4-8 October 2015
Hans Harnack Haus
Europe/Berlin timezone
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Contribution Poster

Poster session

Dust properties from scattering

Speakers

  • Dr. Laurent PAGANI

Primary authors

Co-authors

Content

Dust properties are a key issue to explore the content and structure of molecular clouds. However, they remain difficult to determine because deducing their composition and size distribution from observations is a highly degenerate problem. In particular, emission alone is not able to retrieve the coldest dust (Pagani, 2015). Other information are needed and some insights come from extinction (absorption+scattering) at shorter wavelengths (from visible to mid--infrared). Though extinction curves are efficient to investigate outer parts, they are limited in range, while the coreshine effect (Pagani, 2010) has turned out to be a powerful tool to investigate the densest parts of molecular clouds (Lefèvre, 2014). Moreover, this scattering process at 3.6 and 4.5 $\mu$m, requires the appropriate balance between scattering and absorption. Only 3D radiative transfer modeling combining several wavelengths is able to relate this balance to the composition of the grains, to their shape (compact or porous) and to the spatial evolution of the dust population inside clouds. Modeling wavelengths of interest include effects from :

  • The local strength of the radiation field possibly affected by the reddening of embedded sources (Lefèvre, 2014),

  • H$_2$O 3.0 $\mu$m feature influenced by growth (Whittet, 2013),

  • Absorption at 9.7 $\mu$m by silicate grains sensitive to their incorporation into larger composite aggregates (Min, 2008),

  • The variation of the scattering over absorption ratio with the grain shape for a given dust mass (including porosity and fractal structure).

By testing all these aspects into the modeling, we show that normal interstellar radiation field conditions are sufficient to find suitable grain models for starless cores. The standard diffuse interstellar grains are not able to reproduce our observations and dust has to evolve to explain all the wavelengths. Though the coreshine effect is visible only in the 3-5 $\mu$m range, we found that scattering also has a profound impact at longer wavelength, adding new constraints to the grain properties. The scattering importance has been overlooked in former works, especially considering extinction measurements in the 8 $\mu$m band. Our modeling, including scattering, is compatible with growth via fractal coagulation of the dust. Such properties are expected at later stages in disks as well as to explain far--infrared observations (Köhler, 2012). The influence of fractal coagulation, ices and dust composition fraction will be discussed and we will illustrate the scattering importance at all wavelengths with the L183 molecular cloud as an exemple.

References

Köhler, M., Stepnik, B., Jones, A. P., et al. 2012, A&A, 548, 61

Lefèvre, C., Pagani, L., Juvela, M., et al. 2014, A&A, 572, A20

Min, M., Waters, L. B., de Koter, A., et al. 2008, A&A, 486, 779

Pagani, L., Lefèvre, C., Juvela, M., Pelkonen, V.-M., & Schuller, F. 2015, A&A, 574, L5

Pagani, L., Steinacker, J., Bacmann, A., Stutz, A., & Henning, T. 2010, Science, 329, 1622

Whittet, D. C. B., Poteet, C. A., Chiar, J. E., et al. 2013, ApJ, 774, 102