The dust distribution inside the Large Magellanic Cloud

The distribution of the interstellar dust inside the LMC has been investigated on the basis of UBV photometry and spectral classifications of 1507 luminous O-A type stars. The mean internal reddening has been found to be rather high with $E_{{B-V}} = 0\hbox{$.\!\!^{\rm m}$}16$. The frequency distribution of the reddenings is strongly asymmetric, so that it is useless to give an error of the mean. Reddenings up to 08 have been observed. A strong selection effect is caused by the restricted visibility of strongly reddened stars. The intrinsically faint stars ($V_{\rm{0}} > 13\hbox{$.\!\!^{\rm m}$}3$) of our sample show no or low reddening $E_{B-V} < 0\hbox{$.\!\!^{\rm m}$}2$. Stars of intermediate brightness ($12\hbox{$.\!\!^{\rm m}$}3 < V_{\rm{0}} \leq 13\hbox{$.\!\!^{\rm m}$}3$) are reddened by up to more than 03. Reddenings between 04 and 08 occur for the intrinsically brightest stars ($V_{\rm{0}} \leq 12\hbox{$.\!\!^{\rm m}$}3$) only. According to the luminosity function the completeness limit of our sample has been found to be about 12. To determine the reddening without bias, all intrinsically fainter stars have to be rejected. A comparison of the positions of highly reddened stars with $E_{{B-V}} > 0.3$ with those of molecular clouds detected by Cohen et al. (1998) shows few coincidences. Contrary to the dark cloud catalogues of Hodge (1972) and van den Bergh (1974) we detect highly reddened stars almost everywhere in the LMC. The frequency distribution of the reddenings can be fitted by a two cloud model. The mean reddenings caused by small and large dark clouds are $0\hbox{$.\!\!^{\rm m}$}04 \pm 0\hbox{$.\!\!^{\rm m}$}01$ and $0\hbox{$.\!\!^{\rm m}$}40 \pm 0\hbox{$.\!\!^{\rm m}$}10$, respectively. In the LMC, dust cloud properties are quite similar to those in the Galaxy. The number of small clouds on line of sight in comparison to that of large clouds is, however, higher than in the Milky Way. For the LMC we observe a number ratio of $45 \pm 10$ which may be compared with the galactic value of $9 \pm 1$. Finally we have determined a map of the internal reddening in the LMC. It shows overall agreement with that of Isserstedt & Kohl (1984), but gives higher absolute values, dominated by 30 Dor and the supershell LMC 2. Rather high reddenings have been found in the HII region N 11 and around the supershell LMC 6, too. Our map shows the same structure of the interstellar medium in the LMC as the HI map derived by Luks & Rohlfs (1992) and the IR emissio