Formation from molecular clouds

In truth, star formation from molecular clouds is no easy subject to study. This is because the processes involved change the density from 10 g cm to about 1 g cm within a space of only a few tens of millions of years. Only the force of gravity, whose long range plays a key role, is able to produce such staggering compression rates. 

Kaiser, R.I. Balucani, N. Asvany, O. Lee, Y.T. Crossed molecular beam experiments of radical-neutral reactions relevant to the formation of hydrogen deficient molecules in extraterrestrial environments. In Astrochemistry from Molecular Clouds to Planetary Systems. Mihn, Y.C., van Dishoek, E.F., Eds., Astronomical Society of the Pacific - lAU Series, Volume 197, 2000, 251-264. 

Aside from molecular maser emission of OH, HpO and CH OH there exist several additional indicators of star formation in molecular clouds ... 

Giant molecular clouds the GMCs have a lifetime of order 106—10s years and are the regions of new star formation. The Orion nebula (Orion molecular cloud, OMC) is some 50 ly in diameter and 1500 ly from Earth. The temperature within the cloud is of order 10 K and the atomic density is 106 cm-3. The chemical composition is diverse and contains small diatomic molecules, large polyatomic molecules and dust particles covered with a thick ice mantle. 

The lifetime of the molecular cloud is considered to be a time line running from cloud formation, star evolution and finally dispersion in a period that is several tci. The chemistry of the TMC and, to a good approximation, all molecular clouds must then be propagated over a timescale of at most 20 million years. The model must then investigate the chemistry as a function of the age of the cloud, opening the possibility of early-time chemistry and hence species present in the cloud being diagnostic of the age of the cloud. The model should then expect to produce an estimated lifetime and the appropriate column densities for the known species in the cloud. For TMC-1 the species list and concentrations are shown in Table 5.4. 

The revelatory power of the new astronomy, especially astronomy associated with the extreme forms of radiation, resides in its capacity to expose previously unknown processes to reason and understanding gamma astronomy, the most violent phenomena in the Universe, such as the rupture and destruction of stars, and infrared astronomy, the gentle events, such as the birth of stars. Optical astronomy fills the relatively calm gap between stellar birth and death, whilst millimetre radioastronomy opens our minds to the formation of molecular structure in great clouds of cold gases and opaque dusts, far from any devastating light. 

In recent years, a new source of information about stellar nucleosynthesis and the history of the elements between their ejection from stars and their incorporation into the solar system has become available. This source is the tiny dust grains that condensed from gas ejected from stars at the end of their lives and that survived unaltered to be incorporated into solar system materials. These presolar grains (Fig. 5.1) originated before the solar system formed and were part of the raw materials for the Sun, the planets, and other solar-system objects. They survived the collapse of the Sun s parent molecular cloud and the formation of the accretion disk and were incorporated essentially unchanged into the parent bodies of the chondritic meteorites. They are found in the fine-grained matrix of the least metamorphosed chondrites and in interplanetary dust particles (IDPs), materials that were not processed by high-temperature events in the solar system. 

Stars form in dense cores within giant molecular clouds (see Fig. 1.4, Alves et al. 2001). About 1 % of their mass is in dust grains, produced in the final phases of stellar evolution. Molecular clouds are complex entities with extreme density variations, whose nature and scales are defined by turbulence. These transient environments provide dynamic reservoirs that thoroughly mix dust grains of diverse origins and composition before the violent star-formation process passes them on to young stars and planets. Remnants of this primitive dust from the Solar System formation exist as presolar grains in primitive chondritic meteorites and IDPs. 

The density and temperature distribution of interstellar matter, contrary to its elemental composition, is strongly inhomogeneous. At least three different phases exist (e.g. Tielens 2005) (i) extended low-density bubbles of hot ionized gas (hot interstellar medium or HIM, mass fraction 0.003, volume fraction 0.5), resulting from series of SN explosions in mass-rich stellar clusters (ii) cold and dense clouds of neutral gas (cold and neutral interstellar medium or CNM, mass fraction 0.3, volume fraction 0.01), resulting from sweeping up of warm gas and (iii) a warm, either ionized or neutral, medium in between (warm interstellar medium or WIM, mass fraction 0.5, volume fraction 0.5). The essential properties of the three phases are indicated in Fig. 2.4. The coolest and most massive of the clouds are the molecular clouds (MC, mass fraction 0.2, volume fraction 0.0005), a separate component, that are the places of star formation, where new stars are formed as stellar clusters with total masses between about 200 and several 106 M0. 

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