The 9.7 and 18 μm silicate absorption profiles towards diffuse and molecular cloud lines-of-sight
Astronomical Institute “Anton Pannekoek”, University of
Science Park 904,
2 Astronomical Institute Utrecht, University of Utrecht, PO Box 80000, 3508 TA Utrecht, The Netherlands
3 SETI Institute, 515 North Whisman Road, Mountain View, CA 94043, USA
4 SRON Netherlands Institute for Space Research, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands
5 Jodrell Bank Centre for Astrophysics, Alan Turing Building, School of Physics and Astronomy, The University of Manchester, Oxford Road, Manchester, M13 9PL, UK
6 Academia Sinica Institute of Astronomy and Astrophysics, PO Box 23-141, Taipei 10617, Taiwan
7 IPAC, NASA Herschel Science Center, Mail Code 100-22, California Institute of Technology, Pasadena, CA 91125, USA
8 Physics and Astronomy Department, University of Western Ontario, London ON N6A 3K7, Canada
9 Department of Physics and Astronomy, Institute for Astronomy, K.U. Leuven, Celestijnenlaan 200B, 3001 Leuven, Belgium
10 Department of Astronomy, University of Maryland, College Park, MD 20742, USA
11 Cornell University, Astronomy Department, 108 Space Sciences Bldg., Ithaca, NY 14853-6801, USA
12 Leiden Observatory, PO Box 9513, 2300 RA Leiden, The Netherlands
Received: 13 October 2008
Accepted: 9 December 2010
Context. Studying the composition of dust in the interstellar medium (ISM) is crucial for understanding the cycle of dust in our galaxy.
Aims. The mid-infrared spectral signature of amorphous silicates, the most abundant dust species in the ISM, is studied in different lines-of-sight through the Galactic plane, thus probing different conditions in the ISM.
Methods. We have analysed ten spectra from the Spitzer archive, of which six lines-of-sight probe diffuse interstellar medium material and four probe molecular cloud material. The 9.7 μm silicate absorption features in seven of these spectra were studied in terms of their shape and strength. In addition, the shape of the 18 μm silicate absorption features in four of the diffuse sightline spectra were analysed.
Results. The 9.7 μm silicate absorption bands in the diffuse sightlines show a strikingly similar band shape. This is also the case for all but one of the 18 μm silicate absorption bands observed in diffuse lines-of-sight. The 9.7 μm bands in the four molecular sightlines show small variations in shape. These modest variations in the band shape are inconsistent with the interpretation of the large variations in τ9.7/E(J − K) between diffuse and molecular sightlines in terms of silicate grain growth. Instead, we suggest that the large changes in τ9.7/E(J − K) must be due to changes in E(J − K).
Key words: dust, extinction / evolution / techniques: spectroscopic / infrared: ISM / ISM: clouds
© ESO, 2011