HuBi 1

Lore & Background

HuBi 1 possesses an outer shell emitting H and He recombination lines, weak heavy-elements, and excited lines, indicating that the outer shell is recombining. This outer shell has a radius of ~10" or ~15", making it the largest component. It is also the oldest structure and was ejected ~6700 years ago, expanding at a velocity of 50 km s⁻¹. Notably, the O III λ5007 emission line in the outer shell may be more intense than the Hβ in the inner shell.

The nebula features an inner shell whose emission of intense excited lines of N+, O+, and S+ demonstrates an inverted ionization structure, caused by the C-rich population of stars at the center. The inner shell has a smaller radius of ~3" and was ejected ~1700 years ago. Its expansion velocity of 40 km sec⁻¹ provides evidence that an intense N II λ6548,λ6584 emission produced by shocks occurred. O III emission lines were discovered along the Northeast direction, uniquely defining the inner shell. This emission has a ring-like appearance extending out to a radius of ~2."5, with two distinct peaks along the west-east direction. The O III emission also surrounds both the S II and N II emission zones, while a He II emission line encompasses the O III emission. Among these features, the He II λ4686 possesses the most ionization potential and is also the most spatially extended. The inner shell encompasses emission lines such as Hα moving at -14 to +22 sec⁻¹ and Hβ moving at -96 to +171 sec⁻¹. The Hα emission region is obscured by the nearby N ii λ6584 line.

Obscured at the center of the nebula is a fast outflow or jet spanning ~2". It is the fastest kinematic component, traveling at 150 km s⁻¹, and a possible maximum velocity of 250 km s⁻¹. It is also the youngest feature, having been ejected ~335 years ago.

Reader's Guide

HuBi 1 is notable as a very rare born-again planetary nebula, a class of objects that undergo a late thermal pulse (or more likely a very late thermal pulse) rather than evolving directly to a typical planetary nebula. This event forces the star's evolutionary track to reverse, causing its outer atmosphere to expand and cool. After looping back in the H-R diagram through the AGB zone, the star enters the PN zone a second time. It has been proposed that after returning to the PN stage the star becomes a Wolf–Rayet star, ejecting a H-poor nebula. Interaction with the older, H-rich old outer shell hindered the H-poor nature of the nebula. Alternatively, the system has been proposed to be a binary star system. The central star of HuBi 1 exhibits an unusually late, WC10 spectral type, corresponding to an effective temperature of ~35,000 K, which supports observations that the star has been fading significantly over the past 40–50 years. The nebula's multiple shells and jet provide a rare laboratory for studying the late stages of stellar evolution, including recombination, shocks, and ionization structures.

Did You Know?

• HuBi 1 was discovered in 1990 using data from the Nordic Optical Telescope.
• The outer shell was ejected ~6700 years ago, while the inner shell was ejected ~1700 years ago.
• The central star has a WC10 spectral type and an effective temperature of ~35,000 K.
• The fast outflow or jet at the center travels at 150 km s⁻¹ and is the youngest feature, ejected ~335 years ago.

Discovery & Cataloguing

In 1990, astronomers J. Y. Hu and E. A. Bibo identified an extraordinarily rare object: a born-again planetary nebula that would carry their names. The detection relied on data gathered with the Nordic Optical Telescope, situated at the Roque de los Muchachos Observatory on the island of La Palma in the Canary Islands. The nebula resides at a heliocentric distance of approximately 6.88 kiloparsecs, with a stated uncertainty of 2.19 kpc. What sets HuBi 1 apart from the vast majority of known planetary nebulae is its born-again classification—a designation reserved for systems whose central star has undergone a late or very late thermal pulse, effectively restarting its final evolutionary episode. This rarity, coupled with the multi-layered shell architecture and the fast central outflow revealed in detailed spectroscopic studies, has made HuBi 1 a particularly valuable case for probing how a star can loop back through the asymptotic-giant-branch region of the Hertzsprung-Russell diagram and re-enter the planetary-nebula phase a second time, producing a structure far more complex than a single ejection event would predict.

A Two-Shell Architecture

Observations reveal that HuBi 1 is built from at least two concentric shells of very different ages and physical conditions. The outer shell, the largest and oldest component, spans a radius of roughly 10 to 15 arcseconds and was flung outward approximately 6,700 years ago. It is now expanding at about 50 km/s and is in a recombining state, as shown by hydrogen and helium recombination lines alongside weak heavy-element emission. Notably, the O III λ5007 line in this outer shell can appear more intense than the Hβ line in the inner shell. The inner shell is far more compact—only about 3 arcseconds across—and was ejected roughly 1,700 years ago. It displays an inverted ionization structure with intense excited lines of N+, O+, and S+, a pattern attributed to the carbon-rich stellar population at the core. Its 40 km/s expansion speed, together with strong N II λ6548 and λ6584 emission, points to shock-driven excitation. Along the northeast direction, O III emission traces a ring-like feature extending to about 2.5 arcseconds with two peaks along the west-east axis, enclosing the S II and N II zones, while He II λ4686—the line with the highest ionization potential—forms the most spatially extended envelope of all.

The Central Star and the Born-Again Puzzle

At the heart of HuBi 1 sits a star that has defied the usual one-way march toward a white-dwarf fate. Instead of following a standard planetary-nebula track, the central star appears to have suffered a late or very late thermal pulse. This episode forces the star's evolutionary path to reverse: its outer atmosphere inflates and cools, the star loops back through the asymptotic-giant-branch region of the Hertzsprung-Russell diagram, and it re-enters the planetary-nebula zone a second time. One working model proposes that, upon returning to the nebula stage, the star briefly assumes a Wolf-Rayet character, ejecting a hydrogen-poor wind whose signature is later diluted by interaction with the older, hydrogen-rich outer shell. An alternative hypothesis invokes a binary companion to explain the unusual behavior. The central star's spectrum is classified as WC10—an unusually late Wolf-Rayet type—with an effective temperature near 35,000 K. Observations over the past four to five decades indicate the star has been fading significantly, consistent with the post-pulse cooling expected of a born-again object.

The Fast Central Jet

Buried at the very center of HuBi 1, partially obscured by the surrounding shells, lies the youngest and most energetic component of the entire system: a fast outflow or jet spanning roughly 2 arcseconds. Kinematic measurements place its bulk velocity at about 150 km/s, with a possible peak reaching 250 km/s—far exceeding the 40–50 km/s expansion rates of the two shells. Based on its velocity and spatial extent, the jet was launched only around 335 years ago, making it the most recent ejection event in the nebula's layered history. Its presence is a direct signature of the born-again episode: the very late thermal pulse that reignited the central star produced a brief, powerful wind that punched through the pre-existing inner and outer shells. The jet's high speed and compact size contrast sharply with the broad, slower shells around it, offering a clear temporal sequence—from the 6,700-year-old outer shell, to the 1,700-year-old inner shell, to this 335-year-old central outflow—of at least three distinct mass-loss events in the star's final act.