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Cm-wave continuum emission from molecular clouds

Final Report Summary - CMWAVECLOUDS (Cm-wave continuum emission from molecular clouds)

The goal of this project is to gain physical understanding of the intriguing radio continuum luminosities detected in interstellar dust clouds. This information is also of interest to take into account (and remove) diffuse galactic foregrounds in the signals coming from cosmic microwave background experiments, such as Planck.

S. Casassus, Roueff and collaborators have carried out a variety of astronomical observations involving world-class instruments. These observational results are being compared to astrophysical models, in the framework of a continuing international collaboration forged by the present Marie Curie initiative.

During 2009-2010 the following datasets were collected and partially analysed:
High-angular resolution radio images of the Rho Ophiuchi cloud (ROPH) were obtained using the Australia Telescope Compact Array (ATCA) interferometer. The heterodyne array CHAMP+, mounted on the APEX antenna, was used as well to map a slice of the ROPH cloud. All of our proposals for European Southern Observatory ESO) instrumentation were supported: near infra-red (IR) spectroscopy in M17 and ROPH using Sinfoni, combined to wide field near IR H2 imaging with HAWKI. A high-resolution IR spectroscopy programme (with Cripes), assigned top-priority by ESO, is postponed due to an instrumental breakdown.

Several points can already be emphasised:

The radio emissivity field, i.e. the power radiated by dust grains, varies by at least two orders of magnitude in ROPH. No radio signal is seen in the direction of the brightest IR source in ROPH. The ATCA data reveal spectral variations even in the radio peak. The radio morphology of the filament is parallel to that of the near IR, but is shifted in position towards the exciting star with increasing frequency. Thus the radio spectrum peaks at higher frequencies in the illuminated side of the filament. Another clue is provided by H2 imaging and spectroscopy. There exists a correlation between the radio continuum and IR H2 emission. Interestingly, IR spectra acquired bear signatures which may be due to excitation of H2 during the formation process. A preliminary tentative interpretation of the data as a whole is thus that the energy release during the formation of H2 on grain surfaces is transferred to the grain, resulting in spinning dust excitation. This result, along with numerous other detections (the latest in M78, by members of our team), highlights the ubiquity and importance of spinning dust as a window on ISM physics.

In parallel to these observational results, theoretical efforts are pursued on different aspects. The spectral peak is observed at a frequency around 30 GHz, while it is predicted to occur at 100 GHz in dense regions such as ROPH. This can be interpreted in terms of a revision of grain properties (involving their size and electric dipole). Another important issue undertaken within this project is to produce a physical model of ROPH that describes the gas-phase density, temperature, and radiation intensity fields. The Meudon PDR code can provide such physical quantities given appropriate inputs, which have been collected either through literature searches or archive mining.

Finally, this study of spinning dust in specific regions has provided new constraints on the physics of grain spin-up. According to the proposed work plan, Dr Casassus has now returned to Chile after one year at Paris Observatory. The year 2011 should now see the publication of these results.

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