Evolution of Macromolecular Dust from HUT FUV Observations of Stars

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Session 92 -- Dust and Gas in the ISM
Oral presentation, Friday, January 14, 10:15-11:45, Salon VI Room (Crystal Gateway)

[92.06] Evolution of Macromolecular Dust from HUT FUV Observations of Stars

R. H. Buss, Jr. (Johns Hopkins University), M. Allen (Johns Hopkins University), S. McCandliss (Johns Hopkins University), J. Kruk (Johns Hopkins University), J.-C. Liu (Johns Hopkins University), T. Brown (Johns Hopkins University)

With far-ultraviolet spectra of B stars taken with the Hopkins Ultraviolet Telescope (HUT) aboard the ASTRO-1 space observatory, we determine the far-ultraviolet extinction by galactic dust and hydrogen absorption in the local spiral-arm clouds toward HD~25443 (B0.5~III), HD~37903 (B1.5~V), and HD~200775 (B3~Ve). Respectively, the number fractions of H-atoms in molecular hydrogen relative to the total hydrogen are $f = 0.31$, 0.56, and $> 0.8$, showing that the Galactic media are different. The shape and strength of the HD~25443 FUV extinction is the same as a diffuse-medium mean extinction extrapolated from mid-UV wavelengths (Cardelli, Clayton, and Mathis 1989) [normal]. The HD~37903 FUV extinction through a bright \htwo\ photodissociation region is higher than a mean FUV extinction, 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 but steeper shape.

The ``normal'' FUV-extinction of HD~25443 implies that the small FUV-extinction dust in the diffuse medium forms in part from larger grains as the grains shatter under shocks. The normal strength of the $\rho$~Oph FUV-extinction indicates that the minute dust condenses onto or coaggulates into larger grains in the dense medium. The high extinction of the HD~37903 PDR (and HD~200775 PDR) shows that FUV radiation from hot stars with $T_{eff} \approx$~21,000--23,000~K can evaporate some of the FUV-extinction dust from grain surfaces in photodissociation regions. In contrast, previously-measured extinctions in Galactic \hion\ (H~II) regions shows that FUV radiation at $T_{eff} > 26,000$~K can destroy some of the dust in \hion\ (H~II) regions. Thus, some of the FUV-extinction dust is highly volatile.

We find that the dust carrier of the variable FUV extinction is correlated with the mid-IR (12~\um) radiation from hydrocarbon dust.

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