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

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ID: 329060
2026
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Abstract
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 ± 5○ and an average lobe opening angle of ~53○. 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 ± 4 km s−1) and western (46 ± 3 km s−1) 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 ~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.
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Authors Jaroslav Merc, J Mikołajewska, K Iłkiewicz, A Udalski
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Journal monthly notices of the royal astronomical society
Year 2026
DOI
10.1093/mnras/stag1771
URL
https://doi.org/10.1093/mnras/stag1771
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