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Low-ionisation structures (LISs) are commonly found in planetary nebulae (PNe) but they are still poorly understood. The recent discovery of unforeseen molecular hydrogen gas (H2) has changed what we think we know about these microstructures and PNe. For an overall understanding of LISs, an Fe II 1. 644m imagery survey in PNe with LISs was carried out with the aim to detect the Fe II 1. 644m emission line, a common tracer of shocks. We present the first detection of Fe II 1. 644m line directly associated with the LISs in four out of five PNe. The theoretical H I 12-4 recombination line is also computed either from the Br or the H line and subtracted from the observed narrowband line fluxes. Fe II 1. 644m flux ranges from 1 to 40x10^-15 ergs cm^-2 s^-1 and the surface brightness from 2 to 90x10^-5 erg cm^-2 s^-1 sr^-1. The R (Fe) =Fe II 1. 644m/Br line ratio is also computed and varies between 0. 5 and 7. In particular, the Fe II 1. 644m line is detected in NGC 6543 (R (Fe) <0. 15), the outer pairs of LISs in NGC 7009 (R (Fe) <0. 25), the jet-like LISs in IC 4634 (R (Fe) 1) and in several LISs in NGC 6571 (2<R (Fe) <7). The low R (Fe) in NGC 6543 is attributed to the UV radiation from the central star. Contrarily, the higher values in NGC 6571 and IC 4634 are indicative to shocks. The moderate R (Fe) in NGC 7009 likely indicates the contribution of both mechanisms.
Akras et al. (Tue,) studied this question.