Evidence for cold plasma in planetary nebulae from radio observations with the LOw Frequency ARray (LOFAR)

TL;DR

This study uses LOFAR radio observations to detect a cold plasma component in planetary nebulae, revealing lower electron temperatures than optical methods suggest, which impacts nebula modeling and mass estimates.

Contribution

First direct radio evidence of cold plasma in planetary nebulae, highlighting its significance for accurate modeling and interpretation of nebular properties.

Findings

Cold plasma coexists with hot plasma in planetary nebulae.

Electron temperatures from radio are 20-60% lower than optical estimates.

Accounting for cold plasma improves extinction and mass estimates.

Abstract

We present observations of planetary nebulae with the LOw Frequency ARray (LOFAR) between 120 and 168 MHz. The images show thermal free-free emission from the nebular shells. We have determined the electron temperatures for spatially resolved, optically thick nebulae. These temperatures are 20 to 60% lower than those estimated from collisionally excited optical emission lines. This strongly supports the existence of a cold plasma component, which co-exists with hot plasma in planetary nebulae. This cold plasma does not contribute to the collisionally excited lines, but does contribute to recombination lines and radio flux. Neither of the plasma components are spatially resolved in our images, although we infer that the cold plasma extends to the outer radii of planetary nebulae. However, more cold plasma appears to exist at smaller radii. The presence of cold plasma should be taken into account in modeling of radio emission of planetary nebulae. Modelling of radio emission usually uses electron temperatures calculated from collisionally excited optical and/or infrared lines. This may lead to an underestimate of the ionized mass and an overestimate of the extinction correction from planetary nebulae when derived from the radio flux alone. The correction improves the consistency of extinction derived from the radio fluxes when compared to estimates from the Balmer decrement flux ratios.