CH3NH2, CH3CHO, CH2CHCN, C6H.
More Than Eight Atoms
CH3OCHO, CH3C3N, HC7N, CH3OCH3, CH3CH2OH, CH3CH2CN, CH3C4H, HC9N,
HC11N, CH3C5N, (CH3)2CO.
On the large scale the cosmos is chemically controlled.
All visible matter in the Universe has cooled to temperatures well below
those at the Earth's surface at least once since the Big Bang.
Many of the astrophysical objects that have temperatures less than several
thousands of kelvins contain large abundances of molecules, just as the
Earth's atmosphere at 300° above absolute zero is almost entirely molecular.
Molecules have influenced the births and distributions of all stars and
galaxies.
Temperatures in the envelopes of many old stars drop to several thousand
kelvin inducing molecule formation which triggers the production of dust
grains; these grains transmit the pressure of the stellar light which they
absorb to the gaseous envelopes, powering strong winds which remove stellar
mass, so that an active star is converted into a dwarf.
Each molecule absorbs and emits radiation at wavelengths that are
characteristic of its species. The response of the molecules to the
physical conditions affects the observed radiation in ways that permit
the diagnosis of conditions in those objects.
Chemistry controls the evolution of astronomical objects and is a diagnostic
of conditions in them.
Astrochemistry produces species that sometimes have never been manufactured
in detectable quantities in terrestrial laboratories.
Chemistry controls the properties and evolution of the astronomical
environments in which it takes place.
The masses of interstellar gas clouds are known to be up to a million
times that of the Sun, so that the largest clouds are the most massive
objects in the Galaxy. They are also the sites of current stellar birth.
Ages and Lifetimes: Years and A-Days
(million years = one astronomical day)
Age of the Universe
15 x 10**9 y = 41 A-years
Age of the Galaxy
12 x 10**9 y = 33 A-years
Age of the Sun
5 x 10**9 y = 14 A-years
Age of the Earth
4.5 x 10**9 y = 12 A-years
Rotation Period of the Galaxy
10**8 y = 3 A-months
Life of a Molecular Cloud
10**8 y = 3 A-months
Life of a Bright Star
10**6 y = 1 A-day
Duration of a Cool Envelope
10**4 y = 14 A-minutes
Duration of a Planetary Nebula
10**4 y = 14 A-minutes
Duration of Human Civilization
5 x 10**3 y = 7 A-minutes
Duration of Human Technological Era
10**2 y = 9 A-seconds
Duration of a Supernova
1 y = 0.1 A-seconds
Bright stars come and go like flowers that bloom for an A-day or two
while each of the 'plants' that prodcue them - the molecular clouds -
remain in being for several A-months. Low mass stars like the sun
- like the grass in the garden lawn - can survive for A-decades.
The chemical timescales are also very important. At the lowest densities,
collisons of one H atom with another occur on the average at intervals of
10**13 seconds, nearly a million years (~1 year = 30 million seconds).
Collisions with other elements are much rarer than this. In stellar
atmospheres each molecule may be subjected to hundreds of collisons per
second, and chemistry can be rapid.
Distances in the Universe
Thickness of Galaxy
1,000 light years
Galaxy Radius
50,000 l.y.
Sun-Galactic Centre
27,000 l.y.
Radius of Visible Universe
15 billion l.y.
Mean Earth Radius
0.02 light seconds
Mean Earth Moon
1.3 light seconds
Mean Solar Radius
2.3 light seconds
Mean Earth-Sun Distance
8.3 light minutes
Mean Jupiter-Sun
43 light minutes
Mean PLuto-Sun
5.5 light hours
There are 100 billion, or 10**11 stars in our galaxy.
Molecules do not generally survive in gas that is more than about 10 times
hotter than the Earth's atmosphere. Sunspots and the envelopes of many
highly evolved stars are marginally cool enough for molecules to exist.
Much of the interstellar gas that is molecular has temperatures of only
some tens of kelvins.
The temperature of a gas is proportional to the average kinetic energy of
a free electron, ion, atom or molecule. The average thermal speed rises
proportionately to the square root of the temperature and is around
1 km/s for a H molecule in a 100 kelvin gas.
The number of molecules in a cubic metre of air under standard atmospheric
conditions on Earth is about 3 x 10**25. In interstellar clouds the density
is 10**8-10**9/cubic m. Protosolar disk near where Jupiter formed was
10**2-/cubic metre.
