Astronomy and Astrophysics – Astrophysics
Scientific paper
2005-09-11
Astronomy and Astrophysics
Astrophysics
Astronomy and Astrophysics (accepted; 11 pages, 11 figures)
Scientific paper
10.1051/0004-6361:20053371
In many models of dusty objects in space the grains are assumed to be composite or fluffy. However, the computation of the optical properties of such particles is still a very difficult problem. We analyze how the increase of grain porosity influences basic features of cosmic dust -- interstellar extinction, dust temperature, infrared bands and millimeter opacity. Porous grains can reproduce the flat extinction across the $3 - 8 \mkm$ wavelength range measured for several lines of sight by {\it ISO} and {\it Spitzer}. Porous grains are generally cooler than compact grains. At the same time, the temperature of very porous grains becomes slightly larger in the case of the EMT-Mie calculations in comparison with the results found from the layered-sphere model. The layered-sphere model predicts a broadening of infrared bands and a shift of the peak position to larger wavelengths as porosity grows. In the case of the EMT-Mie model variations of the feature profile are less significant. It is also shown that the millimeter mass absorption coefficients grow as porosity increases with a faster growth occurring for particles with Rayleigh/non-Rayleigh inclusions. As a result, for very porous particles the coefficients given by two models can differ by a factor of about 3. It is found that an increase of porosity leads to an increase of extinction cross sections at some wavelengths and a decrease at others depending on the grain model. However, this behaviour is sufficient to reproduce the extinction curve in the direction of the star $\sigma$ Sco using current solar abundances. In the case of the star $\zeta$ Oph our model requires larger amounts of carbon and iron in the dust-phase than is available.
Dubkova D. N.
Henning Th
Il'in Vladimir B.
Voshchinnikov Nikolai V.
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