arXiv:0807.0857v1 [astro-ph] 5 Jul 2008
Organic Matter in Space
Proceedings IAU Symposium No. 251, 2008
A.C. Editor, B.D. Editor & C.E. Editor, eds.
c ? 2008 International Astronomical Union
Organic Matter in Space - an Overview
Ewine F. van Dishoeck1,2
1Leiden Observatory, Leiden University
P.O. Box 9513, NL–2300 RA Leiden, the Netherlands
2Max-Planck Institute f¨ ur Extraterrestrische Physik, Garching, Germany
Abstract. Organic compounds are ubiquitous in space: they are found in diffuse clouds, in the
envelopes of evolved stars, in dense star-forming regions, in protoplanetary disks, in comets,
on the surfaces of minor planets, and in meteorites and interplanetary dust particles. This
brief overview summarizes the observational evidence for the types of organics found in these
regions, with emphasis on recent developments. The Stardust sample-return mission provides the
first opportunity to study primitive cometary material with sophisticated equipment on Earth.
Similarities and differences between the types of compounds in different regions are discussed in
the context of the processes that can modify them. The importance of laboratory astrophysics
Keywords. astrochemistry, molecular data, circumstellar matter, methods: laboratory, comets:
general, minor planets, ISM: abundances, ISM: molecules, infrared: ISM, stars: AGB and post-
Organic matter is defined as chemical compounds containing carbon-hydrogen bonds
of covalent character, i.e., with the carbon and hydrogen forming a true chemical bond.
Observations over the last century have established that these molecules are ubiquitous
throughout the universe, not only in our Galaxy (Kwok 2007b) but even out to high
redshifts (Yan et al. 2005). With the detection of more than 200 exoplanetary systems, a
major question is whether these organic compounds can be delivered in tact to new plan-
etary systems where they could form the basis for the origin of life. The answer requires
a good understanding of the entire lifecycle of organic molecules from their formation in
the outflows of evolved stars to the diffuse interstellar medium (ISM) and subsequently
through the star-forming clouds to protoplanetary disks. Ehrenfreund & Charnley (2000)
and Ehrenfreund et al. (2002) summarized our understanding of this cycle several years
ago, but since then new observations (e.g., with the Spitzer Space Telescope), labora-
tory experiments, and in-situ space missions (in particular the Stardust mission) have
occurred. Thus, a symposium such as this reviewing these recent developments is timely.
In this opening paper, a broad overview will be given where organic matter is found
in space and which species have been identified. Subsequently, observational evidence of
how and where these molecules are modified will be summarized. Finally, a number of
questions to address at the symposium and in the future are raised. The importance of
laboratory astrophysics in providing the basic data to interpret astronomical and solar-
system observations and analyse meteoretic and cometary samples is emphasized.
2. The need for laboratory astrophysics
Several decades ago, Henk van de Hulst accused cosmologists of ‘playing tennis with-
out a net’, when they were putting forward many models that could not be tested by
any observations. Similarly, much of astrochemistry (and, in fact, much of astronomy as
a whole) would be ‘playing tennis without a net’ if there were no laboratory data avail-
able to analyse and interpret the observational data of astronomical sources. The list of
required data for organic compunds is extensive, and getting such information for even
a single molecule often involves the building up of sophisticated laboratory equipment
followed by years of painstaking data taking.
The most basic required information is spectroscopy of organic molecules from UV
to millimeter wavelengths to identify the sharp lines and broad bands observed toward
astronomical sources. One recent development in this area is the use of cavity ringdown
spectroscopy to increase the sensitivity compared with classic absorption spectroscopy
by orders of magnitude, which has allowed measurements of rare species that can be
produced only in small amounts. Examples include gaseous polycyclic aromatic hydro-
carbons (PAH) (e.g., Tan & Salama 2005, Rouille et al. 2007) or carbon chains (e.g.,
Dzhonson et al. 2007, Linnartz et al. 2000), in addition to matrix-isolation studies of
large samples of PAHs (e.g., Hudgins & Allamandola 1999). Spectroscopy data bases of
solids, including silicates (e.g., Jaeger et al. 1998, 2003), carbonates (e.g., Posch et al. 2007),
ices (e.g., Bisschop et al. 2007c, Bernstein et al. 2005) and carbonaceous material (e.g.,
Mennella et al. 1997, Jaeger et al. 2006, Mu˜ noz-Caro et al. 2006) continue to grow.
