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

TL;DR

This paper reports on HuBi 1, a unique planetary nebula with a double-shell structure caused by a born-again star, providing insights into stellar evolution, shock excitation, and the future of solar-mass stars.

Contribution

It presents the first detailed analysis of HuBi 1's unusual ionization structure and links it to a born-again event in a low-mass star, expanding understanding of planetary nebulae evolution.

Findings

Inner shell excited by shocks

Outer shell is recombining

Star experienced a born-again event

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 descendent of a low-mass star ($\simeq$1.1 $M_\odot$) that has experienced a born-again event 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. Coming from a solar-mass progenitor, HuBi 1 represents a potential end-state for our Sun.