Analysis of the emission of very small dust particles from Spitzer spectro-imagery data using blind signal separation methods ^*,**

¹ Centre d'Étude Spatiale des Rayonnements, CNRS et Université Paul Sabatier Toulouse 3, Observatoire Midi-Pyrénées, 9 Av. du Colonel Roche, 31028 Toulouse Cedex 04, France e-mail: olivier.berne@cesr.fr
² Laboratoire d'Astrophysique de Toulouse-Tarbes, CNRS et Université Paul Sabatier Toulouse 3, Observatoire Midi-Pyrénées, 14 Av. Edouard Belin, 31400 Toulouse, France
³ Steward Observatory, University of Arizona, Tucson, AZ 85721, USA
⁴ Technische Universität Dresden, Institut für Physikalische Chemie und Elektrochemie, 01062 Dresden, Germany
⁵ Spitzer Science Center, MS 220-6, California Institue of Technology, Pasedena, CA 91125, USA
⁶ Institut d'Astrophysique Spatiale, CNRS et Université de Paris Sud, Bât. 109, 91405 Orsay Cedex, France

Received: 21 August 2006
Accepted: 6 March 2007

Abstract

Context.This work was conducted as part of the SPECPDR program, dedicated to the study of very small particles and astrochemistry, in Photo-Dissociation Regions (PDRs).

Aims.We present the analysis of the mid-IR spectro-imagery observations of Ced 201, NCG 7023 East and North-West and ρ  Ophiuchi West filament.

Methods.Using the data from all four modules of the InfraRed Spectrograph onboard the Spitzer Space Telescope, we produced a spectral cube ranging from 5 to 35 μm, for each one of the observed PDRs. The resulting cubes were analysed using Blind Signal Separation methods (NMF and FastICA).

Results.For Ced 201, ρ Ophiuchi West filament and NGC 7023 East, we find that two signals can be extracted from the original data cubes, which are 5 to 35 μm spectra. The main features of the first spectrum are a strong continuum emission at long wavelengths, and a broad 7.8 μm band. On the contrary, the second spectrum exhibits the classical Aromatic Infrared Bands (AIBs) and no continuum. The reconstructed spatial distribution maps show that the latter spectrum is mainly present at the cloud surface, close to the star whereas the first one is located slightly deeper inside the PDR. The study of the spectral energy distribution of Ced 201 up to 100 μm suggests that, in cool PDRs, the 5–25 μm continuum is carried by Very Small Grains (VSGs). The AIB spectra in the observed objects can be interpreted as the contribution of neutral and positively-charged Polycyclic Aromatic Hydrocarbons (PAHs).

Conclusions.We extracted the 5 to 25 μm emission spectrum of VSGs in cool PDRs, these grains being most likely carbonaceous. We show that the variations of the mid-IR (5–35 μm) spectra of PDRs can be explained by the photo-chemical processing of these VSGs and PAHs, VSGs being the progenitors of free PAHs.