The next step in understanding organics is to obtain rates for the various reactions
that are expected to form and destroy organics under space conditions. Here recent de-
velopments include measurements and theory of gaseous neutral-neutral rate coefficients
at low temperatures (e.g., Chastaing et al. 2001, Smith et al. 2006), branching ratios for
dissociative recombination (e.g., Geppert et al. 2004), and rates for photodissociation of
molecules exposed to different radiation fields (van Hemert & van Dishoeck 2008). Sur-
face science techniques at ultra high-vacuum conditions are now being applied to study
thermal- and photo-desorption (e.g., Collings et al. 2004,¨Oberg et al. 2007) and forma-
tion of simple organic ices at temperatures down to 10 K (e.g., Watanabe et al. 2004,
Bisschop et al. 2007a), while more traditional set-ups continue to provide useful informa-
tion on the formation of complex organics in ices exposed to UV (e.g., Elsila et al. 2007,
Mu˜ noz-Caro & Schutte 2003) and to higher energy particle bombardment (e.g., Hudson & Moore 2000).
There is also a wealth of new literature on the formation of carbonaceous material in dis-
charges (e.g., Imanaka et al. 2004) and its processing at higher temperatures and when
exposed to UV (e.g., Dartois et al. 2007).
Finally, the techniques to analyze meteoritic and cometary material in the laboratory
have improved enormously in the last decade, and now allow studies of samples on submi-
crometer scale. Examples include ultra-L2MS and nano-SIMS (e.g., Messenger et al. 2007),
XANES (e.g., Flynn et al. 2006) and NMR (e.g., Cody et al. 2005). Their development
was essential for analysis of the samples returned by Stardust.
3. Which organic compounds are found where?
In the following sections, the observational evidence for organic material is summa-
rized, together with the identification of the type of material, where possible (Fig. 1).
For small gas-phase molecules, the identification is unambiguous, but for larger com-
pounds often only the types of carbon bonds making up the material can be specified.
Carbon can be bonded in several ways: a triple CC bond with single H on the side (e.g.,
HC≡CH, denoted as sp hybridization); a double CC bond with two H’s on each side
(e.g, H2C=CH2, denoted as sp2hybridization) and a single CC bond with three H’s
on each side (e.g., H3C-CH3, denoted as sp3hybridization). In aromatic material, the
electrons are delocalized over the entire molecule such as in a benzene ring or a polyethy-
Figure 1. Examples of different types of carbonaceous material which are likely present in the
ISM and solar system (from: Ehrenfreund & Charnley 2000)
lene chain with alternating double and single CC bonds. The molecule thus contains sp2
bonds. In aliphatic material, no double bonds occur, and only sp3hybridization is found.
Spectral signatures of organic compounds include strong electronic transitions at optical
and UV wavelengths, vibrational transitions at infrared wavelengths, and pure rotational
transitions at millimeter wavelengths. In the latter case, the molecule needs to have a
permanent dipole moment to be detected.
The majority of carbon in interstellar clouds (at least 50%) is in some form of carbona-
ceous solids with grain sizes large enough (∼0.1 µm) not to have any clear spectroscopic
signature, other than continuous opacity. Another fraction of the carbon (up to 30%)
can be in gaseous C, C+and/or CO, or in CO and CO2ices. Most of the discussion in
this paper concerns the remaining ∼20% of carbon present in carbonaceous molecules,
ices and small grains.
3.1. Diffuse and translucent clouds
Diffuse and translucent molecular clouds are concentrations of the interstellar gas with
extinctions up to a few mag (for review, see Snow & McCall 2006). Typical temperatures
range from 15–80 K and densities from a few 100–1000 cm−3. These are the only types
of clouds for which high quality optical and UV spectra can be obtained by measuring
the electronic transitions in absorption against bright background stars. In addition to
the simplest organics CH, CH+and CN detected in 1937–1941, a series of more diffuse
features called the ’diffuse interstellar bands’ (DIBs) has been known since 1922. Nearly
300 DIBs are now known in the 4000–10000˚ A range (e.g., Hobbs et al. 2008), but not
a single one has yet been firmly identified despite numerous suggestions by the world’s
leading spectroscopists. Two bands at 9577 and 9632˚ A are consistent with features of
first fullerene in interstellar space (Foing & Ehrenfreund 1997). Long carbon chains with
n < 10 and small PAHs have been excluded (Maier et al. 2004, Ruiterkamp et al. 2005),
but larger versions are possible. The amount of carbon locked up in the DIB carriers is
likely small, < 1%, assuming typical oscillator strengths for large molecules.
