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