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Abstract
Molecular hydrogen (H 2 ) emission is commonly detected in planetary nebulae (PNe), specially in objects with bipolar morphologies. New studies showed that H 2 gas is also packed in microstructures embedded in PNe of any morphological type. Despite the presence of H 2 in cometary knots being known for years, only in the last five years, much deeper imagery of PNe have revealed that H 2 also exists in other types of low-ionisation microstructures (LISs). Significant differences are found between the host PNe of cometary knots and other types of LISs, such as nebula age, central star temperature (evolutionary stage) and the absolute sizes of the microstructure itself.
Highlights
Planetary nebulae (PNe) represent the final evolutionary stage of low- and intermediate-mass stars (1–8 M ), and they are formed by the interaction of two or more mass-loss events during the previous stages
We present H2 1-0 S(1) narrow-band images of planetary nebulae (PNe) with low-ionisation structures to discuss the different patterns of the host PNe
Besides the presence of H2 gas in the cometary knots of nearby PNe, new observations over the last five years have unveiled that more low-ionisation microstructures (LISs) are composed of H2 gas
Summary
Planetary nebulae (PNe) represent the final evolutionary stage of low- and intermediate-mass stars (1–8 M ), and they are formed by the interaction of two or more mass-loss events during the previous stages. None of the available formation models are able to sufficiently explain all their properties Due to their substantial different kinematic properties, all the microstructures were included under one class called low-ionisation structures (LISs, [5]). Gonçalves and co-workers claimed that LISs should be dense structures composed of molecular gas, similar to CKs [13]. This could explain their systematic lower electron densities compared to the surrounding nebula values (e.g., [6,7,13]). We present H2 1-0 S(1) narrow-band images of PNe with low-ionisation structures to discuss the different patterns of the host PNe
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