Determine the classification of Hen 2-57

Determine whether Hen 2-57 is a classical planetary nebula or a bipolar nebula powered by a dust-enshrouded symbiotic binary system.

Background

Hen 2-57 has traditionally been catalogued as a planetary nebula, but its dense central region, symbiotic-like emission-line diagnostics, strong infrared excess, extremely red near-infrared colours, and long-term mid-infrared variability indicate that it may instead contain a dust-enshrouded D-type symbiotic nucleus. The extended lobes retain properties typical of planetary-nebula gas, creating an unresolved classification problem at the boundary between planetary nebulae and symbiotic outflows.

The authors conclude that the combined evidence favours the symbiotic interpretation, while acknowledging that none of the available diagnostics independently provides definitive proof of a symbiotic binary. Higher-quality spatially resolved spectroscopy and further observations would be needed to establish the nature of the central engine conclusively.

References

The classification of Hen 2-57 has remained uncertain for several decades.

— Highly bipolar planetary nebula Hen 2-57 and its possible symbiotic core  (2609.19874 - Merc et al., 17 Sep 2026) in Section “Is Hen~2-57 a planetary nebula with a symbiotic core?”