The Universe probably began with a Big Bang. When it was only about
3 minutes old nuclear reactions, some of which formed most of the
Universe's helium prevailed. After 500,000 years cooled enough so that
electrons interacted with positive ions to produce neutral matter, mostly
atomic H and He.
Some of this matter accumulated into pregalactic gas clouds through the
influence of molecular H which acted as an effective cooling agent. Within
these regions, the first generation of massive, short-lived stars were
formed. They "burned" H to make the heavier elements of C, N and O. When
these first stars exploded they seeded the Early Universe with trace
amounts of these elements so that the pregalactic gas gradually became
eneriched in C, N and O. In this new situation a wider variety of stars
could form: the cloud of gas became a galaxy of stars, gas clouds and
dust.
The explosions of the supernovae maintain a pressure in the interstellar
medium which helps to accumulate very tenuous gas into somewhat denser
clouds. When these clouds become massive enough, gravitational collapse
ensues and new stars form. Some of these will end in supernovae, but most
of them as stars of lower mass. Low-mass stars develop winds which stir
the gas and help to create conditions in which new stars can form. Older
low-mass stars, towards the end of their lives are unable to hold on to
their atmospheres which begin to drift away in enormous cool envelopes.
Around 80 percent of the mass of visible matter in the Universe is H.
A H atom is the simplest type of atom and consists of a proton and an
electron interacting electromagnetically. Quantum mechanically, a H atom
is veiwed as an electron-proton system that can be in particular states in
which the energy, electron angular momentum and the sum of the spins of the
proton and electron are conserved.
A property of a hot gas is that it radiates at wavelengths that are
characteristic of the atoms or molecules present. The ability to identify
atoms and molecules through features in the spectrum is the basic tool of
astrophysics. These features recur in all regions of the electromagnetic
spectrum, not only in the visible.
Helium is the second most abundant element in the Universe, after H.
(Sodium chloride in a flame gives rise to an oragne glow, corresponding
to a wavelength of ~ 589 nm.)
At large distances two H atoms attract each other weakly, and as they
come together they attract each other even more strongly, but ultimately
at close range they repel each other. What happens when two hydrogen atoms
approach each other from a great distance. Since each atom is made up of a
proton and an electron, each has an electric field that is felt by the
other. Each atom readjusts very slightly in this field in such a way that
the atoms attract each other. If the atoms are far from one another, then
on average only one of the electrons is between the two protons while the
other electron is on the far side of one of the protons. this 'in-between'
electron partly screens the protons from each other, but - on average - in
the region between the two protons the electric field is slightly dominated
by the protons and, hence, attractive to electrons. This attraction for
the electrons slightly distorts the electron distributions enhancing the
maximum of the distributions near the midplanc of points equidistant from
the two protons. The increase in the electron concentration between the
protons further shields that proton from one another and even creates a
pull, towards the midplane, on the protons. As the protons become closer
the distortion of the electron distribution becomes greater, and the pull
towards the midplane on the protoms increases. As the atoms become closer,
the electrons on each respond more to both protons. They no longer 'belong'
to one of the atoms but to the pair, since they, and the protons, are
identical. If the protons come even closer together there is a high
probability that neither electron is between the protons; then the protons
are not screened from one another and their electric interaction caused
them to repel one another.
It is possible to trap the atoms so that they remain bound to each other,
forming a molecule. This molecule now has vibrational and rotational
motions that are not shared by atoms. The total energy of a molecule is
the sum of the electronic, vibrational and rotational energies.
Photodissociation (absorbs radiation and then falls apart) is very often
the major means of destroying molecules in astronomical environments. It
generally requires radiation in the ultraviolet region of the spectrum.
Atoms and molecules may be photoionized but only molecules may be
photodissociated.
Photodissociation:
OH + radiation => OH* => O + H
Molecular Photoionization:
OH + radiation => OH+ + e-
Atomic Photoionization:
H + radiation => H+ + e-
The vibrational energy of a system of atoms bound together in a molecule is
restricted to certain discrete values, just as is the energy of an electron
bound to a proton.
Even in the lowest vibrational energy state there is still some energy so
the molecule is still vibrating in the lowest possible vibrational energy
state. The molecule never sits entirely still, without vibrating.