Measurements of the UV extinction curve show a bump at 2175˚ A, characteristic of the
π → π∗transition in carbonaceous material. The precise identification is still uncertain,
60, but laboratory spectroscopy of gaseous C+
60is needed for firm identifcation of the
122 van Dishoeck
Figure 2. Spitzer and ground-based IR spectrum toward the galaxy IRAS 08572+3915 com-
pared with a laboratory spectrum of amorphous carbon a-C:H. The spectrum in the 5–8 µm
range excludes large amounts of O- and N- containing groups. It suggests a larger ratio of
aliphatic over aromatic content than thought before (Dartois et al. 2007)
with both graphitic and hydrogenated amorphous carbon (HAC, a material consisting of
islands of aromatic C joined by a variety of peripheral sp2- and sp3-bonded hydrocarbons)
leading candidates (e.g., Draine 2003). A major puzzle is why no DIB features have yet
been seen at UV wavelengths, since small carbon-bearing molecules should have strong
electronic transitions in this region.
At IR wavelengths, the characteristic ‘unidentified infrared bands’ (UIR) bands are
seen in emission throughout the diffuse ISM, even in clouds exposed to the normal inter-
stellar radiation field (see review by Tielens 2008). The commonly accepted identification
is with PAHs of sizes small enough (NC≈50) to be excited by UV and emit in discrete
bands. It is clear that the fraction of O and N in PAHs is very low, but a small amount
of N (few % w.r.t. carbon) has been proposed to explain the small shift of the interstel-
lar 6.2 µm feature compared with laboratory data of pure PAHs (Hudgins et al. 2005).
Alternative explanations include the presence of small side chains (Sloan et al. 2005).
The fraction of carbon locked up in small PAHs is estimated to be ∼4%. Large PAHs
or PAH clusters may be responsible for the plateaus underlying the discrete PAH bands,
containing of order 2% of the carbon, whereas the Very Small Grains (VSGs) responsible
for the 12 µm IRAS emission contain a similar amount (Tielens 2008).
In lines of sight which sample large columns of diffuse gas (e.g., toward the Galactic cen-
ter, external galaxies), a feature at 3.4 µm has been seen (see review by Pendleton 2004).
This wavelength is characteristic of the stretching modes of -CH2 and -CH3 groups in
aliphatic material (Duley & Williams 1983). Many materials have been proposed, rang-
ing from HAC to Quenched Carbonaceous Composites (QCC), Kerogen (a coal-like ma-
terial consisting of arrays of aromatic carbon sites, aliphatic chains and linear chains of
benzenic rings) and photoprocessed carbon-containing ices. New constraints come from
Spitzer observations of the 5–8 µm region (Dartois et al. 2007) (Fig. 2). The absence
of strong CO and CN bands points again to little N and O, but the -CH2 and -CH3
bending modes are clearly seen and can be well fitted by a HAC or a-C:H (amorphous
carbon) material, which typically contains about 15% of the available carbon. A possible
representation of the carrier of the 3.4 µm feature is presented in Fig. 3.
Finally, small carbonaceous molecules like C3H2 have been detected in absorption
Figure 3. Proposed structure of the carbonaceous interstellar dust in the diffuse ISM by
Pendleton & Allamandola (2002). The structure is a kerogen-type aromatic network bridged
by aliphatic chains, including side groups and hetero-atoms. A typical 0.1 µm carbonaceous
grain would contain ∼ 104of these fragments. One of the challenges for this meeting is to de-
termine whether this picture is still valid, or whether, for example, more aliphatic chains have
to be added.
at mm wavelengths toward distant quasars whose line of sight passes through a galac-
tic molecular cloud (Lucas & Liszt 2000). These molecules are also seen in emission in
Photon-Dominated Regions (PDRs) with abundances that are much higher than expected
from standard ion-molecule reactions. One possible explanation is that they result from
photodissociation of larger carbonaceous molecules such as PAHs (Pety et al. 2005).
