Leonard Kelley holds a bachelor's in physics with a minor in mathematics. He loves the academic world and strives to constantly explore it.
Stars below 8 solar masses typically go through their lives fusing to lower elements like hydrogen and helium but being unable to fuse past carbon and oxygen. The stars swell up to giant proportions, then expel their outer layers as a planetary nebula, with a white dwarf as a remnant. Astronomy, however, is seldom as simple as our models project it to be, and sometimes these white dwarfs get a second chance to be giant stars. How it gets there depends on its surroundings, but no matter what, we end up with a born-again giant (BAG) star (Asplund).
The First Helium Flash
These stars first become a red giant upon transitioning from a primarily hydrogen core to a helium core. The core contracts, raising temperatures around it due to greater density conditions and thus outward pressure ensues. This causes the outer layers to expand but cool off due to a greater surface area. Meanwhile, fusing helium causes a flash of energy outward but never reaches the surface. It does sustain the outward expansion for some time. The star now uses hydrogen and helium as its energy sources. This is the red giant phase of the star (Asplund).
Over a star’s life, several helium flashes can occur (about once every 10,000 to 100,000 years) and swelling and contracting backward ensue. Eventually we make the same transition from a helium core to an oxygen/carbon one, with the same results happening. The star then enters the asymptotic giant branch (AGB) part of the H-R diagram upon reaching this phase (Asplund).
The Final Helium Flash
After this movement, the star lacks sufficient mass to start converting the oxygen and carbon to higher elements, and so upon a certain point the outer layers begin to thin out but are still fusing material. They get blown away by solar wind (driven each time by a helium flash) and form a planetary nebula around a white dwarf, the remains of the mainly oxygen/carbon core... And that’s it…supposedly (Asplund).
But if the white dwarf has enough leftover hydrogen and helium and retains enough warmth, it could have thin shells of both fusing material. Given the right amount of both of these happening, a final helium flash could occur, causing the white dwarf to again achieve giant proportions and appear as a BAG star, over 100 times the size of the Sun (Asplund, Tafoya 1).
This BAG phase should last only 10 to 100 years before resuming its white dwarf life. It is because of this short span of time that we rarely spot this event, despite models pointing to nearly 20% of AGB stars going through this. At the end, a new inner planetary nebula (deficient in hydrogen but rich in carbon) moving at 40 to 300 kilometers a second is formed inside of the old one (with many materials) and the white dwarf resumes its former career but now with elevated oxygen and carbon levels (Asplund, Tafoya 1-2).
Common Envelope Merger Theory
One thing to point out of great importance is this model applies to single stars. When you introduce a binary to the potential BAG scenario, things get more complicated. Discrepancies in chemical traces, such as higher than expected neon levels and low carbon-to-oxygen levels, pointed to an ONeMg (oxygen-neon-magnesium) white dwarf at the heart of this event (Lau 1871).
These are the result of super-AGB stars ending their lives, but not before burning some carbon for a little bit before the end. These stars can end their lives either as an ONeMg white dwarf or as an electron capture supernova (for more on one of those, check out my article on them). For a planetary nebula of this composition to be made, the star had to start making the nebula as it leaves the super-AGB phase, then become the hydrogen-defiant giant in just a few years. Somehow, you have to then include for the final burst seen afterward (Lau 1871).
The most likely way to account for this is to have a low-mass main sequence star in proximity to our AGB star. If the AGB star is large enough (6-8 solar masses) then it can evolve into a super-AGB star which will lose its hydrogen layer via those helium flashes and expel a planetary nebula. The companion star interacts with this new material that acts as a common envelope between them and via frictional force drags the stars together as angular momentum is shed off. The merger leads to new material being shed off both by new fusion and by the merger itself, with a new planetary nebula being formed (Lau 1872).
If the super-AGB star was large enough to have an ONeMg core, then the ejecta from this event could have both neon and the lower carbon-to -oxygen ratio as we have seen in several BAG stars. But some qualifiers are needed here. To get the two distinct planetary nebulae we need the interaction between the stars to take more than a few years, a slow common envelope event. This could happen if the secondary star starts far away from the AGB star but falls in very fast at the merging moment. This would give a hydrogen-rich planetary nebula from before the merger and the hydrogen-deficient planetary nebula from the final merger (Lau 1872).
Another possibility for a binary system is a nova. Here we have an ONeMg white dwarf primary and an AGB secondary. Solar wind from the AGB star sends material to the white dwarf, not necessarily from the star evolving past its Roche limit (which sends material as gravitational forces transition). Eventually, the AGB star loses its hydrogen layer, makes a planetary nebula, and becomes a white dwarf itself (Lau 1872).
Now that the secondary star has transitioned, it is no longer giving off as much material as before, is much smaller, and not fusing as much on its outer layers. But it could still have its final helium flash and go BAG, sending new material to the primary white dwarf. The amount of new material causes the primary to go nova, releasing a new planetary nebula and the final one as the BAG slowly returns to its white dwarf roots (Lau 1872).
