Radio Spectral Index Analysis of Southern Hemisphere Symbiotic Stars

TL;DR

This study analyzes the radio spectral indices of Southern Hemisphere symbiotic stars, linking radio luminosity to shell burning activity on white dwarfs, and providing insights into their physical properties.

Contribution

It presents new radio spectral index measurements of symbiotic stars and correlates radio luminosity with shell burning activity, enhancing understanding of their emission mechanisms.

Findings

Radio luminosity varies by a factor of 10^4 among symbiotic stars.

Linear size of emission regions varies by a factor of 100.

Radio luminosity indicates whether shell burning occurs on the white dwarf.

Abstract

Symbiotic stars show emission across the electromagnetic spectrum from a wide array of physical processes. At cm-waves both synchrotron and thermal emission is seen, often highly variable and associated with outbursts in the optical and X-rays. Most models of the radio emission include an ionized region within the dense wind of the red giant star, that is kept ionized by activity on the white dwarf companion or its accretion disk. In some cases there is on-going shell burning on the white dwarf due to its high mass accretion rate or a prior nova eruption, in other cases nuclear fusion occurs only occasionally as recurrent nova events. In this study we measure the spectral indices of a sample of symbiotic systems in the Southern Hemisphere using the Australia Telescope Compact Array. Putting our data together with results from other surveys, we derive the optical depths and brightness temperatures of some well-known symbiotic stars. Using parallax distances from Gaia Data Release 3, we determine the sizes and characteristic electron densities in the radio emission regions. The results show a range of a factor of 10^4 in radio luminosity, and a factor of 100 in linear size. These numbers are consistent with a picture where the rate of shell burning on the white dwarf determines the radio luminosity. Therefore, our findings also suggest that radio luminosity can be used to determine whether a symbiotic star is powered by accretion alone or also by shell burning.