The origin of the chemical elements in the Universe is one of the most fascinating and enduring mysteries in astronomy. Progress so far has come from studies of stars, but here only elemental abundances can be determined reliably. Isotopic ratios are more difficult to obtain. Writing in Nature, Schmidt et al.1 study the composition of the young planetary nebula K4-47 — a glowing shell of gas and dust that formed from the outer layer of a Sun-like star and that was thrown off during the final stages of the star’s evolution. The authors find that the nebula is unusually enriched in rare isotopes of carbon (13C), nitrogen (15N) and oxygen (17O). The measured composition of K4-47 shows that this object is more enriched in these isotopes than is almost any other nebula or star examined so far.
Why is Schmidt and colleagues’ finding such a big deal? For one thing, it seems to suggest that stars similar to the Sun can make these rare isotopes — a result that was not expected. Computer simulations2 of Sun-like stars have shown that they can be factories for carbon and nitrogen, but only in the form of the dominant isotopes 12C and 14N. Furthermore, theory2 predicts that the rarer isotopes are not made inside stars that become planetary nebulae. What about direct observations of ageing Sun-like stars, as opposed to planetary nebulae? Such observations are difficult, but the available data3,4 mostly agree with theory, making K4-47 a particularly unusual object.
The only instance in which the isotopes 13C, 15N and 17O are synthesized at the same time is in explosions. CNO cycles are a collection of thermonuclear reactions that involve the capture of protons by isotopes of carbon, nitrogen and oxygen. These reactions are the workhorse of stellar energy production but do not make much 13C, 15N and 17O. Generating these isotopes requires conditions of high temperature and density, as well as plenty of protons. Such a mechanism is known as the hot CNO cycle. So far, the products of the hot CNO cycle have been found only in classical novae5 — nuclear explosions that occur in certain binary star systems.
So how can the presence of the rare isotopes in K4-47 be explained? One mechanism proposed by Schmidt et al. is that the progenitor of K4-47 underwent an explosive event called a helium-shell flash immediately before it became a planetary nebula. This is, in essence, a mixing event that causes hot material from the core of a star, rich in 12C, to be moved to a cooler region, where hydrogen-fusion reactions are occurring. The mixing elevates the temperature of the cooler region, enabling reactions of the hot CNO cycle to proceed before the material is expelled to space.
Although the explosive nature of this scenario is unusual, similar mixing has previously been proposed to explain the composition of other chemically peculiar stars, such as Sakurai’s object6 (also known as V4334 Sagittarii). Detailed computer simulations are needed to test this mechanism. If it can be verified, it will be evidence of previously unknown stellar behaviour that provides insight into how rare isotopes of common elements are generated.
But there are other possible explanations. The isotopic composition of K4-47 is similar to that of J-type carbon stars4, which have ratios of 12C to 13C of less than 15. The sequence of events that lead to a J-type star is unknown, and their existence is not predicted by the theory that describes the evolution of single stars. It has been suggested that J-type stars instead result from binary evolution7, in which two stars orbit each other and interact.
Such interactions have been proposed for all planetary nebulae that, like K4-47, have an hourglass (bipolar) shape and highly collimated outflows of material8,9 (Fig. 1). Observations show that the central stars of planetary nebulae are more likely to be binary stars than was previously thought, giving further credence to this idea. K4-47 could therefore be the product of an interaction or merger between two stars.
Alternatively, K4-47 might not be a planetary nebula at all. It has been speculated that it could be a planetary-nebula mimic, in which the extended nebula was ejected by a pair of interacting binary stars during an explosion10. The isotopic composition of K4-47 could be explained if the interaction of these stars resulted in an explosion akin to a classical nova that would allow for the hot CNO cycle. One prediction of this scenario is that gas would be ejected at high velocities. Has such ejection been observed?
Schmidt and colleagues say they have not seen these high-velocity outflows of material, so they rule out a nova-like explosion as an explanation. But this finding is in contrast to previous studies that have observed high-velocity bullets of material ploughing through the surrounding medium11,12. So who is right? Answering this question will require follow-up observations of K4-47 using astronomical instruments that can extract high-resolution spatial, dynamical and chemical information about the object.
Either way, K4-47, which is rich in the products normally associated with a nova but is embedded in something that looks like a planetary nebula, is one of the most isotopically unusual astronomical objects studied so far (along with CK Vulpeculae13). Detailed computer modelling and follow-up observations are required to tease out the true nature of the progenitor of K4-47. Such work could tell us something about how the rare isotopes of carbon, nitrogen and oxygen are made in stars.
Nature 564, 353-354 (2018)