Proto-stellar objects

Astronomical observations of molecular clouds and young stellar objects provide the basis for our understanding of the early solar system (Cameron, 1995 Alexander et al., 2001). The first stage in this process is when a fragment of an interstellar molecular cloud collapses to form a disk-like nebula, or proto-planetary disk. This process normally takes... 

Abstract We discuss models that astrochemists have developed to study the chemical composition of the interstellar medium. These models aim at computing the evolution of the chemical composition of a mixture of gas and dust under astrophysical conditions. These conditions, as well as the geometry and the physical dynamics, have to be adapted to the objects being studied because different classes of objects have very different characteristics (temperatures, densities, UV radiation fields, geometry, history etc) e.g., proto-planetary disks do not have the same characteristics as proto-stellar envelopes. Chemical models are being improved continually thanks to comparisons with observations but also thanks to laboratory and theoretical work in which the individual processes are studied. 

Class I obj ects also have bipolar outflows, but they are less powerful and less well collimated than those of Class 0 objects. This stage lasts 100 000 to 200 000 years. Class //objects, also known as classical T Tauri stars, are pre-main-sequence stars with optically thick proto-planetary disks. They are no longer embedded in their parent cloud, and they are observed in optical and infrared wavelengths. They still exhibit bipolar outflows and strong stellar winds. This stage lasts from 1-10 million years. Class ///objects are the so-called weak line or naked T-Tauri stars. They have optically thin disks, perhaps debris disks in some cases, and there are no outflows or other evidence of accretion. They are observed in the visible and near infrared and have strong X-ray emission. These stars may have planets around them, although they cannot be observed. 

Molecules are frequently observed in the outer envelopes of red supergiants with surface temperatures less than about 5000 K. These stars have strong stellar winds, of order 10 to 10 M yr with velocities typically less than 50 km s . A few hotter stars, such as 89 Herculis and HD 161796, have also been found to show CO emission these and related stars are proto-planetary nebula objects in the process of becoming white dwarf stars. For some very highly evolved dusty stars, strong far-IR emission, indicative of dust, is accompanied by maser emission. These are the so-called OH/IR stars, which are most frequently Mira variables. Some evolved stars also show SiO masers. A few stars, such as the extreme supergiant IRC -t-10216, are veritable chemical factories, displaying almost all of the molecular species observed in comets and in dense interstellar molecular clouds. 

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