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Astron. Astrophys. 319, 948-966 (1997)

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Magnitude and color distribution of background stars as a probe of small-scale structure in the spatial distribution of interstellar dust *

S. Thoraval 1, P. Boissé 1 and G. Duvert 2

1 Radioastronomie, UA CNRS 336, Ecole Normale Supérieure, 24, rue Lhomond, F-75231 Paris Cedex 05, France
2 Laboratoire d'Astrophysique, Observatoire de Grenoble, B.P. 53X, F-38041 Grenoble Cedex, France

Received 18 December 1995 / Accepted 1 March 1996


In order to determine the amplitude of small scale fluctuations of the extinction induced by molecular material, we have reconsidered the star count method in the context of fragmented clouds. We show that by studying the full magnitude distribution it is possible not only to estimate extinctions in a more reliable way than by using the classical procedure, but also to derive constraints on cloud structure at very small scales (down to few µarcsec). This technique has been applied to a low latitude cloud located at a distance of about 200 pc in the IC 5146 complex. Analyses of CCD B, V, R, i photometric data on more than one thousand stars indicate that the cloud extinction is close to uniform over the 45 arcmin2 field.

To better constrain the fluctuations of the extinction, the distribution of the colors of background stars is considered. Analysis of the [FORMULA] data implies that variations are present but at a level no larger than [FORMULA] %. The accurate [FORMULA] (corresponding to 0.04 pc) resolution map of the [FORMULA] color excess obtained indicates that most of these variations occur at scales larger than [FORMULA] arcmin. We therefore conclude that the contribution of small scales to extinction fluctuations is quite small and that, at these scales, molecular material behaves as if it were homogeneous regarding the transfer of continuum UV radiation.

A comparison of our extinction data with a high resolution map of the same field in the [FORMULA] -0 and [FORMULA] -1 CO lines shows a complete absence of correlation between [FORMULA] and I (CO) at these small scales, which implies an upper limit for the H2 -I (CO) conversion factor (defined as [FORMULA] (H2)/ [FORMULA] (CO)) of about [FORMULA] H2 molecules cm-2 /K km s-1. We discuss the implications of this lack of correlation on the structure of molecular clouds and on the origin of the large spatial variability displayed by CO emission.

Key words: dust, extinction – ISM: clouds, structure, molecules – ISM: IC 5146

* Based on observations made at Observatoire de Haute-Provence (France, operated by the CNRS), Observatoire du Pic du Midi and at the 30m IRAM telescope (Pico Veleta, Spain)

Send offprint requests to: S. Thoraval

© European Southern Observatory (ESO) 1997

Online publication: July 3, 1998