Transitions between various vibrational energy levels of the same
electronic state leading to the emission or absorption of radiation tend
to fall in the IR rather than the visual or UV regions of the spectrum
associated with transitions between the electronic states.
Molecules can rotate end-over-end. Quantum mechanics tells us that the
energy of rotation for molecules can have only certain permitted
values.
The total energy of a molecule depends on its particular electronic state,
its vibrational state and its rotational state. Each electronic state has
a ladder of vibrational levels. Each of these vibrational levels has a
family of rotational levels associated with it. When transitions occur in
molecules, they occur from one particular electronic + vibrational
+ rotational state to another. The energy of each state is the sum of
energies of each part, and when a photon is emitted it carries away an
energy equal to the difference between the energy of the initial state
and the energy of the final state. In dense enough gas the temperature of
the gas determines the population in the various levels.
On Earth most matter is molecular rather than atomic. In many astronomical
situations the gas tends to be atomic rather than molecular and often it
is the intense UV radiation field from very hot stars that is reponsible
for driving the system towards atoms rather than molecules, by
photodissociating the molecules.
For the interaction of two H atoms, as they approach they speed up, then
separate, and slow down and unless they have lost some energy they will
be able to separate completely. At high density, such as in the Earth's
atmosphere, when a third H atom collides with the interacting pair,
removes some energy and leaves them in a bound state. So high density
helps to convert atoms to molecules, because high density leads to
frequent collisions. This is the way that many reactions occur when the
density is comparable to that of the air. But in nearly all astronomical
situations the density is very much lower and the atom Z hardly ever finds
the colliding pair XY and so three-body collisions do not contribute to
the chemistry.
Ion-molecule collisions are very effective in forming new molecules.
Reactions occur almost every time an ion and molecule meet, and they are
drawn into interaction from relatively large separations. The positive
ion polarizes the molecule, i.e. it pulls negative charges slightly
towards it. The attraction between these opposite charges is then greater
than the repulsion between the positive charges so that a net attractive
force results. The partners spiral in towards each other from separations
of about ten times a typical atomic radius, and interact with sufficient
energy to create a complex of atoms. In the reaction of O+ and H2 the OH+
molecule forms, with a stronger bond than H2, and the H2 molecule is lost
and an H atoms is expelled, carrying off excess energy. This is only the
first in a sequence of reactions. Eventually H3O+ is formed and it can add
no more hydrogen; the oxygen atom is unable to bind any more hydrogen to
itself. 
In nearly all astronomical situations, the molecule is H2 and the primary
step in astrochemistry is usually its formation. Once H2 is available,
then the effectiveness of ion-molecule chemistry in astronomy is often
linked to the rate at which the ions can be created.
Ionization can occur in various ways. Ultraviolet radiation with
wavelengths of around 100 nm can ionize some atoms, eg.
C + radiation => C+ + e-. Another possibility is that cosmic rays, which
are energetic particles (mostly protons) collide with atoms or molecules
and eject an electron, eg. H2 + c.r. => H2+ + e- + c.r. Chemically the
most influential cosmic rays in the interstellar gas, star forming regions,
and protostellar disks are probably those travelling at about a few tenths
of the speed of light. By cosmic ray standards these are not very energetic
particles, but they are prevalent, having an abundance of the order of
10-4 m-3 in interstellar space.
What will happen to ions such as H3O+? They may be dissociated by the
radiation field. Another important loss mechanism is the reaction with
negative electrons. Since the gas is neutral overall, for each positive
ion there is a negative electron. The attraction between positive ions and
electrons is strong and neutralization usually occurs, but this very often
creates an unstable molecule which then falls apart. For instance, H3O
formed in the recombination of an electron with H3O+ dissociates to
produce OH or H2O.
Though this fast dissociative type of recombination is a common fate for
molecular ions. Atomic ions, on the other hand, recombine quite slowly
with electrons, eg. C+ + E- <=> C* => C + radiation.
When two atoms (A & B) collide they are more likely to bounce off each other
than to emit radiation and stabilize the molecule AB. When one of the
partners is a molecule a new molecule may be formed. In ion-molecule
reactions the electrical interactions are strong and pull the partners
together so they collide quite energetically, often giving rise to
rearrangement. In the case of neutral partners, the force of neutral
partners is weak and there may be a barrier hindering interaction.
and therefore rearrangement.
