Bok Globules: Stability and Star Formation

Statistics – Methodology

Scientific paper

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Dark Clouds, Star Formation, Bok Globules

Scientific paper

In this Thesis, we study a collection of Southern Bok globules, with the purpose of analyzing the different evolutionary states and to determine the physical conditions that precede an eventual low-mass star formation. To do this, we selected 21 globules, where 11 of them are classified as starless and 10 are associated whith IRAS point sources. In a first stage of the analysis, we used the methodology of the extinction determined from infrared observations, that differently from the technique in the milimetre and sub-milimetre, does not depend on the dust temperature. For this, we used for the first time the 2MASS catalogue to map the extinction distribution in Bok globules, and by means of a stability analysis, we found effectively that the two groups of globules are in different evolutionary states. The starless globules would be associated to a pre-stellar stage, while the IRAS globules would correspond to a more evolved stage, in the direction to the star formation. The extinction technique allowed us to detect embedded dense cores, with absorption peaks invisible in optical images. In this sense, the technique based on 2MASS colors appears to be very sensible and useful for mapping small dark clouds. To study the internal structure of the globules, we pass through some stages, where we determined the distances to the globules, and constructed radial profiles of the embedded dense cores. The integration of these profiles led to the calculation of the masses of the globules. Then, these profiles were modeled with isothermal spheres in hydrostatic equilibrium (Bonnor-Ebert spheres). By solving the Lane-Endem equation numerically, we determined the stability parameter Xi_max and the central extinction for each globule. The most significant results of this work are related to the stability conditions in the two groups. The majority (87%) of the starless globules are found to be in a stable state against gravitational collapse. On the contrary, the majority (62%) of the IRAS globules are found to be in the instability region. To characterize the infrared sources associated with the IRAS globules, we assumed that these sources would correspond to different protostellar classes. For this, we constructed spectral energy distributions (SEDs) and calculated the spectral index Alpha, as well as the bolometric temperature for each source. Out of 13 sources, we found that 10 correspond to Class 0 and I protostars, and 3 to the Class II. The majority of the Class 0 and I sources are associated with gravitationally unstable dense cores, according to the Xi_max parameter, and the more evolved Classe II sources are associated with stable globules. Finally, by means of millimetric observations of 13CO and C18O molecules, we analyzed the globules BHR 138 and BHR 149. As opposed to symmetric lines in BHR 149, we found an assymetry in the case of BHR 138, that is compatible with gravitational collapse motions in the globule. This idea of a probable collapse would be supported by the gravitational instability of the core and by the presence of a Class 0 protostar. Using a simple collapse model (two-layer), it was possible to determine a collapse velocity of 0.25 km/s. However, observations of optically thin molecular lines would be necessary to discard other effects than the gravitational collapse.

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