From pre- to young planetary nebulae: radio continuum variability

Searching for variability, we have observed a sample of hot post-asymptotic giant branch stars and young planetary nebula 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 that the central stars have started ionizing their circumstellar envelopes, and an increase in radio flux with time can be expected as a result of the progression of the ionization front. Such a behaviour has been found in IRAS 18062+2410, whose radio modelling has allowed us to determine that its ionized mass has increased from 10-4 to 3.3 × 10-4 Msun in 8 yr 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 (˜1015 cm) with an electron density of 2 × 106 cm-3. IRAS 22568+6141 and IRAS 17516-2525 have decreased their radio flux densities by about 10 per cent per year over 4 yr. While a linear increase in the flux density with time points to the progression of the ionization 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 ionizing photons.