**OPTICAL-UV DUST
EXTINCTION**

In our model we consider a two-phase dusty interstellar
medium, a dense one to represent molecular clouds associated with
star formation, and a diffuse one associated with older stellar
generations. The radiative transfer of starlight through these
two dusty media is computed exactly by numerically solving the
radiative transfer equation for the dense phase, while we adopt a
simplified treatment for the diffuse gas. In this case we ignore
dust self-absorption and approximate the effects of optical-UV
scattering by means of an effective optical depth, given by the
geometrical mean of the absorption and scattering efficiencies: t^{2}_{eff}=t_{a}(t_{a}+t_{s}). The
latter approximation is rigorously applicable only to an infinite
homogeneus medium and isotropic scattering (see this paper for details).

However we have checked this treatment by comparing our
results with those obtained by the Monte Carlo radiative transfer
models of dust extinction by **(1)** *Witt, A.N.,
Thronson, H.A., & Capuano, J.M., 1992, ApJ, 393, 611*
(WTC), and **(2)** *Ferrara, A., Bianchi, S.,
Cimatti, A., Giovanardi, C., astro-ph/9903078* (FBCG), and we
find good agreement.

**(1) **WTC include anisotropic scattering. The
plot below shows the ratio F/Fo between the dust extinguished
flux and that expected if dust were not present. The points are
by WTC, the dotted line is the result of our code taking into
account only absorption, whereas the solid line includes
scattering with the effective optical depth t^{2}_{eff}=t_{a}(t_{a}+t_{s}). The adopted geometry is that defined by
WTC as *elliptical galaxy*.

**(2) **We have compared the attenuation curves
computed by FBCG with ours for the case of pure disks (B/T=0,
i.e. the easiest choice in order to best reproduce the same
conditions for the two models) with only the diffuse phase of the
ISM, since this is the one for which these Monte Carlo models
were computed.

See in the three plots below the comparison between the
attenuation curves for the case t_{V}=1 and three values of the ratio z_{dust}/z_{star}
(the disk scale-heights for diffuse dust and stars), for disks
observed at different inclination angles, but the results are
general and independent of the values assumed for the
aforementioned parameters: the two models almost perfectly agree
for edge-on disks, while we overestimate the attenuation with
respect to FBCG for diminishing inclination angles. The
differences are never higher than ~20-25%, increase for smaller l, and are always ~10% in the V band.