In this paper we present new and archive radio measurements obtained with the Very Large Array of the magnetic chemically peculiar (MCP) star sigma Ori E. The radio data have been obtained at different frequencies and are well distributed along the r
otational phases. We analyze in detail the radio emission from sigma Ori E with the aim to search evidence of circularly polarized radio pulses. Up to now, among the MCP stars only CU Virginis shows 100% polarized time-stable radio pulses, explained as highly directive electron cyclotron maser emission, visible from Earth at particular rotational phases, like a pulsar. Our analysis shows that there is no hint of coherent emission at frequencies below 15 GHz. We conclude that the presence of a quadrupolar component of the magnetic field, dominant within few stellar radii from the star, where the maser emission should be generated, inhibits the onset of the cyclotron maser instability in sigma Ori E.
We have observed the radio nebula surrounding the Galactic LBV candidate G79.29+0.46 with the EVLA at 6 cm. These new radio observations allow a morphological comparison between the radio emission, which traces the ionized gas component, and the mid-
IR emission, a tracer of the dust component. The IRAC (8 mu m) and MIPS (24 mu m and 70 mu m) images have been reprocessed and compared with the EVLA map. We confirm the presence of a second shell at 24 mu m and also provide evidence for its detection at 70 mu m. The differences between the spatial morphology of the radio and mid-IR maps indicate the existence of two dust populations, the cooler one emitting mostly at longer wavelengths. Analysis of the two dusty, nested shells have provided us with an estimate of the characteristic timescales for shell ejection, providing important constraints for stellar evolutionary models. Finer details of the ionized gas distribution can be appreciated thanks to the improved quality of the new 6 cm image, most notably the highly structured texture of the nebula. Evidence of interaction between the nebula and the surrounding interstellar medium can be seen in the radio map, including brighter features that delineate regions where the shell structure is locally modified. In particular, the brighter filaments in the south-west region appear to frame the shocked southwestern clump reported from CO observations.
Searching for variability, we have observed a sample of hot post-AGB stars and young Planetary Nebulae candidates with the Very Large Array at 4.8, 8.4, and 22.4 GHz. The sources had been previously detected in the radio continuum, which is a proof t
hat the central stars have started ionising their circumstellar envelopes and an increase in radio flux with time can be expected as a result of the progression of the ionisation front. Such a behaviour has been found in IRAS 18062+2410, whose radio modelling has allowed us to determine that its ionised mass has increased from 10^{-4} to 3.3 10^{-4} M_sun in 8 years and its envelope has become optically thin at lower frequencies. Different temporal behaviours have been found for three other sources. IRAS 17423-1755 has shown a possibly periodic pattern and an inversion of its radio spectral index, as expected from a varying stellar wind. We estimate that the radio flux arises from a very compact region around the central star (10^{15} cm) with an electron density of 2 10^6 cm^{-3}. IRAS 22568+6141 and 17516-2525 have decreased their radio flux densities of about 10% per year over 4 years. While a linear increase of the flux density with time points out to the progression of the ionisation front in the envelope, decreases as well as quasi-periodic patterns may indicate the presence of unstable stellar winds/jets or thick dusty envelopes absorbing ionising photons.
The radio emission from the youngest known Planetary nebula, SAO244567, has been mapped at 1384, 2368, 4800, 8640, 16832 and 18752 MHz by using the Australian Telescope Compact Array (ATCA). These observations constitute the first detailed radio stud
y of this very interesting object, as they allow us to obtain the overall radio morphology of the source and to compute, for the first time, the radio spectrum up to millimetre range. Radio emission is consistent with free-free from a wind-like shell, which is also the region where most of the [OIII] comes from as revealed by HST images. Physical parameters of the radio nebula and of the central star were derived, all consistent with SAO 244567 being a very young Planetary Nebula still embedded in the dusty remnant of the AGB phase. The optically thin radio flux density appear to decrease when compared to data from the literature. Even very appealing, the variability of the radio emission, probably related to the evolution of the central object, needs further investigations.