3.2. Evolved stars
The spectra of carbon-rich (proto-)planetary nebulae (PPN) are full of IR emission fea-
tures, with both the aromatic PAH and the aliphatic 3.4 µm bands commonly observed in
emission (e.g., Goto et al. 2007). Interestingly, the aromatics are stronger and aliphatics
weaker in planetary nebulae (PN) compared with earlier evolutionary stages. Also, they
appear to be larger than in the diffuse ISM, with NC ≈ 100 − 200. In the AGB phase
leading up to the extreme carbon stars, C2H2absorption at 13.7 µm (the building block
to make benzene) and emission features at 21 and 30 µm are seen, with the latter two
still unidentified (see overview by Kwok 2007a).
A particularly interesting object is the Red Rectangle, a protoplanetary nebula which
shows prominent Extended Red Emission (ERE), both as a continuum at 5000–7000
˚ A and in discrete bands (van Winckel et al. 2002). Some of the latter are close to the
positions of strong DIBs seen in diffuse clouds. The positions and shapes of both the
IR and optical bands may vary with distance from the star, suggesting a change in the
composition of the material with UV dose (Song et al. 2003).
3.3. Dense star-forming regions
Cold dense cores are the realm of the long unsaturated carbon chains such as HC9N,
discovered in the 1970’s. Recent developments include the identification of negative ions
such as C6H−and C8H−(McCarthy et al. 2006) as well as more saturated chains such as
124 van Dishoeck
CH2CHCH3(Marcelino et al. 2007). Taken together, these chains make up only a small
fraction, <0.1%, of the total carbon budget.
Saturated complex organic molecules such as CH3OH, CH3OCH3 and C2H5CN are
commonly seen in high abundances toward warm star-forming regions such as Orion
and SgrB2, which have been surveyed at (sub)mm wavelengths for more than 30 years
(e.g., Schilke et al. 2001). Such ‘hot cores’ have been detected around most massive pro-
tostars and are now commonly used as a signpost of the earliest stages of star for-
mation. One recent development is that they are also found around low-mass proto-
stars, with IRAS 16293–2422 as the prototypical example (Cazaux et al. 2003). Abun-
dance ratios from source to source are remarkably constant (e.g., Bisschop et al. 2007b,
Bottinelli et al. 2007) pointing to an origin in grain surface chemistry, although some vari-
ations between low- and high-mass sources are found. Also, a clear segregation of oxygen-
and nitrogen-rich organics is seen (e.g., Wyrowski et al. 1999). One of the major ques-
tions is whether all observed complex organics are produced in the ice or whether some
of them are formed in the hot gas following evaporation of ices (Charnley et al. 1992).
Each organic molecule has an abundance of typically 10−9− 10−7with respect to H2,
but the total fraction of carbon locked up in these complex molecules can amount to a
More complex organic molecules such as amino acids and bases, which are relevant for
pre-biotic material, have not yet firmly been identified. Indeed, the spectra of hot cores
are so crowded that line confusion is a serious issue. Ethylene glycol, CH2OHCH2OH, a
complex organic found in comets, has been claimed in SgrB2 (Hollis et al. 2002), but its
detection is not yet fully secure.
The largest reservoir of volatile carbonaceous material is in the ices, whose strong mid-
infrared absorption bands are seen not only toward most massive protostars (Gibb et al. 2004)
but also toward a wide variety of low-mass YSOs (e.g., Boogert et al. 2008). Besides H2O
ice, CO, CO2, OCN−, CH4, HCOOH, CH3OH and NH3 ice have been identified. The
recent surveys of low-mass YSOs show that some molecules like CH4have relatively con-
stant abundances of ∼5% with respect to H2O ice (¨Oberg et al. 2008), whereas those of
CH3OH vary from < 1 to 25% (see Bottinelli et al., this volume). Altogether, the known
organic molecules (excluding CO and CO2) may lock up to 10% of the available carbon.
