The paper in Nature Astronomy is here: go.nature.com/2N1LN2R

One more night at the Nordic Optical Telescope. Perfect observing conditions, as usual during the short summer nights in La Palma island. One after another, the planetary nebulae in my target list are being imaged through narrowband filters. Then it was the turn of HuBi 1, a low-excitation planetary nebula surrounding a cool 38,000 K central star (for these stars, such a temperature is low). The first image in the H-alpha line of the hydrogen Balmer series showed an almost round, low surface brightness nebula. Then it was the turn of the emission line from singly ionized atoms of nitrogen N⁺, which traces low-ionization material. The image came up. Either I had used the wrong filter or there was something very unusual with this planetary nebula.

The Nordic Optical Telescope (NOT) at the Observatorio de El Roque de los Muchachos on the island of La Palma (Spain). Picture taken on a summer night by one of the authors.

Planetary nebulae are clouds of gas ionized by the strong flux of UV photons from their hot central stars. As such, material closer to the central star has higher ionization degree than material far from the star. The image of a planetary nebula is typically that of IC 418 and the Ring Nebula, with emission from low-ionization species in the outermost nebular regions, embracing emission from higher ionization species inside. The image of HuBi 1 was exactly opposite: the emission from the low-excitation N⁺ was found closer to the central star.

Colour-composite pictures of the planetary nebulae IC 418 (left), HuBi 1 (center), and NGC 6720, the Ring Nebula (right). The emission from low-ionization species is in all cases shown in red colors, whilst that of high-ionization species is shown in blue and green.

The observing run was over and I had to wait for next season. Then, favored once again by exceptional observing conditions, I obtained long-slit optical spectra of this source. This technique allows the acquisition of spectra along a line crossing the nebula. The analysis of these spectra confirmed and amplified its peculiarities. The outer shell emits basically recombination lines of hydrogen and helium, whereas the inner shell exhibits an inverted ionization structure: low excitation lines of O⁺ were found inside and the high excitation line of He⁺⁺ outside. The emission line of He⁺⁺? It can’t be! The star is too cool to kick off the two electrons from the helium atom …

Wait a minute! Where did the central star go? The star was barely detected in our spectrum. We then compared it with photometric measurements and spectra obtained in the period 1970 to 1995 and found that the star has declined by 10 magnitudes, i.e. it is now 10,000 times fainter than it was in 1971.

Our simulations to model the spectra of the inner and outer shells of HuBi 1 provided us with important clues. The outer shell is recombining, because the flux of UV photons is not sufficient to keep the ionization balance. The inner shell, on the other hand, is not photo-ionized, but it is shock-excited. Apparently, ejecta from the central star is traveling at high speed through the nebula, shock-exciting it, and at the same time blocking the light from the central star.

These late ejections of material from central stars of planetary nebulae are unusual, but not impossible. A few central stars (the most famous being Sakurai’s object) are known to have experienced a so-called born-again event, when the thin helium layer below its hydrogen-rich atmosphere reaches a critical mass and ignites on a very late thermal pulse. The case of HuBi 1 was somehow different to those, slower. We investigated different late evolutionary sequences for these events and realized that only low-mass progenitor stars could produce these events.

Evolution of the central star of HuBi 1 in the Hertzprung-Russell (luminosity vs temperature) diagram. The star experienced a very late thermal pulse and returned back to the up-right corner. Nowadays, it is tracing a loop at high luminosity and relatively low temperature.

This event may occur to our Sun, some 5,000 million years from now. What it is really important is that we are witnessing in real time the transition from a hydrogen-rich central star of a planetary nebula to a carbon-rich one. This event is sought to have happened in all planetary nebulae with carbon-rich central stars, but this is the first time we see how highly processed material and mechanical energy is injected into their nebulae. 

HuBi 1 is evolving in real time. Its spectrum is changing, its inner nebula is expanding, and its outer nebula is being switched off. More summer nights are coming with HuBi 1 at the focal plane of our telescopes …