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Complex Processes in Simple Ices
The dawn of chemistry
Fourteen billion years ago, the Big Bang created the universe hot, dense and filled with
radiation – too hot for even atoms to exist at any length. With time the universe expanded,
the energy spread out and it got colder and it got darker. Eventually it was cold and dark
enough for the first atoms to form. The lack of photons during the next period in the
Universe’s history has provided the name of the epoch, the Dark Ages; it was to be the
dawn of chemistry. In the beginning of this age, a Helium atom and a hydrogen atom
found each other and bonded together long enough to be considered a stable species. The
Universe’s first molecule had formed. Other molecules followed, even if the chemistry
was rather primitive compared to what we are used to on earth – carbon, oxygen and
all other larger elements had yet to form. These elements are all the products of stars
and there was none, yet. Exactly how the first stars formed millions of years later is a
mystery, but form they did. Their light filled the Universe; the Dark Ages came to an
end. Most importantly for this thesis, the new-born stars converted hydrogen into carbon
and oxygen and even heavier species, which was then ejected into space during the stars’
violent deaths. This enrichment of the interstellar medium, the matter between stars, with
more and more heavy atoms and also with small dust grains continued for the next billions
of years and is still continuing today. The organic chemistry observed here on earth, on
other bodies in our solar system, around new-born stars and in distant galaxies all has its
origin in these blasts when the Universe was only a little more than a billion years old.
A star is born
The study of stars seems to be almost as old as humanity itself. The study of the matter
between stars is only a century old. At visible wavelengths the densest parts of this interstellar matter appears as dark patches on the night sky, which blocks out the light of the
stars, much as atmospheric clouds block out light of the sun. They are aptly named dark
clouds and consist of gas and dust. These clouds are the birth places of new stars (Fig. 1.
Some of these dark clouds are dense enough to start to collapse under their own gravity.
Before collapse the clouds are cold (-263 centigrades) and rotate slowly. As the cloud
collapse it becomes hotter and starts to rotate faster to preserve the original energy and
angular momentum (the rotation of a body multiplied by the distance from its centre of
gravity) of the original cloud. Eventually the collapsing cloud is dense and hot enough for
a protostar to turn on. The young star is still embedded in some cloud material. This envelope of material eventually accretes onto the star or is blown away by the stellar radiation.
To preserve angular momentum without spinning up the star, large amounts of material
is carried away from the protostar by outflows of material, and by the creation of a disk
around the star. This disk of dust and gas is the birthplace of planets; dust particles in the
disks meet, stick together and form pebbles, boulders and eventually comets and planets.
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b)
c)
a)
1 pc
10 pc
f)
10000 AU
d)
1 AU
e)
200 AU
50 AU
Figure 1 – Star formation starts in interstellar clouds (b) where cloud cores collapse under their
own gravity to form a protostar (c). As the cloud disperses, the pre-main sequence star and an
accompanying disk is exposed (d). The disk material is incorporated into planets (e). When the star
dies, new material is thrown into the interstellar medium (f), eventually becomes incorporated into
interstellar clouds and the cycle starts over.
The chemical composition of this planet forming disk thus sets the stage for future prebiotic and biotic chemistry, the origin of life, on the planets themselves. Understanding
its chemistry requires an understanding of the chemical evolution preceding it.
Ice and gas around new-born stars
The interstellar clouds and planet forming disks consist of a mixture of gas and microscopic dust grains. The composition of both the gas and dust evolves during star formation, from the very exotic to the very familiar. In the early stages, before the cloud
collapse, most atoms and molecules such as molecular hydrogen, carbon atoms, oxygen
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a) t=0
b) t=10 5 yrs
i)
i)
ii)
iii)
silicate
ii)
iii)
d) t=107 yrs
c) t=106 yrs
Figure 2 – The proposed ice evolution during star and planet formation starting with simple ice
formation in dark clouds and cloud cores (a). Heat and UV from the protostar may result in a more
complex ice mixture (b.ii), which evaporates close to the protostar (b.iii). Some of this ice becomes
incorporated in the protoplanetary disk instead (c), and further into comets and planets (d).
atoms and carbon monoxide are in the gas phase. On earth gas phase chemistry is dominated by three-body collisions, where two of the species stick forming a new molecules
and the third species transports away the energy released from forming a chemical bond.
In space the atoms and molecules are few, even in the clouds, and thus the probability
of three of them colliding at once is very low. Even two is rare unless one of them is
charged, i.e. an ion, and can thus attract a reaction partner from a distance. The chemistry
therefore proceeds through mainly ion-molecule reactions where a neutral and charged
species bind together forming e.g. long carbon chains which are not found naturally in
our own atmosphere.
