Highly bipolar planetary nebula Hen 2-57 and its possible symbiotic core

Abstract: We present a multi-wavelength analysis of the highly bipolar object Hen 2-57 based on new optical spectroscopy from the Southern African Large Telescope (SALT), complemented by archival imaging, and long-term time-series photometry from OGLE and NEOWISE. The nebula exhibits a pronounced hourglass morphology with a symmetry axis at a position angle of $110\pm5^{{\circ}$ and an average lobe opening angle of $\sim$53^{\circ}.}Wealsoidentifypronouncednorth−−southbrightnessasymmetry,withthesouthernpartsoflobesconsistentlybrighter.Thespectralenergydistributionshowsastrongmid−infraredexcessthatcannotbereproducedbyasingle−temperatureblackbody,indicatingthepresenceofadensecircumstellarduststructure,likelyintheformofadiskortorus,analogoustothatobservedinM2−9.Opticalspectroscopyrevealsacomplexkinematicstructureinwhichthecentralcoreisredshiftedrelativetoboththeeastern(.</sup> We also identify pronounced north--south brightness asymmetry, with the southern parts of lobes consistently brighter. The spectral energy distribution shows a strong mid-infrared excess that cannot be reproduced by a single-temperature blackbody, indicating the presence of a dense circumstellar dust structure, likely in the form of a disk or torus, analogous to that observed in M 2-9. Optical spectroscopy reveals a complex kinematic structure in which the central core is redshifted relative to both the eastern (55\pm4\ \mathrm{km\,s^{-1}})</sup>andwestern()</sup> and western (46\pm3\ \mathrm{km\,s^{-1}})</sup>lobes.Whilestandarddiagnosticdiagramsplacetheextendedlobesintheplanetarynebularegime,emission−linediagnosticsofthecentralcoreareconsistentwiththoseofsymbioticstars.ThesameregionalsoexhibitsasteepBalmerdecrement,indicativeofsignificantinternaldustextinction.Long−termOGLEphotometryshowsnoevidenceforcoherentstellarpulsations,whereasNEOWISEphotometryrevealsagradualmid−infraredfadingof)</sup> lobes. While standard diagnostic diagrams place the extended lobes in the planetary nebula regime, emission-line diagnostics of the central core are consistent with those of symbiotic stars. The same region also exhibits a steep Balmer decrement, indicative of significant internal dust extinction. Long-term OGLE photometry shows no evidence for coherent stellar pulsations, whereas NEOWISE photometry reveals a gradual mid-infrared fading of \sim$0.25 mag between 2010 and 2024. Together, these properties support a scenario in which Hen 2-57 hosts a dust-enshrouded D-type symbiotic central engine actively shaping the surrounding bipolar nebula, making it a close analogue of M 2-9.