A&A 470, 941-955 (2007)
DOI: 10.1051/0004-6361:20065261

Interstellar gas, dust and diffuse bands in the SMC

N. L. J. Cox^{1, 2}, M. A. Cordiner^{3, 4}, P. Ehrenfreund⁵, L. Kaper², P. J. Sarre⁴, B. H. Foing⁶, M. Spaans⁷, J. Cami⁸, U. J. Sofia⁹, G. C. Clayton¹⁰, K. D. Gordon¹¹, and F. Salama<sup>12

1</sup>  Herschel Science Centre, European Space Astronomy Centre, Research and Scientific Support Department of ESA, Villafranca del Castillo, Ap. de Correos 50727, 28080 Madrid, Spain
    e-mail: Nick.Cox@esa.int
²  Astronomical Institute, University of Amsterdam, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands
³  Astrophysics Research Centre, School of Mathematics and Physics, Queen's University Belfast, Belfast, BT7 1NN, Northern Ireland
⁴  School of Chemistry, The University of Nottingham, University Park, Nottingham, NG7 2RD, UK
⁵  Astrobiology Group, Leiden Institute of Chemistry, Leiden University, Einsteinweg 55, 2300 RA Leiden, The Netherlands
⁶  ESA/SCI-SR, ESTEC, PO Box 299, 2200 AG Noordwijk, The Netherlands
⁷  Kapteyn Astronomical Institute, University of Groningen, PO Box 800, 9700 AV Groningen, The Netherlands
⁸  SETI Institute, 515 North Whisman Road, Mountain View, CA 94043, USA
⁹  Astronomy Department, Whitman College, Walla Walla, WA 99362, USA
¹⁰  Dept. of Physics & Astronomy, Louisiana State University, Baton Rouge, LA 7080, USA
¹¹  Steward Observatory, University of Arizona, Tucson, AZ 85721, USA
¹²  Space Science Division, NASA Ames Research Center, Mail Stop 245-6, Moffett Field, California 94035, USA

(Received 23 March 2006 / Accepted 19 April 2007)

Abstract
Aims.In order to gain new insight into the unidentified identity of the diffuse interstellar band (DIB) carriers, this paper describes research into possible links between the shape of the interstellar extinction curve (including the 2175 Å bump and far-UV rise), the presence or absence of DIBs, and physical and chemical conditions of the diffuse interstellar medium (gas and dust) in the Small Magellanic Cloud (SMC).
Methods.We searched for DIB absorption features in VLT/UVES spectra of early-type stars in the SMC whose reddened lines-of-sight probe the diffuse interstellar medium of the SMC. Apparent column density profiles of interstellar atomic species (Na I, K I, Ca II and Ti II) are constructed to provide information on the distribution and conditions of the interstellar gas.
Results.The characteristics of eight DIBs detected toward the SMC wing target AzV 456 are studied and upper limits are derived for the DIB equivalent widths toward the SMC stars AzV 398 , AzV 214 , AzV 18 , AzV 65 and Sk 191 . The amount of reddening is derived for these SMC sightlines, and, using and the H I column density, converted into a gas-to-dust ratio. From the atomic column density ratios we infer an indication of the strength of the interstellar radiation field, the titanium depletion level and a relative measure of turbulence/quiescence. The presence or absence of DIBs appears to be related to the shape of the extinction curve, in particular with respect to the presence or absence of the 2175 Å feature. Our measurements indicate that the DIB characteristics depend on the local physical conditions and chemical composition of the interstellar medium of the SMC, which apparently determine the rate of formation (and/or) destruction of the DIB carriers. The UV radiation field (via photoionisation and photo-destruction) and the metallicity (i.e. carbon abundance) are important factors in determining diffuse band strengths which can differ greatly both between and within galaxies.

Key words: astrochemistry -- galaxies: Magellanic Clouds -- ISM: dust, extinction -- ISM: lines and bands -- ISM: molecules -- ISM: atoms

© ESO 2007

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