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

S. Casassus ; Lars-Åke Nyman (Institutionen för radio- och rymdvetenskap, Nationella anläggningen för radioastronomi) ; C. Dickinson ; T. J. Pearson
Monthly Notices of the Royal Astronomical Society (0035-8711). Vol. 382 (2007), 4, p. 1607-1622.
[Artikel, refereegranskad vetenskaplig]

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 = -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.

Nyckelord: radiation mechanisms : general, planetary nebulae : general, radio, continuum : ISM, submillimetre, RADIO-SYNTHESIS OBSERVATIONS, COSMIC BACKGROUND IMAGER, CONTINUUM, OBSERVATIONS, CENTRAL STARS, ABSORPTION-COEFFICIENTS, INTERSTELLAR-MEDIUM, METALLIC NEEDLES, MAGNETIC-FIELDS, GAUNT FACTORS, DUST GRAINS



Denna post skapades 2008-12-12.
CPL Pubid: 81276

 

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Institutioner (Chalmers)

Institutionen för radio- och rymdvetenskap, Nationella anläggningen för radioastronomi (2005-2010)

Ämnesområden

Astronomi, astrofysik och kosmologi

Chalmers infrastruktur