INTERSTELLAR EXTINCTION CURVE IN THE FAR AND EXTREME ULTRAVIOLET

The optical properties of the silicate and graphite dust grains of var ious sizes are calculated for the wavelength range lambdalambda 100-35 00 angstrom (proposed working region of the Spectrum-UV project). The interstellar extinction curves for silicate - graphite mixture with a power-law size distribution are also computed. The main features of th ese curves, besides the famous bump near lambda 2200 angstrom, are a s trong maximum near lambda almost-equal-to 700 angstrom and a plateau a t lambda less-than-or-equal-to 300 angstrom. It is shown that the abso rptions of radiation by dust and by gas become comparable at lambda le ss-than-or-equal-to 150 if there is a considerable fraction of very sm all particles in the interstellar medium and/or the dust/gas ratio is higher than the mean interstellar one. Indirect opportunity to investi gate the bump near lambda almost-equal-to 700 angstrom are the observa tions of the fluorescent lines CII, NI, NeI, SiII, excited by radiatio n within the range 600-900 angstrom, in spectra of planetary nebulae a nd HII regions.