EVOLUTION OF MACROMOLECULAR DUST - FAR-ULTRAVIOLET SPECTRAL DUST EXTINCTION AND GAS-ABSORPTION OF STELLAR LIGHT AS MEASURED WITH THE HOPKINS ULTRAVIOLET TELESCOPE

From far-ultraviolet (FUV) spectra of B stars taken with the Hopkins U ltraviolet Telescope (HUT) and Voyager space observations, we determin e the far-ultraviolet extinction by Galactic dust and hydrogen absorpt ion in the local spiral-arm clouds towards HD 25443 (B0.5 III), HD 379 03 (B1.5 V), and HD 200775 (B3 Ve). We find that the [n(H)] = 1.3 cm-3 and number fraction (f = 0.31) of H atoms in molecular hydrogen relat ive to the total hydrogen indicates a mostly diffuse medium toward HD 25443. We also determine f = 0.56 for the gas in the photodissociation region (PDR) in front of HD 37903 and f almost-equal-to 0.36 for the HD 200775 PDR. The inferred ratios of N(H2)/I(CO) = 2.4-2.8 x 10(20) K -1 (km s-1)-1 cm-2 for cool gas in dark clouds agree with previous can onical estimates of N(H2)/I(CO) = 2-3 x 10(20) K-1 (km s-1)-1 cm-2 and support the use of the ratio for determining masses of molecular clou ds. We find that the shape and strength of the HD 25443 FUV extinction is the same as a diffuse-medium mean extinction (R(V) = 2.95) extrapo lated from mid-UV wavelengths (normal). The HD 37903 FUV extinction th rough a bright H-2 photodissociation region is higher than a mean FUV extinction (R(V) = 4.11), as is the HD 200775 extinction. Another star from the literature, rho Oph, probes the dense medium and exhibits an FUV extinction of normal mean strength (R(V) = 4.55) but steeper shap e. The ''normal'' FUV-extinction of HD 25443 implies that the small FU V-extinction dust in the diffuse medium forms in part from larger grai ns as the grains shatter under shocks. The normal strength of the rho Oph FUV-extinction indicates that the minute dust condenses onto or co agulates into larger grains in the dense medium. The high extinction d eviation of the HD 37903 PDR (and HD 200775 PDR) shows that FUV radiat ion from hot stars with T(eff) almost-equal-to 20,000-23,000 K in PDRs can evaporate some of the FUV-extinction dust from grain surfaces. In contrast, previously measured extinctions in Galactic H+ (H II) regio ns shows that FUV radiation at T(eff) > 25,000 K can destroy some of t he dust in H+ regions. Thus, some of the FUV-extinction dust is highly volatile. We find that the dust carrier of the variable FUV extinctio n is correlated (r = 0.99) with the mid-IR (12 mum) radiation from hyd rocarbon dust. The volatile FUV-extinction component could thus be due to minute hydrocarbon dust particles or macromolecules, probably PAHs , which are known to condense onto larger grains in the dense medium. Larger grains composed of PAHs clustered into a solid.could thus produ ce the steep FUV extinction of the rho Oph dense medium. After exposur e to FUV photons within approximately 1 pc of HD 37903, PAHs with abou t 30 carbon atoms would evaporate off the larger grains, increasing ga seous PAH abundance by approximately 14%. Also, after receiving shocks in the diffuse medium toward HD 25443, the larger hydrocarbon grains would shatter into PAH molecules. In summary, as the Galactic medium c ycles between the diffuse and dense medium, hydrocarbon dust cycles in to and out of larger grains under the influence of shocks, radiation, and condensation.