As the cloud becomes denser a different kind of chemistry takes over as more and
more of the gas phase species stick to the cold (-263 centigrades) grains, forming an
icy layer; at these low temperatures even carbon monoxide can exist as an ice. Atomic
and molecular hydrogen requires even lower temperatures to form ices, however, and
while it may spend some time on a grain surface it is highly mobile. This is the basis of
the new chemistry which consist of reactions between hydrogen and other species (and
itself) on the grain surfaces resulting in the production of such ‘common’ molecules as
water, from oxygen and hydrogen, and methanol form carbon monoxide and hydrogen.
When the protostar turns on these ices are dominated by water ice but also contains large
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amounts of carbon monoxide, carbon dioxide, and less methane, methanol and ammonia;
the building blocks of a complex organic chemistry are present on these grains.
These ices may further evolve into more complex species through the interaction with
ultraviolet light and with heat from the protostar forming for example ethanol, sugars and
amino acids. The ices that get too close to the protostar in the embedded stage or later
in the disk will evaporate, fuelling a new kind of gas phase chemistry based on reactions
between the evaporation products. The reminder of the ice stays with the dust grains and
become incorporated into comets and planets (Fig. 2). Pre- and proto-stellar ice chemistry
may thus hold the key to the chemistry in planetary systems.
This chemical evolution has partly been observed through the use of spectroscopy.
Spectroscopy builds on the wave-length specific absorption and emission pattern of all
molecules. By measuring the absorption of star light passing through a cloud or disk as a
function wavelength, the amount of a certain molecule in the environment can be derived.
This requires known absorption pattern or absorption spectra of the molecules in question,
which requires laboratory measurements. The laboratory is also the site for investigating
the chemical processes producing the molecules in the first place. In space the chemistry
seldom changes on time scales less than a hundred to a million years. Thus even the best
observations only provide snap-shots of the chemical evolution towards a certain young
star or a sample of stars. Laboratory simulations of the suggested chemical processes are
necessary to connect these snapshots into a chemical evolution from interstellar clouds to
planets.
Laboratory solid-state astrophysics
Laboratory solid-state astrophysics thus have two aims: to provide spectra of ices that
can be compared with observations of interstellar ices and to simulate chemical processes
predicted to be important during star and planet formation. Ice spectroscopy is typically
done at infrared wavelengths. The infrared spectra of an ice depends both on the kind of
ice, e.g. water or carbon monoxide, and its environment, that is, whether the ice is pure
or mixed with other species. Thus comparison between laboratory and interstellar spectra
reveals both what kind of ices are most common during star formation and whether these
ices have formed sequentially in pure layers or mixed together.
The second aim of laboratory solid-state astrophysics, to simulate chemical processes
that occurs in space and in particular in ices around young stars, requires a recreation of
the high vacuum and low temperatures in these environments. Even the dense cloud cores
have particle concentrations that are comparable to the best vacuums reached on earth.
This recreation is done in ultra-high vacuum chambers containing a surface that can be
cooled to -260 centigrades using liquid Helium. By observing the effects of heat or irradiation or both on an ice at the cold surface it is possible to predict how a similar ice will
react to heat or irradiation in space. These prediction can then be tested against observations. In general astrochemical observations thus inspire experiments, whose relevance in
a particular astrophysical setting is tested by new observations. This is also the storyline
of this thesis.
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This thesis: Complex processes in simple ices
Ices are present at all stages of star and planet formation, from interstellar clouds to comets
in our own solar system. Their evolution in the past set the stage for the chemistry we have
today on this planet. Their evolution is right now setting the stage for the chemistry in
extra-solar systems. Its investigation thus sets the stage for how life came in to being here
and how likely it is to arise elsewhere. The overarching aim of this thesis is to combine
observations and laboratory studies to constrain how ices form, how they change and
how they evaporate during star and planet formation. The overarching result is that while
even simple ices display a complex interaction with heat, UV irradiation and other ice
species, these complex processes can be quantified. Once quantified they can be included
into astrochemical models and tested against observations. With several new exciting
telescopes coming online in the next few years, such predictions are absolutely crucial to
constrain the evolution of ices from dark clouds to comets.
With the advent of the Spitzer Space Telescope a few years ago, it became for the
first possible to investigate ices around new born stars of a similar mass to our own sun
– before high-mass stars alone had been accessible. Analysis of a large sample of ices
towards protostars reveals that simple ices – from water and methane to methanol – form
sequentially during the dark cloud, cloud core and prestellar stages because of hydrogenation of atoms, direct freeze-out, hydrogenation of carbon monoxide, oxygen and nitrogen
additions to carbon monoxide and finally ice diffusion close to the protostar. The later
an ice forms in this sequence, the more its abundance tends to vary between different
sources. Methanol is one of the later species and the organic chemistry may therefore be
quite different in exo-planetary systems compared to our own (Chapters 2, 3).
