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The mid-infrared extinction in molecular clouds: Case study of B 335
Stockholm University, Faculty of Science, Department of Astronomy.
Stockholm University, Faculty of Science, Department of Astronomy.
2011 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 534, A127- p.Article in journal (Refereed) Published
Abstract [en]

 Field stars behind a molecular cloud can be used to probe the cloud extinction for both the reddening and the absorption features. By combining multi-colour photometry and IR spectroscopy the spectral class of the star can be determined as can the extinction curve, including the vibrational bands of ices and silicates. Results. Based on observations of field stars behind the dark globule B335, we determine the reddening curve from 0.35 to 24 mu m. The water ice band at 3.1 mu m is weaker (tau(3.1) = 0.4) than expected from the cloud extinction (A(V) approximate to 10 for the sightline to the most obscured star). On the other hand, the CO ice band at 4.7 mu m is strong (tau(4.67) = 0.7) and indicates that the mass column density of frozen CO is about the same as that of water ice. We fit the observations to model calculations and find that the thin ice coatings on the silicate and carbon grains (assumed to be spherical) lower the optical extinction by a few percent. We show that the reddening curves for the two background stars, for which the silicate band has been measured, can be accurately modelled from the UV to 24 mu m. These models only include graphite and silicate grains (plus thin ice mantles for the most obscured star), so there is no need for any additional major grain component to explain the slow decline of the reddening curve beyond the K band. As expected, the dust model for the dense part of the cloud has more large grains than for the outer regions. We propose that the well established shallow reddening curve beyond the K band has two different explanations: larger graphite grains in dense regions and relatively small grains in the diffuse ISM, giving rise to substantially less extinction beyond the K band than previously thought. Conclusions. For the sight line towards the most obscured star, we derive the relation A(Ks) = 0.97 . E(J - K(Ks)), and assuming that all silicon is bound in silicates, N(2H(2)+H) approximate to 1.5 x 10(21) . A(V) approximate to 9 x 10(21) . A(Ks). For the rim of the cloud we get A(Ks) = 0.51 . E(J -K(s)), which is close to recent determinations for the diffuse ISM. The corresponding gas column density is N(2H(2)+H) approximate to 2.3 x 10(21) . A(V) approximate to 3 x 10(22) . A(Ks).

Place, publisher, year, edition, pages
EDP Sciences, 2011. Vol. 534, A127- p.
Keyword [en]
ISM:molecules; ISM:clouds-dust;extinction-scattering-opacity
National Category
Astronomy, Astrophysics and Cosmology
Research subject
URN: urn:nbn:se:su:diva-68066DOI: 10.1051/0004-6361/201015564ISI: 000296554800007OAI: diva2:471742
Available from: 2012-01-02 Created: 2012-01-02 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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