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

GUERRERO, MARTÍN A.; FANG, XUAN; MILLER BERTOLAMI, MARCELO M.; RAMOS-LARIOS, GERARDO; TODT, HELGE; ALARIE, ALEXANDRE; SABIN, LAURENCE; MIRANDA, LUIS F.; MORISSET, CHRISTOPHE; KEHRIG, CAROLINA; ZAVALA, SAÚL A.

Nature Astronomy

Nature Publishing Group

Año: 2018 vol. 2 p. 784 - 789

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? event 1 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 object 2 . Coming from a solar-mass progenitor, HuBi 1 represents a potential end-state for our Sun.