The impact of heat and ice composition on dynamics in water-rich ices is the topic of
Chapters 4–6, which present laboratory studies on ice spectroscopy, segregation and desorption. Mixing in carbon dioxide with water ice severely disrupts the hydrogen-bonding
network, resulting in significantly different spectroscopic water-ice features compared to
pure ice.
When heating water-rich ice mixtures they first segregate and then evaporate. Segregation is a potential probe of protostellar ice heating during star formation and thus of
the physics of star formation. It also offers the best constraint of ice surface and bulk diffusion, which governs all ice chemistry and is currently poorly understood. Segregation
of ice mixtures with water and carbon monoxide and carbon dioxide proceeds through
first fats surface diffusion followed by slower bulk diffusion. The water-carbon dioxide
surface diffusion is much faster than previously assumed and becomes important already
at 30 K or -243 centigrades in protostellar envelopes. This is low enough to explain the
presence of the observed pure carbon dioxide ice towards protostars.
The second effect of heating, ice evaporation, drives much of the gas phase chemistry
during star formation. It is well known from laboratory studies that mixed ices evaporate
very differently from pure ices and trapping of volatile species in water-ice is thought to
be important both for the gas composition in disks and for the ice composition in comets,
though their is a general lack of quantification of this process. By systematically studying trapping of carbon monoxide and carbon dioxide in water ices we have developed a
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model that reproduces this process under a range of laboratory conditions and can also
be directly applied to astrophysical conditions, thus for the first time bridging the gap
between experiment and astrophysical modeling of evaporation of ice mixtures.
In addition to thermal evaporation, ices can also evaporate because of UV irradiation.
If efficient, this mechanism can maintain a fraction of the molecules formed on grains
in the gas phase at all times, even in the coldest stages of star formation. This may
both change the gas phase chemistry and provide sign posts of ice processes. Theory
predicted that UV evaporation is efficient following UV dissociation, but prohibitively
slow otherwise. Chapter 7 and 8 shows, however, that UV desorption is equally efficient
for carbon monoxide, which is not dissociated by the UV rays, as for carbon dioxide,
which is. Water and methanol ice are also efficiently UV evaporated (Chapter 9–10).
With all major ice species having such high evaporation efficiencies, UV evaporation can
explain previously puzzling observation of water, carbon monoxide and methanol gas in
cold and dense regions where all molecules, except for hydrogen, should exist as ice.
UV irradiation can also induce a complex chemistry in ices through dissociation
followed by chemical reactions. This is investigated quantitatively for the first time in
methanol ice and in different ammonia-rich ice mixtures (Chapters 10–11). Photochemistry in methanol ice is the proposed origin of most complex molecules around protostars
– gas phase observations has revealed a rich chemistry close to protostars where ices have
evaporated, including the production of organic acids, esters, ethers and sugars (Fig. 2).
These molecules probably form in the ices and then appear in the gas phase due to thermal
ice evaporation. The lack of quantitative laboratory data has so far prevented this hypothesis from being tested. Chapter 10 shows that methanol ice photochemistry both produces
all the commonly observed complex molecules and that their observed ratios towards a
range of objects can be explained by sequential formation of different species as the grains
heat up while falling in towards the protostar. The laboratory results are also consistent
with the relative abundances of complex molecules in comets and may thus also be the
answer to the pre-biotic chemistry during the formation of our own solar-system.
Chapter 11 follows up on these results by investigating similar processes in ammoniarich ices to probe the pre-biotically important nitrogen chemistry. The main result here
is that more complex ice formation, up to simple amino acids, can be predicted from
the simpler and more easily studied chemistry in ice mixtures with only two components
compared to the six major mixture components in space. This opens the door for predicting the pre-biotic ice evolution up to and beyond organic sugars and amino acids – the
molecules of life.
A major problem when constraining complex ice chemistry in space is that ice spectroscopy is only possible for the most abundant ice species. More complex ice products
can thus not be detected directly. Instead their production is inferred from gas phase
species, proposed to be evaporated ice products. Thermal ice evaporation implies, however, high enough temperatures that gas phase reactions become efficient, which were not
possible during the cold stages of star formation. The gas phase reactions will modify the
evaporated ices and thus gas phase observations are challenging to use as evidence for
ice formation pathways. UV evaporation and other kinds of non-thermal evaporation require no heat and thus observations of UV evaporation products may provide a gas phase
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finger print of the ice composition before gas phase reactions become efficient. This is
tested in a small pilot study in Chapter 12, where the gas and ice phase abundances of two
molecules are compared towards a sample of protostars. The study is consistent with a
constant release of ice into the gas phase through UV evaporation, but a larger sample is
required to show that this is the case towards all protostars.