The Synthesis of Organic and
Inorganic Compounds in the
Circumstellar Environment
I. Introduction
The carbon atom is the basic element in organic
molecules and therefore crucial to the origin of life.
Carbon in the universe is produced by thermal nuclear
reactions by asymptotic giant branch (AGB) stars during the very late stages of stellar evolution. In the past
30 years, advances in infrared and millimetre wave
spectroscopy have led to the detection of over 50 gasphase molecules in the stellar winds from AGB stars.
In addition to molecules, solid-state species, both amorFigure 1. Organic compounds
are routinely produced in planetary nebulae, a brief phase in
the late stages of stellar evolu-
phous and crystalline, are also found. The most common solid-state condensate is
amorphous silicates and silicon carbide (SiC). These solid particles are believed to
condense directly from gas-phase molecules, as the gas temperature cools with the
tion. This spectrum of the planetary nebula NGC 7027 was
expansion of the wind. These observations provide direct evidence that both mole-
obtained with the Infrared
cules and solids can be made in old stars over time scales as short as a few hundred
Space Observatory and it
shows a number of emission
years.
features (as marked in the legend) due to aromatic compounds.
II. Infrared spectroscopy from space
The launch of the Infrared Space Observatory (ISO) by the European Space
Agency allowed for spectroscopic observations in the mid-infrared, which are not
,
possible from ground-based telescopes due to absorption by the Earth s atmosphere.
ISO observations of planetary nebulae, descendents of AGB stars, show emission
features due to aromatic compounds. Figure 1 shows ISO spectrum
of the carbon-rich planetary nebula NGC 7027. Strong features at
3.3, 6.2, 7.7, 8.6, and 11.3 ɢm can be identified as stretching and
bending modes of aromatic C-H and C-C bonds. By observing
proto-planetary nebulae, transition objects between AGB stars and
planetary nebulae, we found that their ISO spectra show emission
features at 3.4 and 6.9 ɢm. We are able to identify these features as
arising from the stretching and bending modes of aliphatic side
groups attached to aromatic rings. These results show that complex
Fig. 2 The cotton candy nebula was discovered by
the author with the Hubble Space Telescope and
organic compounds can be synthesized in the environment of evolved
stars. Furthermore, by comparing the spectra of objects of different
is one of the proto-planetary nebulae showing the
mineral amorphous silicates in the spectrum.
6
ages, we are able to hypothesize on the chemical processes leading to
these compounds.
In addition to organics, inorganic minerals such as amorphous and crystalline silicates (Fig. 2), various refractory oxides
(corundum, spinel, rutile, etc.) are also detected. Furthermore, a
strong feature at 21ɢm was discovered by us in the spectra of
proto-planetary nebulae (Fig. 3a). This feature has no known laboratory counterpart and its origin is a mystery. The only clue we
have is that all stars showing the 21ɢm feature are carbon-rich, therefore suggesting
that the carrier of this feature is carbon-based (Fig. 3b).
Fig. 3a This picture of the
Water Lily Nebula was taken
by the author with the Hubble
Space Telescope. We found
this nebula to be a prolific pro-
III. Stardust in the solar system
Through stellar winds, these inorganic and organic compounds are spread over
ducer of aromatic compounds
as well as an unknown compound emitting at 21 microns.
the interstellar medium. Infrared spectroscopic observations of comets have identified the presence of amorphous and crystalline silicates, and the 3.4 ɢm aliphatic features that we have found in proto-planetary nebulae are also observed in comets and
interplanetary dust.
Analysis of meteoric material has identified grains with isotopic ratios suggesting that they are stellar in origin. This presents the clearest evidence that star dust can
travel through the interstellar medium without
being destroyed. Through these results, we suggest that the early solar system was chemically
enriched by molecular material synthesized in
evolved stars, with the possibility that exogenous
delivery may play a role in the origin of life.
At the Academia Sinica, scientists in the
Institute of Astronomy and Astrophysics and the
Institute of Atomic and Molecular Sciences are
working together to use laboratory spectroscopy to
seek the carrier of unidentified emission features in
proto-planetary nebulae. Through this work, we hope to tie the ingredients in mete-
Fig. 3b
orites to stellar material and give further confirmation to the stellar-solar system con-
responsible for the strong
nection.
A carbon-based
material of unknown nature is
emission
feature
at
21
microns seen in this spectrum
of proto-planetary nebulae
Sun Kwok
IRAS 07134+1005 obtained
Institute of Astronomy & Astrophysics, Academia Sinica
Observatory by the author.
with the Infrared Space
Nature 430, 985-991 (2004)
7