Dust properties of the dark cloud IC 5146

Submillimeter and NIR imaging

¹ I. Physikalisches Institut, Universität zu Köln, Zülpicher Straße 77, 50937 Köln, Germany e-mail: bhaswati@ph1.uni-koeln.de
² Mullard Radio Astronomy Observatory, Cavendish Laboratory, Madingley Road, Cambridge CB3 0HE, UK e-mail: jsr@mrao.cam.ac.uk
³ European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching, Germany e-mail: jalves@eso.org
⁴ Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA e-mail: clada@cfa.harvard.edu

Corresponding author: C. Kramer, kramer@ph1.uni-koeln.de

Received: 31 May 2002
Accepted: 9 December 2002

Abstract

We present the results of a submillimeter dust continuum study of a molecular ridge in IC 5146 carried out at 850 μm and 450 μm with SCUBA on the James Clerk Maxwell Telescope (JCMT). The mapped region is in size () and consists of at least four dense cores which are likely to be prestellar in nature. To study the dust properties of the ridge and its embedded cores, we combined the dust emission data with dust extinction data obtained by Lada et al. (1999) from the NIR colors of background giant stars. The ridge shows dust extinctions above ~10 mag, rising up to 35 mag in the cores.
A map of dust temperatures, constructed from the continuum flux ratios, shows significant temperature gradients: we find temperatures of up to ~20 K in the outskirts and between the cores, and down to ~10 K in the cores themselves. The cores themselves are almost isothermal, although their average temperatures vary between 10 K and 18 K. We used the extinction data to derive in addition a map of the dust emissivity parametrized by . The average value of agrees well with the canonical value of Mathis (1990). We find that increases by a factor of from to when the dust temperature decreases from  K to  K. A Monte Carlo simulation shows that this change is significant with regard to the estimated calibration uncertainties. This is consistent with models of dust evolution in prestellar cores by Ossenkopf & Henning (1994) which predict that grain coagulation and the formation of ices on grain surfaces in the cold, dense cloud interiors lead to a significant increase of the m dust opacity. This interpretation is furthermore supported by the previous detection of gas-phase depletion of CO in one of the IC 5146 cores (Kramer et al. 1999). Observations of dust fluxes at short wavelengths are however needed to verify this result.