Electron temperature fluctuations in planetary nebulae

Abstract

An observational study of the spatial variation of the electron temperature and density in 10 galactic planetary nebulae is presented. The data consist of long-slit spectra of high signal-to-noise ratio in the 3100 to 6900 A range. Electron temperatures were determined from the [Oiii](λ 4959 + λ 5007)/λ 4363 and [Nii](λ 6548 + λ 6583)/λ 5755 ratios and from the Balmer discontinuity. Electron densities were estimated from the [Sii]λ 6716/λ 6731, [Cliii]λ 5517/λ 5537, and [Ariv]λ 4711/λ 4740 ratios. Electron temperature variations of low amplitude were found across the nebular surface in the planetary nebulae studied. The temperature distribution across each nebula presents a variance relative to the mean corresponding to 0.0003 ≤ t 2 (Bal) ≤ 0.0078, 0.0003 ≤ t 2 (Nii) ≤ 0.0097, and 0.0011 ≤ t 2 (Oiii) ≤ 0.0050. A systematic spatial variation of electron density has been detected in most of objects (NGC 1535, NGC 2438, NGC 2440, NGC 3132, NGC 3242, NGC 6302, NGC 6563, and NGC 7009). The remaining objects (NGC 6781 and NGC 6853) have not shown any significant electron density dependence on position. NGC 2438, NGC 6563, NGC 6781, and NGC 6853 are in general the most diffuse and probably evolved objects studied here, with low mean densities in the range Ne(Sii) ≈ 95–158 cm −3 . An anti-correlation between temperature and density was found for NGC 2438 and NGC 3132, with the electron temperature increasing with the decrease of electron density and a correlation between temperature and density was found for NGC 2440, NGC 3242, NGC 6302, and NGC 7009, with the electron temperature increasing with the increase of electron density. These relationships seem to be associated with the structure of the nebula. The nebulae in which the correlation between temperature and density is present are ring shaped. The anti-correlation between temperature and density is found in bipolar planetary nebulae that are denser in the centre of the nebula.