Anomalous radio emission from dust in the Helix

Abstract

A by-product of experiments designed to map the cosmic microwave background is the recent detection of a new component of foreground Galactic emission. The anomalous foreground at similar to 10 - 30 GHz, unexplained by traditional emission mechanisms, correlates with 100 mum dust emission. We use planetary nebulae (PNs) as astrophysical laboratories to test known radio emission processes and report that in the Helix the emission at 31 GHz and 100 mum are well correlated and exhibit similar features on sky images, which are absent in Hbeta. Upper limits on the 250 GHz continuum emission in the Helix rule out cold grains as candidates for the 31 GHz emission and provide spectroscopic evidence for an excess at 31 GHz over bremsstrahlung. We estimate that the 100 mum - correlated radio emission, presumably due to dust, accounts for at least 20% of the 31 GHz emission in the Helix. This result strengthens previous tentative interpretations of diffuse interstellar medium spectra involving a new dust emission mechanism at radio frequencies. Very small grains, thought not to survive in evolved PNs, have not been detected in the Helix, which hampers interpreting the new component in terms of electric dipole emission from spinning grains. The observed iron depletion in the Helix favors considering the identity of this new component to be magnetic dipole emission from hot ferromagnetic grains. The reduced level of free-free continuum that we report also implies an electronic temperature of T-e = 4600 +/- 1200 K for the free-free emitting material, which is significantly lower than the temperature of 9500 +/- 500 K inferred from collisionally excited lines.