It is a fabled place, the 鈥�island of stability鈥� at the edge of the periodic table, where superheavy elements are thought to live long lives without decaying. Now, a trio of astrophysicists suggest the place to look for such elements is in certain unusual stars.
Each element has an atomic number corresponding to the number of protons in its nucleus. Most elements heavier than lead, atomic number 82, are unstable and radioactive. But physicists have long speculated that there might be long-lived elements with 114 or more protons in their nuclei.
To create these superheavy elements, we smash atoms together to form larger nuclei. But particle accelerator technology, pushed to its limits, has so far only built atoms that decay in as little as a fraction of a microsecond.
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Stars are natural nuclear reactors, and most heavy elements are forged in supernovae, whose explosions spread them to the next generation of stars. Now, at the University of New South Wales, Australia, and his colleagues think an oddball star called (HD 101065) could be harbouring superheavy elements.
Chemically weird
The star鈥檚 discoverer, Antoni Przybylski, who found it in 1961, saw that it was chemically weird from the get-go. A variable star some 370 light years away in the constellation Centaurus, it has little iron or nickel, but a lot of heavy elements.
It is the only star believed to contain short-lived radioactive elements called actinides, with atomic numbers ranging from 89 to 103, such as actinium, plutonium, americium and einsteinium. Only one other star, HD 25354, has even come close, but its hints of americium and curium are on shakier footing.
It鈥檚 hard to explain how these heavy elements could form there in the first place. One possible explanation was that Przybylski鈥檚 star had a neutron star companion, which could bombard it with particles and create heavy elements in its atmosphere. But no companion was ever found.
鈥淚f such elements are indeed confirmed, it will remain a great challenge for nucleosynthesis models to explain their origin,鈥� says astrophysicist at the Free University of Brussels (ULB).
Decay chain
Dzuba suggests that the actinides are a sign that the predicted island of stability elements exist there, and that actinides are in fact the products of their slow decay. The half-lives are a clue: the observed elements all decay quickly relative to a star鈥檚 lifetime. After millions of years, they ought to be gone unless there is some mechanism to replenish them.
鈥淲e can say that we already have indirect indication,鈥� Dzuba says.
To find out if they are right, Dzuba鈥檚 team suggests searching the star鈥檚 spectrum for five elements with atomic numbers of 102 or more: nobelium (102), lawrencium (103), nihonium (113) and flerovium (114). These could be intermediate steps in the radioactive decay chain between an island of stability element and the actinides.
It sounds simple enough, but it is hard to work out what the spectral signatures of superheavy elements would look like, because their half-lives are so short. Nobelium鈥檚 most stable isotope鈥檚 half-life is under an hour, for instance. That means some spectra aren鈥檛 well defined yet.
鈥淚f and when such [spectral] lines are found, that would be very strong evidence for the existence of the long-living superheavy elements somewhere in our universe,鈥� Dzuba says.
But Goriely doesn鈥檛 think the evidence for actinides is that strong.
鈥淧rzybylski鈥檚 stellar atmosphere is highly magnetic, stratified and chemically peculiar, so that the interpretation of its spectrum remains extremely complex,鈥� he says. 鈥淭he presence of such nuclei remains to be confirmed.鈥�
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