Most significant is the absence of PAH and 3.4 µm emission or absorption in the cold
cores and deeply embedded stages of star formation. Indeed, a recent Spitzer and VLT
survey of low-mass embedded YSOs shows no detections, indicating PAH abundances
that are at least a factor of 10 lower than in the diffuse gas, perhaps due to freeze-out
(Geers et al. 2008). No absorptions due to PAHs in ices have yet been found, but the
lack of basic spectroscopy prevents quantitative limits.
3.4. Protoplanetary disks
Once the collapsing cloud has been dissipated, a young star emerges which can be seen
at visible wavelengths but is still surrounded by a protoplanetary disk. PAH emission
has been detected in roughly half of the disks surrounding Herbig Ae stars, i.e., in-
termediate mass young stars (Acke & van den Ancker 2004). More recently, PAHs have
also been seen in a small fraction (∼ 10%) of disks around solar mass T Tau stars
(Geers et al. 2006). A quantitative analysis of the emission indicates PAH abundances
that are typically factors of 10–100 lower than in the diffuse ISM, either due to freeze-out
or caused by coagulation. The spatial extent of the PAH emission measured with adap-
tive optics on 8m class telescopes is of order 100 AU, i.e., comparable with the size of the
disk, but varying with feature (Habart et al. 2004). Modeling of the spatial extent as well
Figure 4. Left: False color HST image (∼ 4′′× 4′′) of the HR 4796A disk. Blue corresponds to
0.58 µm, green to 1.10 µm and red to 1.71 µm. Right: Disk / stellar flux ratio as function of
wavelength. For comparison, grain models for candidate materials with a n−3.5size distribution
with amin=3 µm and amax =1000 µm are shown, normalized to the 1.10 µm data for HR 4796A
and offset for clarity (from: Debes et al. 2008).
as the destruction of PAHs by the intense UV or X-ray emission from the star indicates
that the PAHs must be large, NC≈ 100 (Geers et al. 2007a, Visser et al. 2007).
Smaller organics are present in high abundances in the inner disk (< 10 AU). Indeed,
hot (400-700 K) C2H2and HCN have been detected in absorption in edge-on disks with
abundances factors of 1000 larger than in cold clouds (Lahuis et al. 2006). Recently,
they have also been seen in emission (Carr & Najita 2008). The observed abundances
are consistent with models of hot dense gas close to LTE (e.g., Markwick et al. 2002).
A particularly intriguing class of disks is formed by the so-called transitional or ‘cold’
disks with large inner holes. An example is Oph IRS 48, in which a large (60 AU radius)
hole is revealed in the large grain 19 µm image. Interestingly, PAHs are present inside
the hole, indicating a clear separation of small and large grains in planet-forming zones
(Geers et al. 2007b). Another intriguing case is formed by the more evolved disk around
HR 4796A, which has likely lost most of its gas and is on its way to the debris-disk stage.
Recent HST imaging shows colors with a steep red slope at 0.5–1.6 µm and subsequent
flattening off (Debes et al. 2008) (Fig. 4). These colors are reminiscent of those of minor
planets in our solar system, such as the Centaur Pholus, where the data are best fitted
with tholins, i.e., complex organics produced in the laboratory in a CH4/N2 discharge
with characteristics similar to Titan’s haze (e.g., Cruikshank et al. 2005).
3.5. Comets and minor planets
Many volatile organics have been detected in bright comets like Hale-Bopp thanks to
improved sensitivity at IR and mm wavelengths (see reviews by Bockel´ ee-Morvan et al.
2000, 2006). Most of them are parent species evaporating directly from the ices. The
list includes HCN, C2H2, C2H6, CH3OH, .... , all of which except C2H6have also been
detected in star forming regions in the ice or gas. Typical abundances are 0.1–few %
with respect to H2O ice. A larger variety of comets originating from both the Oort cloud
and Kuiper Belt have now been sampled, and variations in abundances between comets
are emerging, with organics like CH3OH and C2H2depleted by a factor of 3 or more in
some comets (e.g., Kobayashi et al. 2007). PAHs have not yet been firmly identified by
ground-based 3.3 µm spectra.
Fly-bys through the comae of Comets Halley, Borelly and Wild-2 have provided a
much closer look at cometary material, including in-situ mass spectrometry of the gases.
