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Query: UMLS:C0344329 (collapse)
 28,634 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

Stars are the basic building blocks of our universe, therefore it is one of the most important research topics in astrophysics to understand the origin and the early evolution of these objects. The current picture is that stars are formed during the collapse of a large enough self-gravitating interstellar molecular cloud. The early collapse gives birth to a fetus of a star, which is surrounded by a rotating accretion disk. The proto-star accretes interstellar matter through the disk which in turn transfer the accumulated matter to the central proto-star, then the star gets weight during the process. Observation shows that gorgeous ejection of matter always come along with the accretion process. In the presence of disks, these outflows usually escape from the system along the axis of the disk, forming so called bipolar outflows. Typical tracers of these activities are rich molecules such as CO, SiC2, C3H, C3H2 etc. Observationally, such typical molecular outflows can be detected using Doppler effect by spectroscopic measurements. Using the 13.7 m radio telescope in Delingha station of Purple Mountain Observatory, we performed a survey for 12 low temperature IRAS objects, some of the sources show high velocity properties. Detailed analysis of the Doppler profiles of IRS34 is presented. Star forming activities are clearly seen in this field.
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PMID:[Molecular spectral diagnosis of star forming regions]. 1294 60

Class 0 protostars, the youngest type of young stellar objects, show many signs of rapid development from their initial, spheroidal configurations, and therefore are studied intensively for details of the formation of protoplanetary disks within protostellar envelopes. At millimetre wavelengths, kinematic signatures of collapse have been observed in several such protostars, through observations of molecular lines that probe their outer envelopes. It has been suggested that one or more components of the proto-multiple system NGC 1333-IRAS 4 (refs 1, 2) may display signs of an embedded region that is warmer and denser than the bulk of the envelope. Here we report observations that reveal details of the core on Solar System dimensions. We detect in NGC 1333-IRAS 4B a rich emission spectrum of H2O, at wavelengths 20-37 microm, which indicates an origin in extremely dense, warm gas. We can model the emission as infall from a protostellar envelope onto the surface of a deeply embedded, dense disk, and therefore see the development of a protoplanetary disk. This is the only example of mid-infrared water emission from a sample of 30 class 0 objects, perhaps arising from a favourable orientation; alternatively, this may be an early and short-lived stage in the evolution of a protoplanetary disk.
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PMID:The development of a protoplanetary disk from its natal envelope. 1772 52

The age of dense interstellar cloud cores, where stars and planets form, is a crucial parameter in star formation and difficult to measure. Some models predict rapid collapse, whereas others predict timescales of more than one million years (ref. 3). One possible approach to determining the age is through chemical changes as cloud contraction occurs, in particular through indirect measurements of the ratio of the two spin isomers (ortho/para) of molecular hydrogen, H2, which decreases monotonically with age. This has been done for the dense cloud core L183, for which the deuterium fractionation of diazenylium (N2H(+)) was used as a chemical clock to infer that the core has contracted rapidly (on a timescale of less than 700,000 years). Among astronomically observable molecules, the spin isomers of the deuterated trihydrogen cation, ortho-H2D(+) and para-H2D(+), have the most direct chemical connections to H2 (refs 8, 9, 10, 11, 12) and their abundance ratio provides a chemical clock that is sensitive to greater cloud core ages. So far this ratio has not been determined because para-H2D(+) is very difficult to observe. The detection of its rotational ground-state line has only now become possible thanks to accurate measurements of its transition frequency in the laboratory, and recent progress in instrumentation technology. Here we report observations of ortho- and para-H2D(+) emission and absorption, respectively, from the dense cloud core hosting IRAS 16293-2422 A/B, a group of nascent solar-type stars (with ages of less than 100,000 years). Using the ortho/para ratio in conjunction with chemical models, we find that the dense core has been chemically processed for at least one million years. The apparent discrepancy with the earlier N2H(+) work arises because that chemical clock turns off sooner than the H2D(+) clock, but both results imply that star-forming dense cores have ages of about one million years, rather than 100,000 years.
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PMID:H2D(+) observations give an age of at least one million years for a cloud core forming Sun-like stars. 2540 40