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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 HopkinsUltraviolet Telescope (HUT) and Voyager space observations, we determinethe far-ultraviolet extinction by Galactic dust and hydrogen absorptionin the local spiral-arm clouds towards HD 25443 (B0.5 III), HD 37903(B1.5 V), and HD 200775 (B3 Ve). We find that the (n (H)) = 1.3 cu cmand number fraction (f = 0.31) of H atoms in molecular hydrogen relativeto 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 approximately equals 0.36 for the HD200775 PDR. The inferred ratios of N(H2)/I(CO) = 2.4 to 2.8 x1020/K(km/s)/sq cm for cool gas in dark clouds agree withprevious canonical estimates of N(H2)/I(CO) = 2 to 3 x1020/K(km/s)/sq cm and support the use of the ratio fordetermining masses of molecular clouds. We find that the shape andstrength of the HD 25443 FUV extinction is the same as a diffuse-mediummean extinction (Rnu = 2.95) extrapolated from mid-UVwavelengths (normal). The HD 37903 FUV extinction through a bright H2photodissociation region is higher than a mean FUVextinction(Rnu = 4.11), as is the HD 200775 extinction.Another star from the literature rho Oph, probes the dense medium andexhibits an FUV extinction of normal mean strength (Rnu =4.55) but steeper shape. The normal FUV -extinction of HD 25443 impliesthat the small FUV-extinction dust in the diffuse medium forms in p artfrom larger grains as the grains shatter under shocks. The normalstrength of the rho Oph FUV-extinction indicates that the minute dustcondenses onto or coagulates into larger grains in the dense medium. Thehigh extinction deviation of the HD 37903 PDR (and HD 200775 PDR) showsthat FUV radiation from hot stars with Teff approximatelyequals 20,000 to 23,000 K in PDRs can evaporate some of theFUV-extinction dust from grain surfaces. In contrast, previouslymeasured extinctions in Galactic H(+) (H II) regions shows that FUVradiation at Teff greater than 25,000 K can destroy some ofthe dust in H(+) regions. Thus, some of the FUV-extinction dust ishighly volatile. We find that the dust carrier of the variableFUV-extinction is correlated (r = 0.99) with the mid-IR (12 micrometers)radiation from hydrocarbon dust. The volatile FUV-extinction componentcould thus be due to minute hydrocarbon dust particles ormacromolecules, probably PAHs, which are known to condense onto largergrains in the dense medium. Larger grains compsed of PAHs clustered intoa solid could thus produce the steep FUV extinction of the rho Oph densemedium. After exposure to FUV photons wihtin approximately 1 pc of HD37903, PAHs with about 30 carbon atoms would evaporate off the largergrains, increasing gaseous PAH abundance by approximately 14%. Also,after receiving shocks in the diffuse medium toward HD 25443, the largerhydrocarbon grains would shatter into PAH molecules. In summary, as theGalactic medium cycles betwee n the diffuse and dense medium,hydrocarbon dust cycles into and out of larger grains under theinfluence of shocks, radiation, and condensation.
| A study of UV extinction towards OB associations and star-forming regions based on IUE data
Extinction curves for 115 stars were derived using the IUE data bank:70% of them belong to OB associations. The sample provides good coverageof the galactic plane including objects located as far as 4 kpc from theSun. The sample covers a variety of different areas includinginteresting objects such as the Carina Nebula and the Rho Ophiuchi DarkCloud. While the older associations and field stars show extinctioncurves very close to the MIEC, younger objects show strong variations ofthe amount of the extinction in the far UV and, in some cases, also inthe hump region.
| The S201 far-ultraviolet imaging survey. II - A field in Cygnus
Far-ultraviolet imagery of a region in Cygnus, a 20 deg diameter fieldcentered near (1950) R. A. 21 h 31.2 m decl. +37 deg 25 arcmin, wasobtained by the S201 far-ultraviolet camera during the Apollo 16mission. In a 10 minute exposure covering the 1250-1600 A wavelengthrange (effective wavelength 1400 A), 730 star images can be detected,corresponding to a limiting ultraviolet magnitude of about m (1400) =10. Assuming nominal interstellar extinction values in this region nearthe galactic plane, this result corresponds to the detection of A0 Vstars to a distance of 300 pc and of B0 V stars to 1500 pc.Uncertainties in spectral classification and interstellar extinction forindividual objects are probably more significant than calibration ormeasurement errors. Most of the objects detected are identified withstars in the Smithsonian Astrophysical Observatory Star Catalog (1966),or the Catalog of Stellar Identifications (1979) or both, but 87 objectsremain unidentified (or are identified with late-type stars).
| Be stars and galactic rotation.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1968AJ.....73..338C&db_key=AST
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| Takýmyýldýz: | Kugu |
| Sað Açýklýk: | 21h04m19.17s |
| Yükselim: | +36°08'57.3" |
| Görünürdeki Parlaklýk: | 9.109 |
| özdevim Sað Açýklýk: | -2.2 |
| özdevim Yükselim: | -4.3 |
| B-T magnitude: | 9.071 |
| V-T magnitude: | 9.106 |
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