# Radio variability and non-thermal components in stars evolving toward planetary nebulae

## TL;DR

This study uses multi-frequency radio observations to analyze variability and non-thermal emission in stars transitioning to planetary nebulae, revealing potential links between radio variability and non-thermal mechanisms.

## Contribution

It provides new high-resolution radio maps and models of transition stars, highlighting non-thermal emission and variability patterns not previously documented.

## Key findings

- Detected radio variability in five transition stars.
- Identified non-thermal spectral indices contrary to thermal expectations.
- Suggested a link between radio variability and non-thermal emission mechanisms.

## 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.