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Published December 21, 2007 | Published
Journal Article Open

A centimetre-wave excess over free–free emission in planetary nebulae


We report a centimetre-wave (cm-wave, 5–31 GHz) excess over free–free emission in planetary nebulae (PNe). Accurate 31- and 250-GHz measurements show that the 31-GHz flux densities in our sample are systematically higher than the level of optically thin free–free continuum extrapolated from 250-GHz. The 31-GHz excess is observed, within one standard deviation, in all 18 PNe with reliable 31- and 250-GHz data, and is significant in nine PNe. The only exception is the peculiar object M2-9, whose radio spectrum is that of an optically thick stellar wind. On average the fraction of non-free–free emission represents 51 per cent of the total flux density at 31 GHz, with a scatter of 11 per cent. The average 31–250 GHz spectral index of our sample is〈α^(250)_(31)〉=−0.43 ± 0.03 (in flux density, with a scatter of 0.14). The 31–250 GHz drop is reminiscent of the anomalous foreground observed in the diffuse interstellar medium (ISM) by cosmic microwave background (CMB) anisotropy experiments. The 5–31 GHz spectral indices are consistent with both flat spectra and spinning dust emissivities, given the 10 per cent calibration uncertainty of the comparison 5-GHz data. However, a detailed study of the objects with the largest cm-excess, including the low-frequency data available in the literature, shows that present spinning dust models alone cannot explain the cm-wave excess in PNe. Although we have no definitive interpretation of our data, the least implausible explanation involves a synchrotron component absorbed by a cold nebular screen. We give flux densities for 37 objects at 31 GHz, and for 26 objects at 250 GHz.

Additional Information

© 2007 Universidad de Chile. Journal compilation © 2007 RAS. Accepted 2007 August 17. Received 2007 August 16; in original form 2007 June 5. We are grateful to Pat Roche for useful discussions and for reminding SC about helium, to Eli Dwek for an e-mail exchange providing extensive information on his needle models, and to Wouter Vlemmings for feedback on the nebular magnetic fields. The Swedish- ESO was operated jointly by ESO and the Swedish National Facility for Radioastronomy, Onsala Space Observatory at Chalmers University of Technology. SC acknowledges support from FONDECYT grant 1060827, and from the Chilean Centre for Astrophysics FONDAP 15010003. We gratefully acknowledge the generous support of Maxine and Ronald Linde, Cecil and Sally Drinkward, Barbara and Stanely Rawn, Jr, Fred Kavli and Rochus Vogt. Part of the research described in this paper was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration. CD thanks B. and S. Rawn, Jr, for funding a fellowship at the California Institute of Technology for part of this work.

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