The close approach of reactants does not ensure that a reaction will take
place. The product of a particular reaction may have a higher total
internal energy than the reactants, and for the reaction to proceed
additional energy must be provided by the kinetic energy of the approaching
reactants.
Neutral reactions are not in general as efficient as ion-molecule reactions.
The charge on the ion means that an ion and a molecule interact
significantly at distances much larger than the size of the electron
cloud around each reactant. The neutral species, on the other hand, have to
experience a close collision before any reaction is possible. Neutral
reactions are therefore only about 1% as likely to occur as ion-molecule
reactions.
In astronomy, catalysis on surfaces is provided by dust grains. While
atoms A & B might only rarely combine in the gas bacause the time they
spend in collision is so short, on a surface there are sites to which both
A & B will be drawn and held long enough for a reaction to occur and for
some of the excess energy liberated to be absorbed by the grain. Dust in
interstellar space can provide surfaces on which such catalysis is
responsible for the formation of molcular H. Molecular H formed on the
surfaces of dust grains can then take part in a variety of gas phase
reactions, such as ion-molecule reactions and neutral exchanges.
The temperature of a gas affects the nature of the chemistry that occurs in
it. High temperatures mean that colisons are more energetic, so that
neutral exchange reactions that are inhibited at low temperatures can occur
at high temperatures. The temperature also affects in a sensitive way the
radiation that is emitted by the gas. In a warm gas, colisions between
molecules help to populate a wider range of rotational and vibrational
levels than is possible in a cold gas. The temperature is clearly an
important parameter in any astronomical situation. Heating and colling of
astronomical gases occur by a variety of processes, with those involving
radiation often being the dominant ones in astronomical sources.
Heating can occur when matter extracts energy from the radiation field. For
example, if ultraviolet radiation (perhaps from a hot star) ionizes a
hydrogen atom then the electron carries off that amount of energy that is
in excess of the minimum energy required to release the electron from the
proton. In subsequent collisions, the electron shares its energy with the
gas and loses energy itself in the process, while the gas of H atoms gains
in energy, i.e. becomes hotter. In this way, energy produced by
thermonuclear processes deep within a star and emerging as ultraviolet
radiation heats the gas surrounding the stars.
Cosmic rays are also energy sources, and are particularly important in
regions where stellar ultraviolet radiation does not penetrate. Cosmic
rays colliding with atoms or molecules ionize them, producing electrons
which carry off some energy which is then transferred to the neutral gas
via collisions.
Cooling of a gas generally occurs by the emission of radiation from atoms
and molecules. The temperature in a gas is the result of a balance between
the heating and cooling mechanisms.
The Electromagnetic Spectrum
Energy is proportional to frequency and is inversely proportional to
wavelength, i.e. long wavelengths mean low energy and vice versa.
At the highest energies (shortest wavelengths) fall the electronic spectra
of ions, atoms and molecules. In the IR we find wavelengths of radiation
corresponding to vibrational transitions of molecules, while at longer
wavelengths, in the radio, lie lines associated with rotational
transitions of molecules.
Temperature is a measure of the average kinetic energy of a particle in a
gas. For example, the visible region corresponds to temperatures of the
order of 10,000 kelvin while gas at a million kelvin or so can be expected
to radiate in the x-ray region. The cool matter in the Universe, such as
the Earth at a few hundred kelvin above absolute zero, radiates in the
infrared.
Chemistry After the Big Bang
The Universe began in a hot Big Bang some 15 billion years ago and
has been
expanding ever since. At a very early stage when the
temperature was
hundreds or thousands of millions of kelvins,
collisions between subatomic
particles created H and He nuclei with
very minor traces of deuterium
(or heavy H), Li, Be, B nuclei. The
matter was almost entirely ionized at
this stage. As the expansion
continued the gas temperature fell to several
thousand K, matter
began to recombine, and the Universe became more neutral
as the
expansion continued. As the expansion continued the wavelengths of
the radiation filling the whole Universe continually became longer,
in
direct proportion to the degree that the Universe had expanded;
most of
the radiation consequently acquired wavelengths longer than
those
necessary for it to ionize H and He.
Due to their self-gravity, certain regions of matter within the
generally
expanding Universe collapsed to form
protogalaxies.
