Title: Radio variability and non-thermal components in stars evolving toward planetary nebulae,
Abstract: We present new JVLA multi-frequency measurements of a set of stars in
transition from the post-AGB to the Planetary Nebula phase monitored in the
radio range over several years. Clear variability is found for five sources.
Their light curves show increasing and decreasing patterns. New radio
observations at high angular resolution are also presented for two sources.
Among these is IRAS 18062+2410, whose radio structure is compared to
near-infrared images available in the literature. With these new maps, we can
estimate inner and outer radii of 0.03$"$ and 0.08$"$ for the ionised shell, an
ionised mass of $3.2\times10^{-4}$ M$_\odot$, and a density at the inner radius
of $7.7\times 10^{-5}$ cm$^{-3}$, obtained by modelling the radio shell with
the new morphological constraints. The combination of multi-frequency data and,
where available, spectral-index maps leads to the detection of spectral indices
not due to thermal emission, contrary to what one would expect in planetary
nebulae. Our results allow us to hypothesise the existence of a link between
radio variability and non-thermal emission mechanisms in the nebulae. This link
seems to hold for IRAS 22568+6141 and may generally hold for those nebulae
where the radio flux decreases over time.