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The inside-out planetary nebula around a born-again star

Abstract

Planetary nebulae are ionized clouds of gas formed by the hydrogen-rich envelopes of low- and intermediate-mass stars ejected at late evolutionary stages. The strong UV flux from their central stars causes a highly stratified ionization structure, with species of higher ionization potential closer to the star. Here, we report on the exceptional case of HuBi 1, a double-shell planetary nebula whose inner shell presents emission from low-ionization species close to the star and emission from high-ionization species farther away. Spectral analysis demonstrates that the inner shell of HuBi 1 is excited by shocks, whereas its outer shell is recombining. The anomalous excitation of these shells can be traced to its low-temperature [WC10] central star whose optical brightness has declined continuously by 10 magnitudes in a period of 46 years. Evolutionary models reveal that this star is the descendant of a low-mass star (1.1 M) that has experienced a ‘born-again’ event1 whose ejecta shock-excite the inner shell. HuBi 1 represents the missing link in the formation of metal-rich central stars of planetary nebulae from low-mass progenitors, offering unique insight regarding the future evolution of the born-again Sakurai’s object2. Coming from a solar-mass progenitor, HuBi 1 represents a potential end-state for our Sun.

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Fig. 1: Colour composite picture and spatial profiles of selected lines of HuBi 1.
Fig. 2: Long-term spectro-photometric evolution of IRAS 17514, the central star of HuBi 1.
Fig. 3: Evolutionary sequence of a PN progenitor with initial mass 1.1 M that experiences a VLTP.

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Acknowledgements

Some of the data presented in this article were obtained with ALFOSC, which is provided by the Instituto de Astrofísica de Andalucía (IAA) under a joint agreement with the University of Copenhagen and NOTSA. The Nordic Optical telescope (NOT) is installed in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofísica de Canarias, in the island of La Palma (Spain). This article is also based on observations carried out at the Observatorio Astronómico Nacional on the Sierra San Pedro Mártir (OAN SPM), Baja California, Mexico. We thank the daytime and night support staff at the OAN SPM for facilitating and helping obtain our observations. A.A. and C.M. acknowledge support through the CONACyT project CONACyT-CB2015-254132. G.R.-L. acknowledges support from Universidad de Guadalajara, Fundación Marcos Moshinsky, ProMoFID2018 and CONACyT (grant A1-S-12258). L.S. acknowledges support from PAPIIT grant IA-101316 (Mexico). L.F.M. is supported by Ministerio de Economía, Industria y Competitividad (Spain) grants AYA2014-57369-C3-3 and AYA2017-84390-C2-1-R (cofunded by FEDER funds). M.A.G. acknowledges support of the grant AYA 2014-57280-P, cofunded with FEDER funds. M.M.M.B. is partially supported through ANPCyT grant PICT-2016-0053 and MinCyT-DAAD bilateral cooperation program through grant DA/16/07. S.A.Z. was supported by the ITE-UNAM agreement 1500-479-3-V-04.

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M.A.G. planned the research project, programmed the observations, wrote the main body of the manuscript, and organized the writing of some subsections. A.A. performed the MAPPINGS simulations, C.M. the CLOUDY ones, and both devised the excitation nature of the inner and outer shells. C.K. estimated the ionizing flux necessary for the inner and outer shells. G.R.-L. reduced the imaging data and contributed to the analysis of the time evolution of the central star. H.T. analysed the spectrum of the central star using PoWR to determine its stellar parameters and abundances. L.S. and G.R.-L. obtained and reduced the high-dispersion spectroscopic observations, and together with L.F.M. and S.A.Z. analysed them using SHAPE. M.M.M.B. devised the post-AGB evolutionary scenario and computed the LPCODE VLTP evolutionary sequences. X.F. reduced the spectroscopic data and carried out the analysis of one-dimensional spectra and spatial profiles of emission lines. All authors contributed to the discussion of the different sections of this work.

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Correspondence to Martín A. Guerrero.

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Supplementary Table 1, Supplementary Figures 1–6

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Guerrero, M.A., Fang, X., Miller Bertolami, M.M. et al. The inside-out planetary nebula around a born-again star. Nat Astron 2, 784–789 (2018). https://doi.org/10.1038/s41550-018-0551-8

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