The gas in the Early Universe was mostly atomic H. He was also
present
with an abundance about one tenth that of H. Deuterium (D or 2H) was less
abundant than H (the main isotope, 1H) and was
present in one part in
100,000. Li, Be and B were also present but
they were less abundant.
Although the gas was initially ionized, as
the expansion proceeded the gas
cooled and the protons and
electrons recombined radiatively
H+ + e- => H + radiation so that
the photons of the radiation did not have
enough energy to ionize
other H atoms. The recombination of electrons and
protons was never
quite complete and the gas remained slightly ionized.
(The Universe
expanded so collisions became less frequent). The
temperature of
the gas was about 3000 K when most of the recombination was
completed. This cooling was simply due to the expansion of the
Universe.
There are two indirect routes which allowed H2 to form in the Early
Universe:
H + e- => H- + H => H2 and H + H+ => H2+ + H => H2
These reactions gave a small H2 density of about one or two
molecules of H2
per million H atoms. Deuterium behaves chemically
as H so reactions
involving H in the Early Universe also applied to
D. The molecule HD was
present at an abundance of about one
molecule of HD to 100,000 of H2.
As gravity caused a protogalaxy to collapse, much of the
gravitational
energy released in the collapse was converted to heat
in the gas. The
pressure of hot gas retards the collapse of
objects. In order for galaxies
to form, cooling mechanisms had to
operate to counteract the input of heat
due to the collapse. Any
gas at tens of thousands of kelvin in the Early
Universe after
recombination cooled rapidly by converting energy of the
atoms'
motions into radiation. The radiation could not be absorbed by
other atoms. The gas was therefore less energetic and therefore
cooler
than before eg. H + H => H + H+ + e-
Cooling was also caused by the collisions of the electrons with
neural H
atoms. A collision induced the excitation of an H atom to
its n=2
electronic state removing thermal energy from the gas and
eventually the
atom radiated and returned to its ground
state.
As the cooling continued to below 10,000 K the rate of cooling
declined.
Molecular H formation introduced new cooling mechanisms
into the Early
Universe and these allowed cooling to continue to
around 100 K. This
cooling enhanced the ability of regions to
collapse. The HD molecule is
actually more efficient, per molecule,
as a coolant at very low
temperatures. Even though it was at a very
low abundance in the Early
Universe, its contribution to cooling
was also important, particularly
when the temperature was below
about 100 K, since its lowest easily
excited rotational level is at
a much lower energy than that of H2.
If gravity is to cause a region to collapse, then the gravitational
attraction must pull inwards harder than the gas pressure pushes
outwards.
If the gas is tenuous the gravitational forces are weak.
Therefore tenuous
hot gases are not likely to collapse. Spherical
regions of gas must have a
mass greater than a critical value,
called the Jeans mass (Mj) if gravity
is to overcome internal pressure. The Jeans mass is approximately
Mj = 2 x 10**4 (T**3/n)**1/2
where T = temperature in kelvins
n = number of H atoms per cubic metre (number density)
mass is calculated in solar masses (approx. 2 x 10**30 kg)
The Jeans mass is larger at higher temperatures, but smaller at
higher
densities.
The sizes of the first regions to collapse in the Early Universe
are
unknown. Electrons and protons recombined so that the Universe was largely
neutral atomic H when the Universe was about 5 x 10**5 years old. The
temperature was approx. 3000 K and the number density, n, was about
10**10 m**-3, corresponding to a Jeans mass of about 10**5 solar masses.
A great deal of cooling had to occur to allow the Jeans mass in collapsing
protogalaxies to fall from its value of 10**5 solar masses
at the time of
recombination to a value of about 1 solar mass so
that stars could form.
It is likely that collapse really got going when the average number density
of the Universe had dropped to 10**4 m**-3, at which time the Universe was
about 5 x 10**8 years old and the
gas had a temperature of somewhat less
than a kelvin.
There had to be a means of removing energy from the gas as the
collapse
proceeded to form the first stars. Emission of radiation
by H2 provided
this cooling in the early Universe. The heat of the
gas was removed in the
collapse time so that the gas never heated
up. Therefore a collapse, once
started, continued and the Jeans
mass became smaller and smaller, because
the temperature did not
increase while the density increased. This means
that during
collapse less and less massive units became gravitationally
unstable. Thus, fragmentation to smaller and smaller objects can occur.
Go to Astrochemistry - Part 2