Dense Interstellar Clouds
In our home galaxy, the Milky Way, as well as in external galaxies, the space between the stars is filled with an Interstellar Medium (ISM) consisting of gas and dust, like the beautiful Orion Nebula. While accounting for only a small fraction of the Galaxy's mass, the interstellar medium is nevertheless an important part of the Galactic ecosystem. The ISM is composed mainly of three types of clouds: dark clouds (Av > 5 mag), translucent clouds (1 mag < Av < 5 mag), and diffuse clouds (visual extinction, Av ~ 1 mag). Interstellar clouds are neither uniform nor dynamically quiescent on long timescales. They display "clumpy" structures, are continually evolving as new stars form, and are enriched by material ejected from dying stars that was formed during stellar nucleosynthesis.
Dense interstellar clouds are the birth sites of stars of all masses and their planetary systems. Interstellar molecules and dust become the building blocks for protostellar disks, from which planets, comets, asteroids, and other macroscopic bodies eventually form. Observations at infrared, radio, millimeter, and sub-millimeter frequencies show that a large variety of gas phase organic molecules are present in the dense interstellar medium. These include classes such as nitriles, aldehydes, alcohols, acids, ethers, ketones, amines, and amides, as well as many long-chain hydrocarbon compounds. Such species display large compositional variations between quiescent dark clouds and star-forming regions, as well as strong abundance gradients on small spatial scales within each type of cloud.
Radioastronomical molecular line surveys of well-known sources, such as the dense star-forming cores in the Orion and Sagittarius molecular clouds, show that molecules of considerable complexity can be found in these regions. Gas phase reactions can be important for forming some of the observed species but the presence of high abundances of saturated species, and the large deuterium fractionation ratios, indicates that catalysis on dust grains also contributes to the composition of these so-called hot molecular cores around protostars, theoretical studies indicate that a combination of gas and surface chemistries is necessary to explain many of their observed characteristics: evaporation of simple molecular mantles drives a complex gas phase chemistry. For example, surface-formed methanol, ethanol, and higher alcohols, can act as precursors of very large interstellar molecules through alkyl cation transfer reactions.
Diffuse clouds have moderate extinctions (< 1 mag) and densities
of roughly 100 - 300 per cm3. They are characterized by an average
temperature of ~ 100 K and a UV radiation field of approximately ~ 10E8
photons per cm2 s. Since the initial discovery of simple diatomic
molecules in interstellar space, CH, CN neutrals and CH ions, many more
molecules have been detected in the photon-dominated diffuse medium,
although at lower abundances than found in dense clouds. Species
detected include: HCO ions, CO, OH, C2, HCN, HNC, CN, CS and H2CO and
recently C2H and c-C3H2 molecules in several extragalactic diffuse
clouds. The C2H abundance varies little from diffuse to dense clouds
whereas the c-C3H2 abundance is markedly higher in dense clouds.
Reactions of the neutral molecules CH and CH2 molecules with C ions can
lead to the formation and build-up of polyatomic hydrocarbons. However,
the presence of large carbon-bearing species is strongly dependent on
their formation and survival rate because the diffuse medium is
controlled by photochemistry. The larger carbonaceous molecules that
enter the diffuse interstellar gas are detected in circumstellar
envelopes around late-type stars.
Fig. 1: ((c) C.R. O'Dell and S.K. Wong, Rice University and NASA)
Planetary systems now forming in Orion The Orion Nebula is a
star-forming region located in the constellation Orion, the Hunter,
about 1500 light years away. The optically visible nebula is excited by
one of the young massive stars that formed here about one million years
ago together with thousands of lower mass stars. Many of the low mass
stars are still surrounded by disks of placental cloud material of gas
and dust that formed during the protostellar collapse. Using the Hubble
Space Telescope, various of such protoplanetary disks have been
detected in silhouette against the nebular emission background. The
above mosaic shows several examples. In the bottom left insert the
relative size of our own solar system is shown for comparison. The
discovery of protoplanetary disks around other stars provides strong
evidence for the paradigm of solar system first proposed by Kant and
Laplace.
About 1 % per mass of the interstellar medium is
in the form of solid dust grains, which may be carbon or silicon-based.
Dust grains act as catalytic surfaces throughout the interstellar
medium, and in general show dimensions on the sub-micron scale. The
starlight is absorbed and scattered by dust grains and reaches the
observer dimmed, a process referred to as extinction. The
extinction-curve of the interstellar medium represents a superposition
of the wavelength-dependent extinction properties of different dust
particles. Dust particles in diffuse clouds and circumstellar envelopes
can be composed of silicates, amorphous carbon (AC), hydrogenated
amorphous carbon (HAC), diamonds, organic refractories, and
carbonaceous networks such as coal, soot, graphite,
quenched-carbonaceous condensates (QCC), and others. The dust size
distribution could be inferred from astronomical observations in the
UV, VIS, and IR. A three-component model of interstellar dust proposed
suggests the coexistence of big grains (silicates with refractory
mantles), very small grains (carbonaceous) and polycyclic aromatic
hydrocarbons (PAHs).
Dust grains form in the cool expanding circumstellar environment of evolved stars. Stellar winds inject dust into the ambient interstellar medium, which is then distributed by supernovae shock waves over large scales through the ISM. During this period, dust particles cycle several times through dense and diffuse clouds, which allows efficient mixing and processing of interstellar dust. UV irradiation and cosmic rays, together with processes such as grain-grain collisions, sputtering, and grain growth, alter and destroy dust in interstellar and circumstellar regions. Therefore, grains probably retain only traces of their origin, as evidenced by the isotopically anomalous composition of presolar grains found in meteorites. Recent observations suggest that the abundance of carbon in the interstellar medium is only two thirds of its solar value. This poses problems for many recent dust models, because only a limited amount of carbon is available for the dust phase.
Life, as we know it, is based on carbon. In the early Universe only light elements, such as H and He (and traces of other light elements) were formed. The formation of heavier elements had to await the formation of stars. Nucleosynthesis of heavy elements in stars, such as carbon, allowed the formation of organic molecules, which are currently widespread in our Galaxy and beyond. Biogenic elements (H, C, N, O, S, P) and organic matter are today some of the major constituents of the Universe.
The only known life in the Universe resides on a planet orbiting a G-type star. Stars like the Sun are born in dense molecular clouds, and these also provide the initial organic inventory available to protostellar disks for the formation of planets.
The interstellar medium, with its molecules and dust particles, represents the raw material for forming future generations of stars which may develop planetary systems like our own. The discoveries of protoplanetary disks around other stars show that our solar system is no longer the only known example of a planetary system in the Universe. This is supported by the detection so far of about 70 exoplanets circling other stars.
Fig. 2: The picture shows the Orion gas nebular which is also a close-by star-forming region.
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