Molecular clouds evolution

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 molecules found to date are composed of the elements H, C, N, O, Si, S, and Cl with the bulk of the molecules containing H, C, N, and O. The light elements H, D, and He are of cosmological origin and are therefore tracers of the early universe. On the other hand the heavier elements C, N, O,... are produced in stars by the processes of stellar nucleosynthesis. In addition to the most abundant isotopic forms many stable isotopes such as D, 13C, 170, lsO, 15N, 30Si, 33S, and 34S have been detected (see Appendix 1). The detailed determination of isotopic ratios — though often beset with formidable difficulties — has become a useful indicator of the chemical evolution of molecular clouds and the past chemical history of the galaxy. 

Allamandola and Hudgins have considered the formation of complex organic species in ice matrices and provided a summary of the photochemical evolution on those ices found in the densest regions of molecular clouds, the regions where stars and planetary systems are formed 42 Ultraviolet photolysis of these ices produces many new compounds, some of which have prebiotic possibilities. These compounds might have played a part in organic chemistry on early Earth. 

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 collapse of rotating molecular cloud cores leads to the formation of massive accretion disks that evolve to more tenuous protoplanetary disks. Disk evolution is driven by a combination of viscous evolution, grain coagulation, photoevaporation, and accretion to the star. The pace of disk evolution can vary substantially, but massive accretion disks are thought to be typical for stars with ages < 1 Myr and lower-mass protoplanetary disks with reduced or no accretion rates are usually 1-8 Myr old. Disks older than 10 Myr are almost exclusively non-accreting debris disks (see Figs. 1.3 and 1.5). 

This describes the final steps of stellar evolution of single stars. Since stars are formed in clusters, the initially unstable region in the molecular cloud has to fragment into many collapsing subunits. How this works is a yet unsolved problem. Additionally, a large fraction of stars are formed with at least one companion star this process is also not well understood. Both problems remain important challenges in our understanding of star formation (McKee Ostriker 2007). 

The Hj molecular ion plays the pivotal role in the ion-neutral reactions scheme now generally believed to be the major mechanism for the chemical evolution of dense molecular clouds. Hj is produced through cosmic ray ionization of H followed by the ion-neutral reaction (1). Since the latter reaction is extremely fast, the rate-determining process for the production of HJ is the cosmic ray ionization, whose rate is generally taken to be f a 10"g-i 63.68 main destruction mechanism of Hj is the proton-hop reaction (2). 

Further increases in sensitivity and resolution of observational spectrometers will lead us to the detection of Hj in interstellar space. If detected, the spectrum will give crucial information on the working of the ion-neutral reaction scheme for the chemical evolution of molecular clouds. 

Observations of star-forming regions have advanced our understanding of the star-formation process considerably in the last few decades. We now can study examples of nearly aU phases of the evolution of a dense molecular cloud core into a nearly fully formed star (i.e., the roughly solar-mass T Tauri stars). As a result, the theory of star formation is relatively mature, with fumre progress expected to center on defining the role played by binary and multiple stars and on refining observations of known phases of evolution. 

Dense molecular clouds, after further contraction, are the places where stars are born. The observation of protostars, stars still embedded in their placental cloud, is a probe of the presence of ices in the clouds the almost black-body continuum emitted from the young object is absorbed by grains whose temperature changes as a function of the distance from the object. These observations, which are mainly obtained by IR spectroscopy, may reveal the evolution of ices due to thermal and/or energetic (e.g. interaction with UV photons and/or stellar particle winds and cosmic rays) processing (e.g. Cox and Kessler [6]). 

Wave Astronomy Satellite (SWAS) show the abundance to be less than 0.1% that of CO. At this level, it does not interfere with organic synthesis. With elemental evolution, an increase of the 0/C ratio is expected. If this is not readily incorporated into the refractory solid phase, production of organic species in the interstellar molecular clouds could well be reduced. CO is an abundant molecule in strongly red shifted quasars (Downes and Solomon, 2003) (z = 2.6-6.4). Thus, it would be expected that its reaction products also are present, but harder to observe. SWAS is instrumented to measure O2 abundances and has not observed any. (O2, although lacking an electric dipole moment, has magnetic dipole transitions.) Note that it has probably not been observed in dense molecular clouds. 

Chemical Evolution within Dense, Dark, Molecular Clouds... 

Originally, the protoplanetary disk contains gas and dust with a composition similar to the parental molecular cloud. During the course of evolution, this material turns into larger bodies such as comets, asteroids, and planets. Because these disks are very opaque in their youngest phase, it is difficult to observe this process directly. However, the stellar radiation is absorbed by the gas and dust in the disk and heats the matter to typical temperatures of a few 1000 K in the inner disk regions to 10 K in the outer regions. 

Molecular clouds. HII and HI regions. Galactic centre. Nature and distribu-. tion of dust. Protostars. Stellar evolution. Development of HII regions. Multiband photometry (X/AX 1-20). XX 20 ym - 1 mm. High spatial resolution mapping. Polarimetry. 

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