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The extinction law for molecular clouds Case study of B335
Stockholm University, Faculty of Science, Department of Astronomy.
Stockholm University, Faculty of Science, Department of Astronomy.
2010 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 522, A84- p.Article in journal (Refereed) Published
Abstract [en]

Context. The large optical and near-IR surveys have made it possible to investigate the properties of dark clouds by means of extinction estimates. There is, however, a need for case studies in more detail in order to investigate the basic assumptions when, say, interpreting reddening in terms of column density. Aims. We determine the extinction curve from the UV to the near-IR for molecular clouds and investigate whether current models can adequately explain this wavelength dependence of the extinction. The aim is also to interpret the extinction in terms of H-2 column density. Methods. We applied five different methods, including a new method for simultaneously determining the reddening law and the classification of the background stars. Our method is based on multicolour observations and a grid of model atmospheres. Results. We confirm that the extinction law can be adequately described by a single parameter, R-V (the selective to absolute extinction), in accordance with earlier findings. The R-V value for B335 is R-V = 4.8. The reddening curve can be accurately reproduced by model calculations. By assuming that all the silicon is bound in silicate grains, we can interpret the reddening in terms of column density, N-H = 4.4 (+/- 0.5) x 10(21) EI-Ks cm(-2), corresponding to N-H = 2.3 (+/- 0.2) x 10(21) . AV cm(-2), close to that of the diffuse ISM, (1.8-2.2) x 10(21) cm(-2). We show that the density of the B335 globule outer shells can be modelled as an evolved Ebert-Bonnor gas sphere with.. rho proportional to r(2), and estimate the mass of this globule to 2.5 M-circle dot

Place, publisher, year, edition, pages
2010. Vol. 522, A84- p.
Keyword [en]
ISM: clouds, dust, extinction
National Category
Astronomy, Astrophysics and Cosmology
Research subject
URN: urn:nbn:se:su:diva-51219DOI: 10.1051/0004-6361/201014174ISI: 000284153100090OAI: diva2:387803
authorCount :2Available from: 2011-01-14 Created: 2011-01-10 Last updated: 2012-02-14Bibliographically approved
In thesis
1. Extinction in Molecular Clouds: Case of Barnard 335
Open this publication in new window or tab >>Extinction in Molecular Clouds: Case of Barnard 335
2012 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The Bok globule B335 is a small molecular cloud in the solar neighbourhood near the galactic plane. The aim for this three-paper-study is to construct and analyze the extinction for this globule.

The method we apply is to use the light from field stars behind the cloud in broadband filters ranging from UV to the mid-infrared. We have observations performed at the ESO telescopes at La Silla and Paranal as well as at the Nordic 2.5 m telescope at La Palma. Together with images and spectra from 2MASS-, ISO- and Spitzer-archives we are able to cover the wavelength range from 0.35 to 24 μm. An important tool to analyze these observations results in order to get the extinction is the grid of synthetic stellar atmospheric spectra provided by Hauschildt (2005).

The extinction so received is a result in itself. From the analysis of the extinction wavelength dependence we derive properties of the dust, especially its composition and grain size distribution. By modeling the grain size distribution we are able to find the extinction from the reddening of the stars.

We find that the extinction in the optical wavelength 0.35 to 2 μm range nicely follows the functional form described by Cardelli et al. (1989). Our result from the wavelength range redward of 2 μm show an extinction dependent on the part of the cloud examined. For the rim of the cloud we get an extinction similar to that reported earlier for the diffuse interstellar medium. From the central parts of the cloud, however, a higher extinction was found. Our grain size model contains a carbonaceous particle distribution and a silicate one. The result can be explained by depletion of carbon onto carbonaceous grains and also by carbon onto all grains including the silicates.

Our modeling of the extinction and our classification of the background stars allow us to

- determine the distance to the globule

- estimate the gas column density ratio

- estimate the mass of globule

- get a handle on the dust conversion processes through the grain size distribution


From the water- and CO-ice spectra we are able to estimate the ice column densities. We find similar ice column densities for the two ices. The estimates differ, when calculated from band strengths or from Lorenz-Mie calculations of ice mantles on the grain size distribution, by a factor of two.

Place, publisher, year, edition, pages
Stockholm: Department of Astronomy, Stockholm University, 2012. 78 p.
ISM, interstellar medium, extinction, molecular cloud
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Astronomy; Astronomy
urn:nbn:se:su:diva-72523 (URN)978-91-7447-430-5 (ISBN)
Public defence
2012-03-16, sal FB52, AlbaNova universitetscentrum, Roslagstullsbacken 21, Stockholm, 13:00 (English)
Available from: 2012-02-23 Created: 2012-02-14 Last updated: 2012-02-21Bibliographically approved

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