This event is oxygen and neon rich and mixes with material from the final helium flash of the secondary star. So long as both stars give off roughly the same material, one from the final helium flash and the other from the nova, then the resulting nebula could match what we see in some BAG stars. Also crucial is the timing: the nova needs to happen relatively soon after the final helium flash. While we could look for two peaks to indicate that this nova scenario took place, ONeMg nova are very dim and thus hard to spot. That, and if the timing is right then you have a very narrow window to spot one (Lau 1872-3).
V4334 Sagittarii (aka Sakurai’s Object)
First spotted by amateur astronomer Yukio Sakurai in February of 1996 and named after him, this star was first thought to be a nova event. But Sakurai’s Object didn’t fit the profile of a nova. First, the spectrum didn’t match and second the object didn’t dim like a nova. Over the years, observational data instead pointed to this star being a BAG. For starters, in the first year the surface temperature dropped from about 8000 K to about 6000 K, while hydrogen and levels dropped but heavy metals increased. This shows the dense wind emanating from Sakurai’s Object, pushing dust around and dimming the actual surface of the star. By 2005, it was practically impossible to see the star in the visible spectrum with most of the light being emitted in the IR portion of the spectrum (Asplund, Geballe 1-2).
With a lack of this visible light, it makes other clues important. The chemical changes were a result of material mixing with the outer layers coming into contact with the uprising interior material, causing both new hydrogen burning (decreasing those amounts) as well as the formation of lithium and other heavy elements via neutron capturing. Before total obstruction, visible light seemed to indicate the inner planetary nebula was asymmetric, with the pattern indicating that gas left Sakurai’s Object in two opposite directions primarily. This created an oblong shell of a sort high in dust and molecule production of C2, CN, 12CO, 13CO and various hydrogenated molecules as temperatures dropped (Asplund, Tafoya 2, Geballe 2).
First spotted in 1919, V605’s spectrum showed little hydrogen in the giant star and over the next two years it increased in brightness. This activity took place inside a 20,000 year old planetary nebula and further observations over the years have shown an asymmetric inner one also. By 2006, the temperature of the star had increased from its first observed 5000 K to 95,000 K, owing to the star leaving the AGB phase and returning to a white dwarf, condensing material and raising temperatures back up. The planetary nebula here is similar in profile to Sakurai’s Object, further fueling the BAG theory. Also supporting the theory is the remaining atmosphere around V605: 40% carbon, 54% helium, 5% oxygen, and the rest trace elements, really no hydrogen to speak of (Asplund, Tafoya 2, Lau 1870-1).
As for what V605 was before its BAG moment, the models point to a common envelope merger scenario. After its outburst, it became a hydrogen defiant giant with a spectral profile similar to an R Coronae Borealis-type star. These are mainly helium stars and often from a merger event. For V605, it could have been an AGB star of 6-8 solar masses in a common envelope with a low-mass main sequence star (Lau 1872).
Located 7000 light-years away, this low mass star was found to have doubled in temperature over a 20 year period and from 1971 to 2002 an overall increase of 40,000 K. This was caused by the ionization of the Stingray planetary nebula surrounding it, with further observations showing the cooling and expanding behavior of both the nebula and the star. This all suggests that the star underwent a helium flash possibly, but more observations are needed to determine if it was indeed the final one (Howell).
This planetary nebula, located 5,500 light-years away, is similar to Sakurai’s Object but has low carbon to oxygen knots at the center and higher than expected neon, implying this event has more in common with an ONeMg nova than that of a BAG. That neon should only have been dredged up from the core, something a final helium flash shouldn’t be capable of doing. This implies either the common envelope merger scenario or the nova scenario as possible explanations. The planetary nebula is about 12,000 years old (Lau 1870, Bauer).
While we have a limited supply of BAG stars known, more will follow. Who knows what further surprises will be out there? Have we found all the possible scenarios? Will we find one close to us for better study? Oftentimes, the most likely surprises are just below the surface…
Asplund, Martin. “A Stellar Swan-Song.” Science, Vol. 308, No. 5719, pg. 210-1.
Bauer, Markus. “Born-again star foreshadows fate of Solar System.” Esa.int. ESA, 15 Nov. 2012. Web. 22 Feb. 2022.
Geballe, T.R. et al. “The Infrared Evolution of Sakurai’s Object.” arXiv:astro-ph/0102043v1 2 Feb 2001
Howell, Elizabeth. “Hubble Telescope Spies Strange ‘Born-Again’ Star After Epic Burn.” Space.com. Future US Inc., 15 Sept. 2016. Web. 22 Feb. 2022.
Lau, Herbert H.B. and Orsola De Marco, X.W. Liu. “V605 Aquilae: a born-again star, a nova, or both?” Mon. Not. R. Astron. Soc. 410, 1870–1876 (2011)
Tafoya, Daniel et al. “First images of the molecular gas around a born-again star revealed by ALMA.” arXiv:2201.04110v1 [astro-ph.GA] 11 Jan 2022.
This content is accurate and true to the best of the author’s knowledge and is not meant to substitute for formal and individualized advice from a qualified professional.
© 2022 Leonard Kelley