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Relation gas-dust in the BLR

The delay obtained for the central component with respect to the UV continuum of tex2html_wrap_inline7853 is similar to that obtained for the K and L bands, where the emission is caused by heated dust (Clavel et al.\ 1989; Sect. 4.3.3). Therefore the dust, and the gas producing the central component, seem to be coexisting in the same region. Even, if the central gas is situated at tex2html_wrap_inline7859 (more compatible with the photoionization models) and the dust at tex2html_wrap_inline7861, it would appear that the outer part of the gas would be (within the errors) associated with the inner part of the dust zone. An upper limit to the dust mass at 400 light-days has been estimated by Clavel et al. in tex2html_wrap_inline7863 and we will estimate the mass-to-dust ratio from the maximum luminosity of both tex2html_wrap_inline7865\ and tex2html_wrap_inline7867. Assuming a typical tex2html_wrap_inline7869 and that at the CIV emission maximum all carbon is tex2html_wrap_inline7871 and all ions are in the lower level, tex2html_wrap_inline7873, with a maximum of tex2html_wrap_inline7875, tex2html_wrap_inline7877. For a solar carbon abundance of tex2html_wrap_inline7879 this gives tex2html_wrap_inline7881 and tex2html_wrap_inline7883 (Aller 1984). Then, the ratio tex2html_wrap_inline7885, similar to the galactic value of 200. Using the tex2html_wrap_inline7887 luminosity, tex2html_wrap_inline7889 (Aller 1984), for the Case B of recombination (tex2html_wrap_inline7891), with tex2html_wrap_inline7893 and the maximum tex2html_wrap_inline7895 luminosity of tex2html_wrap_inline7897 (Wamsteker et al. 1985), we find tex2html_wrap_inline7899. Then tex2html_wrap_inline7901 and the ratio tex2html_wrap_inline7903, four times larger than the galactic value.

Note that the mass of the central gas is in the range of tex2html_wrap_inline7905 from the different lines and the subsequent mass(central gas)/mass(dust) ratio extends from 100 to 750 could indicate that the abundance C/H is less than the solar one. If we take tex2html_wrap_inline7909 from the first calculus and tex2html_wrap_inline7911 from tex2html_wrap_inline7913, an abundance tex2html_wrap_inline7915 is obtained, an order of magnitude lower than the solar abundance assumed previously. Another explanation to the difference in the results from both lines might be that also the mixture of gas-to-dust is stratified, as had been previously found for the gas only. This is a consequence of the significantly different derived gas mass depending on the ionization state of the ion used for the calculation, while the dust can not survive in the inner regions. This would be consistent with the general concept from previous results suggesting stratification in the BLR (e.g. Clavel et al. 1991). However at the same time the coexistence of the gas and dust, and the difference in line delays with respect to the continuum, seem to indicate that we are most likely dealing with a situation where the mixture with dust and the ionization stratification are all taking place in a region at approximately tex2html_wrap_inline7917 from the central source in an anisotropic radiation field.


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