Other
Scientific paper
Jun 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994phdt.......273l&link_type=abstract
PhD Thesis, Laboratoire d'Astrophysique, Observatoire de Grenoble, France, 1994.
Other
Scientific paper
We have conducted a detailed study both numerical and analytical of Cometary Globules (CGs), related to their possible mechanism of formation. CGs are small dense clouds commonly found in the vicinity of O--B stars in HII regions; they consist of a dense head, surrounded by a bright rim, prolonged by a diffuse tail. Recent surveys have shown that CGs are active sites of star formation. One of the models advanced to explain the formation and the evolution of CGs is the ``Radiation-Driven Implosion''(RDI): the UV flux of the O--B association ionizes the external layers of the cloud. The ionised gas expands freely into the interstellar medium while an ionization front preceded by a shock propagates into the cloud. We have built a 2-D radiative hydrodynamical code based on the piece-wise linear method. The equations of radiative transfer are solved using the "on-the-spot" approximation. The equation of state P = P(ρ,x), where x is the ionised fraction per atom, couples the equations of hydrodynamics and radiation. Gravity is neglected. We have shown that photo-ionisation alone can account for the formation and evolution of CGs. For physical parameters typical of H II regions, RDI is a two-stage process: a brief collapse phase (~105 yrs, 10% of the cloud life) followed by a transient phase during which the cloud undergoes a series of radial expansions and re-compressions, leading to the commonly observed cometary phase. The collapse phase is characterised by a double kinematic emission component, the second component being associated with shocked gas. In the cometary phase, the globule is in a quasi-hydrostatic equilibrium and has no remarkable spectroscopic signature. This phase lasts a few 105 to 106 yrs. The results of numerical simulations were confirmed by a simple analytic model and extended to the case of a non-thermal support. It appears that small- and large-scale instabilities, Rayleigh-Taylor like, similar to the surface corrugations observed in CGs of the Gum Nebula, can develop in the cometary phase, eventually leading to the cloud disruption. We found that globules with realistic masses and supported by a non-thermal pressure are gravitationally stable. From simulations, we generated emissivity maps and position-velocity diagrams in order to allow a direct observational confrontation with our model. The maps of emission measure exhibit a striking resemblance to various types of CGs and other bright-rimmed structures found in H II regions. The globule CG7S in IC1848 was observed at the IRAM 30-m radio-telescope (in CO + isotopes and CS) in order to test the RDI model. This globule is remarkable in that a) it has not yet developped the common cometary structure, b) the main body of the cloud is accelerated by an overall velocity gradient of 3 km s-1 pc-1, c) a second blueshifted kinematic component is associated to some dense and brighter surface gas of the front side. The main features of CG7S are easily reproduced numerically, suggesting CG7S is a young "pre-cometary" object which has already collapsed once and is now re-expanding. From a small sample of clouds in the (quasi-equilibrium) cometary stage, some of which exhibiting signs of pre-stellar activity, we found that a) globules are supported against the ionised gas pressure by the pressure of a static magnetic field which seems to obey Heiles'law, b) there is no equipartition between the different terms of energy, c) they are gravitationally stable. Star formation is very likely to be triggered in the initial collapse phase but the way globules get rid of their magnetic flux is still unclear, and star formation remains a puzzle in Cometary Globules.
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