a) Modification of the extinction curves of Ossenkopf & Henning (1994, OH94) for various grain sizes and β-values (see the text). The extinction coefficients κ_abs and κ_sca of model Milky Way A for R_V = 5.5 of Weingartner & Draine (2001) match at 1 μm. b) The extinction curves of Zubko et al. (2004, ZDA04 in red) and Weingartner & Draine (2001, WD01 in black). The modified curve of Ossenkopf & Henning (1994) is shown in blue (OH94 mod). Solid lines depict mass absorption, dashed lines scattering. c) Structure of model II, where the isodensity surface corresponds to 20 percent of the highest density. The surface encloses approximately 30 percent of the total mass of the core. The LOS is along the negative z-axis, i.e. the axis pointing towards the observer. In the sky, right ascension is along the negative x-axis and declination along the positive y-axis. The dominating heating source S 1 is situated near the focus of the paraboloid low-density region. d) The surface density distribution of the dust, Σ_dust in g cm^-2, for model II.3, i.e. for the extinction curve (Weingartner & Draine 2001, R_V = 5.5 with PAHs).