Evidence for Cold Plasma in Planetary Nebulae From Radio Observations With the LOw Frequency ARray (LOFAR)

Marcin Hajduk,Glenn J White,Marco Iacobelli,Marijke Haverkorn,Mateusz Olech,Alexander Drabent,Harish K Vedantham,Joseph R Callingham,Timothy Shimwell

doi:10.3847/1538-4357/ac0fda

Abstract

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%–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.

Highlights

Planetary nebulae (PNe) are detectable at a broad range of wavelengths, from X-rays up to radio frequencies
Equation (1) gives a satisfactory fit to most of the PNe using electron temperatures derived from collisionally excited lines (CELs) given that Ω is smaller than the observed size of the nebula, i.e., when the bulk of the emission comes from a fraction of the solid angle
Using an electron temperature derived from CELs or fixing it to an arbitrary value (e.g., 104K) is a common practice in fitting the radio spectral energy distribution (SED) of PNe (Pazderska et al 2009; Hajduk et al 2018; Bojičić et al 2021)

Summary

Introduction

Planetary nebulae (PNe) are detectable at a broad range of wavelengths, from X-rays up to radio frequencies. CELs are suppressed by collisional de-excitation when the critical density is exceeded in plasma They are weak at low electron temperatures. Liu et al (2000) showed that the inclusion of high-density hydrogen-deficient plasma can explain the RL and CEL temperature and abundance discrepancies. This has been subsequently confirmed by Tsamis et al (2004), Zhang et al (2005), and Wesson et al (2005). Thick free–free radio emission provides an alternative method to assess electron temperatures in PNe. Brightness temperature is equal to electron temperature for optically thick thermal radiation. We report electron temperatures for a sample of PNe using LOFAR observations of optically thick free–free emission

Radio Emission from Planetary Nebulae

Observations and Data Analysis

Spectral Fitting

Electron Temperatures

Comparison of Radio and Optical Emission

Discussion and Conclusions