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TOPIC: Bok globule Density Structure

Blobrana

L

Posts: 131433

Date: Oct 19 04:24 2011

RE: Bok globule Density Structure	Permalink

Title: Tracing the evolutionary stage of Bok globules: CCS and NH3 Authors: C. Marka (1), K. Schreyer (1), R. Launhardt (2), D. A. Semenov (2), Th. Henning (2) ((1) AIU Jena, (2) MPIA Heidelberg) We pursue the investigation of a previously proposed correlation between chemical properties and physical evolutionary stage of isolated low-mass star-forming regions. In the past, the NH3/CCS abundance ratio was suggested to be a potentially useful indicator for the evolutionary stage of cloud cores. We aim to study its applicability for isolated Bok globules. A sample of 42 Bok globules with and without signs of current star formation was searched for CCS(2-1) emission, the observations were complemented with NH3 measurements available in the literature and own observations. The abundance ratio of both molecules is discussed with respect to the evolutionary stage of the objects and in the context of chemical models. The NH3/CCS ratio could be assessed for 18 Bok globules and is found to be moderately high and roughly similar across all evolutionary stages from starless and prestellar cores towards internally heated cores harbouring protostars of Class 0, Class I or later. Bok globules with extremely high CCS abundance analogous to carbon-chain producing regions in dark cloud cores are not found. The observed range of NH3/CCS hints towards a relatively evolved chemical state of all observed Bok globules. Read more (583kb, PDF) __________________

Blobrana

L

Posts: 131433

Date: Apr 7 00:53 2010

Bok globules	Permalink

Title: Looking into the hearts of Bok globules: MM and submm continuum images of isolated star-forming cores Authors: R. Launhardt, D. Nutter, D. Ward-Thompson, T. L. Bourke, Th. Henning, T. Khanzadyan, M. Schmalzl, S. Wolf, R. Zylka We present the results of a comprehensive infrared, submillimetre, and millimetre continuum emission study of isolated low-mass star-forming cores in 32 Bok globules, with the aim to investigate the process of star formation in these regions. The submillimetre and millimetre dust continuum emission maps together with the spectral energy distributions are used to model and derive the physical properties of the star-forming cores, such as luminosities, sizes, masses, densities, etc. Comparisons with ground-based near-infrared and space-based mid and far-infrared images from Spitzer are used to reveal the stellar content of the Bok globules, association of embedded young stellar objects with the submm dust cores, and the evolutionary stages of the individual sources. Submm dust continuum emission was detected in 26 out of the 32 globule cores observed. For 18 globules with detected (sub)mm cores we derive evolutionary stages and physical parameters of the embedded sources. We identify nine starless cores, most of which are presumably prestellar, nine Class 0 protostars, and twelve Class I YSOs. Specific source properties like bolometric temperature, core size, and central densities are discussed as function of evolutionary stage. We find that at least two thirds (16 out of 24) of the star-forming globules studied here show evidence of forming multiple stars on scales between 1,000 and 50,000 AU. However, we also find that most of these small prototstar and star groups are comprised of sources with different evolutionary stages, suggesting a picture of slow and sequential star formation in isolated globules Read more (2143kb, PDF) __________________

Blobrana

L

Posts: 131433

Date: Jun 17 21:19 2005

Bok globule Density Structure	Permalink

Near Infrared Imaging Survey of Bok Globules: Density Structure Authors: Ryo Kandori, Yasushi Nakajima, Motohide Tamura, Ken'ichi Tatematsu, Yuri Aikawa, Takahiro Naoi, Koji Sugitani, Hidehiko Nakaya, Takahiro Nagayama, Tetsuya Nagata, Mikio Kurita, Daisuke Kato, Chie Nagashima, Shuji Sato On the basis of near-infrared imaging observations, researchers derived visual extinction (Av) distribution toward ten Bok globules through measurements of both the colour excess (E_{H-K}) and the stellar density at J, H, and Ks (star count). Radial column density profiles for each globule were analyzed with the Bonnor-Ebert sphere model. Using the data of our ten globules and four globules in the literature, the researchers investigated the stability of globules on the basis of xi_max, which characterizes the Bonnor-Ebert sphere as well as the stability of the equilibrium state against the gravitational collapse. They found that more than half of starless globules are located near the critical state (xi_max = 6.5 +/- 2). Thus, they suggest that a nearly critical Bonnor-Ebert sphere characterizes the typical density structure of starless globules. Remaining starless globules show clearly unstable states (xi_max less than 10). Since unstable equilibrium states are not long maintained, we expect that these globules are on the way to gravitational collapse or that they are stabilized by non-thermal support. It was also found that all the star-forming globules show unstable solutions of xi_max less than 10, which is consistent with the fact that they have started gravitational collapse. They investigated the evolution of a collapsing gas sphere whose initial condition is a nearly critical Bonnor-Ebert sphere, and found that the column density profiles of the collapsing sphere mimic those of the static Bonnor-Ebert spheres in unstable equilibrium. The collapsing gas sphere resembles marginally unstable Bonnor-Ebert spheres for a long time. The frequency distribution of xi_max for the observed starless globules is consistent with that from model calculations of the collapsing sphere. Read More (372kb) Bok globules, named after astronomer Bart Bok who studied them extensively, are small dark clouds made of gas and dust that are typically condensing to form a star or stars. These Bok globules are found in front of a glowing H II region known as IC 2499. __________________

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