A major discovery of the Giotto mission to Halley was the detection of the so-called
Figure 5. µltra-L2MS analysis of one of the Stardust samples, showing the presence of small
PAHs (from: Clemett et al. 2007).
CHON particles: complex, mostly unsaturated, organics with only a small fraction of O
and N atoms (Kissel & Krueger 1987).
A major question is whether the evaporating gases are representative pristine material
unchanged since the comets were formed more than 4 billion yr ago, or whether they have
been changed by ‘weathering’ (e.g., high-energy particle impact) during their long stay
in the outer solar system. The Deep Impact mission to Comet Tempel 1 was specifically
designed to address this question, by liberating pristine ices from deep inside the comet
following impact (A’Hearn et al. 2005). A sigificant increase in the IR emission around
3.5 µm, characteristic of CH3CN and CH-X bands was seen immediate after impact, but
no strong PAH bands were evident.
The Stardust mission has taken a major step forward in the study of primitive solar
system material by returning samples from Comet Wild-2 back to Earth, where they can
be subjected to in-depth laboratory analysis using the most sophisticated experimental
techniques (Brownlee et al. 2006). Wild-2 is a less evolved comet than others, having
spent most of its lifetime in the Kuiper Belt and being captured into its currrent orbit
only 30 years ago. Thus, it should not have suffered much thermal heating close to the
Sun. Many complex organic molecules are found in the analysis of the Stardust particles to
date, with a heterogeneous distribution in abundance and composition between particles.
Many of the organics are PAHs, with typical sizes of just a few rings, i.e., generally smaller
than the PAH size inferred in the ISM (Sandford et al. 2006, Clemett et al. 2007) (Fig.
5). Also, a new class of aromatic poor organic material is found compared with those
seen in IDPs and meteorites, perhaps related to the fact that Wild-2 has had less thermal
processing. The material appears richer in O and N than meteoritic organics. A major
challenge for future studies will be to quantify the organics produced by the particle
impacts inside the aerogel and isolate those from true cometary material.
Evidence for the presence of organics on other minor planets comes from their red
colors at optical and near-IR wavelengths as seen in reflected sunlight. A particularly
well studied case is the surface of the Centaur object Pholus (Cruikshank et al. 2005).
Since discrete spectral features are lacking, identification of the material is not unique,
but energy deposition in gas and ice mixtures containing CH4and N2produces tholins
with colors similar to those observed (Imanaka et al. 2004).
Titan is particularly interesting because its atmosphere is thought to be similar to
that of our (primitive) Earth, with the main difference being that it consists mostly of
N2and CH4rather than N2and CO2. The Cassini mission has studied Titan’s haze in
detail and the descent of the Huygens probe through the atmosphere has indeed revealed
many nitrogen-rich organics (Niemann et al. 2005). Methane in the atmosphere must be
continuously replenished by cryo-volcanism or other processes, since its lifetime due to
photochemistry is short.
Besides tholins, HCN polymers have also been speculated to be part of the dark compo-
nent present on outer solar system bodies, including comets (Matthews & Minard 2006).
It can also contribute to the orange haze in the stratosphere of Titan. Overall, it is clear
that organics are a widespread component of solar system material.
3.6. Meteorites and IDPs
The most primitive and least processed meteorites —the so-called carbonaceous chon-
drites — contain ample organic material. Well known examples are the Murchison,
Orgueil and Tagish Lake meteorites, which contain up to 3% by weight in carbon-rich
material. Most of the organics (60-80%) are in an insoluable macromolecular form, often
described as ‘kerogen-like’. The remaining 20% are in soluable form and have been found
to contain corboxylic acids, PAHs, fullerenes, purines, amides, amides and other pre-
biotic molecules (e.g., Cronin & Chang 1993, Botta & Bada 2002). Amino acids –more
than 80 different types– have also been found, but are likely formed from reactions of
liquid water with HCN and H2CO under the high pressure in the parent body rather
than being primitive solar system material.
Interplanetary dust particles (IDPs) have been collected through stratospheric flights
over the past decade and analyzed in detail in the laboratory. Organic carbon, in-
cluding aliphatic hydrocarbons and the carrier of the 2175˚ A feature, are common
(Flynn et al. 2000, Bradley et al. 2005).
