EDP Sciences
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Volume 397, Number 1, January I 2003
Page(s) 181 - 189
Section Formation, structure and evolution of stars
DOI http://dx.doi.org/10.1051/0004-6361:20021103

A&A 397, 181-189 (2003)
DOI: 10.1051/0004-6361:20021103

Multitransitional observations of the CS core of L673

O. Morata, J. M. Girart and R. Estalella

Departament d'Astronomia i Meteorologia, Universitat de Barcelona, Av. Dia go nal, 647, 08028, Barcelona, Spain
(Received 7 January 2002 / Accepted 29 July 2002 )

A multitransitional study with the BIMA interferometric array was carried out toward the starless core found in the L673 region, in order to study the small-size structure of the cores detected with previous single-dish observations, which provides us with a test of the predictions of the chemical model of Taylor et al. (1996, 1998). We detected emission in the CS ( $J=2\rightarrow 1$), N 2H + ( $J=1\rightarrow0$), and HCO + ( $J=1\rightarrow0$) lines. Several clumps of size $\la$ 0.08 pc were found for each line distributed all over the region where previous single-dish emission was found (Morata et al. 1997). Each molecular transition traces differently the clump distribution, although in some cases the detected clumps are coincident. The distribution of the N 2H + emission and the single-dish NH 3 emission are coincident and compatible with an origin in the same gas. The large fraction of missing flux measured for the CS ( $2\rightarrow1$) transition can be explained if the cloud is formed by a clumpy and heterogeneous medium. Four positions were selected to derive the abundance ratios [ N 2H +/CS] and [ HCO +/CS] from the molecular column density determinations, and to compare them with the values predicted by the chemical model. The model was able to explain the interferometric observations, and, in particular, the chemical differentiation of the detected clumps and the coincidence of the NH 3 and N 2H + emissions. The lack of HCO + towards the two selected positions that trace the more evolved clumps cannot be accounted for by the model, but it is possibly due to strong self-absorption. We propose a classification of the studied clumps according to the stage of chemical evolution indicated by the molecular abundances.

Key words: molecular processes -- ISM: abundances -- ISM: clouds -- ISM: molecules -- ISM: individual objects: L673

Offprint request: O. Morata, oscar@am.ub.es

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