4. Evolution of organic matter
As organic material evolves from the evolved stars to the diffuse and dense ISM, and
subsequently from collapsing envelopes to disks, icy solar sytem bodies and meteorites,
many processescan affect their composition and abundance (Ehrenfreund & Sephton 2006).
From the AGB and PPN phase to the PN phase, UV processing changes aliphatics to
aromatics material (Kwok 2007a). In the subsequent step, the organics can be shat-
tered by shocks as they enter the diffuse ISM and are exposed to passing shocks from
supernovae and winds (Jones et al. 1996). Destruction of graphite produces very small
carbonaceous grains, including presumably the smaller PAHs. When the organics enter
the dense cloud phase, freeze-out will affect all organics, coagulation can occur, and the
changing balance between UV dissociation and re-hydrogenation can modify the aliphatic
to aromatic abundance ratio (e.g., Mu˜ noz-Caro et al. 2001). Also, volatile complex or-
ganics are formed on the grains as ices.
Once the molecules enter the inner part of the collapsing envelope, ices will evaporate
and some of the material is transported into disks, either as ice or gas. UV radiation and
heating can further process the material before it becomes incorporated into cometary or
planetary material. Here weathering and processing over the last 4.5 billion yr can change
the top layers of the parent body, whereas aqueous alteration and thermal processing can
further change the composition of organics in meteorites. Given all this potential pro-
cessing, it is natural to expect the organics in the different sources to vary substantially.
One of the strongest pieces of evidence that some organic material may remain unaltered
through this entire cycle comes from the similarity of the 3.4 µm feature in the diffuse ISM
and in meteorites (Pendleton 2004). Also, the mere presence of PAHs in (proto)planetary
nebulae, the diffuse ISM, comets and meteorites suggests that these molecules are not
fully destroyed during the lifecycle from evolved stars to solar systems.
Other intriguing piece of the puzzle is provided by the similarity in the composition
and abundances of ices in protostellar regions and comets: is this just a coincidence or
are the original ices preserved as they enter the comet-forming regions? The answer likely
depends on the volatility of the species. Also, the similar red colors found in at least one
protoplanetary disk and those seen on minor planets hint at preservation of the more
refractory organics in the transport through disks. Finally, the presence of the 2175˚ A
bump in IDPs and in the ISM suggests a common carbonaceous carrier.
There is also abundant evidence that not all organics seen in the various sources are the
same. The PAH and 3.4 µm aliphatic features are not seen in dense clouds or protostellar
sources, with abundances of the carriers inferred to be lower by factors of 10–100, most
likely due to freeze-out. Also, the sizes of PAHs in meteorites are smaller than those
in the ISM and disks, which in turn are smaller than those found in (proto-)planetary
nebulae. Aliphatic material is transformed into aromatics with increasing UV dose in
evolved stars. Thus, most of the PAHs seen in protoplanetary disks are not the same as
those seen around evolved stars.
In terms of volatiles, some large organics are clearly much more abundant in comets
than in interstellar gases or ices, with ethylene glycol a good example. Organics in comets
are also different compared with IDPs and meteorites, both in terms of PAHs and other
species. This suggests processing during planetary formation or during the journey of the
meteorites and IDPs to Earth.
5. Some open questions
The above summary raises many questions that shoud be addressed in this meeting and
future studies. How is carbonaceous material formed in the envelopes of evolved stars and
how does the composition depend on stellar parameters? What is the main form of solid
carbon? How important is UV processing in modifying organic matter, or even producing
it (e.g., from UV processing of water-poor ices)? How relevant are the volatile complex
organics found in star-forming regions to the origin of life? Do they survive the transport
from clouds to disks to planetary systems? What is the link between interstellar and
solar system refractory macromolecular carbon? Where do the CHON particles found in
comets fit in? How are the interstellar PAHs modified in disks, and where is the ‘soot’
line in relation to the terrestrial planet-forming zones?
The future is bright thanks to upcoming new observational facilities such as ALMA,
further development of infrared interferometers, HST-COS, JWST and ELTs. Comple-
mentary laboratory work will be even more essential to make progress.
The author or grateful to S. Kwok for the invitation and hospitality. This work is
supported by a Spinoza grant from NWO.
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