The contribution of explosions known as novae to the lithium content of the Milky Way is uncertain. Radioactive beryllium, which transforms into lithium, has been detected for the first time in one such explosion. See Letter p.381
The origin of lithium observed in today's Universe is a long-standing problem. It is known that a fraction of this light chemical element was created during the Big Bang, along with hydrogen and helium, and that another fraction has formed since then through nuclear reactions induced by energetic cosmic rays. But comparison of chemical-evolution models and observed stellar lithium abundances in the Milky Way indicates that part of the lithium should also have been synthesized in old low-mass stars, such as red giants, and in stellar explosions known as novae. However, although lithium has been observed in giants, its detection in novae has remained elusive. On page 381 of this issue, Tajitsu et al.1 provide the first observational evidence of lithium synthesis in novae. The authors detected radioactive beryllium-7 (7Be), the parent nucleus of lithium-7 (7Li), during a nova explosion called V339 Del (Nova Delphini 2013).
It has long been known that almost all of the chemical elements are produced in stars by the nuclear fusion of light elements into heavier ones, starting with hydrogen fusion2. The synthesized elements can then be expelled to the interstellar medium — from which new stars will form — either by stellar winds or during supernova explosions and their dimmer relatives, novae. However, the main origin of the light elements lithium, beryllium and boron is not linked to nuclear reactions in stars. Instead, it is related to nucleosynthesis processes that are less efficient than stellar ones. This is why these elements are much less abundant in the Milky Way and the Solar System than heavier elements.
Lithium has a complex origin. It is produced in three ways: by nucleosynthesis during the Big Bang; by nuclear reactions in the interstellar medium that are induced by energetic cosmic rays and are also responsible for the origin of beryllium and boron; and by nuclear reactions in stellar sources, such as red giants3. The stellar sources are required to reproduce the rise of lithium abundance in the Milky Way after the formation of the Solar System about 4.5 billion years ago.
Inside stars, 7Be, the subject of Tajitsu and colleagues' study, is formed by the fusion of helium-3 and helium-4. This radioactive element then captures an electron and transforms into its daughter nucleus, 7Li, within a short timescale (7Be has a half-life of 53.22 days), releasing a 478-kiloelectronvolt-energy photon (Fig. 1). But efficient production of 7Li requires this nuclear reaction to occur in hot, external stellar layers, and requires freshly produced 7Be to be transported into cooler subsurface layers before it transforms into 7Li. In this way, 7Li is immune to destruction once it is created. This process, known as the Cameron–Fowler 7Be transport mechanism, is responsible for 7Li production in stars4,5.
Novae are thermonuclear explosions, and take place on top of white dwarfs that pull hydrogen-rich material from a companion star. As more hydrogen accumulates on the white dwarf, it builds up a shell that reaches pressures and temperatures sufficient to trigger explosive runaway fusion of the hydrogen. This leads to the fast expansion and subsequent ejection of the white dwarf's outer layers, and is accompanied by a sudden large increase in the star's brightness. During this process, 7Li is thought to be produced through the Cameron–Fowler 7Be transport mechanism.
The first studies of lithium production in novae were made in the 1970s6,7, but it was not until 1996 that the details of the process were pinned down8. It was realized that the initial chemical composition of the white dwarf that undergoes a nova was a crucial determinant of the amount of 7Li synthesized in the explosion; depending on the mass of its progenitor star, the white dwarf is made of either carbon and oxygen (CO novae) or oxygen and neon (ONe novae).
In CO novae, the carbon content makes the hydrogen fusion proceed faster than in ONe novae, owing to the operation of the CNO cycle of fusion reactions. Such faster evolution prevents the destruction of 3He and 7Be (Fig. 1), and so results in a larger production of 7Be and 7Li. The amount of 7Li produced by a CO nova corresponds to about 10−10 of the Sun's mass, but this value largely depends on the total ejected mass.
In their study, Tajitsu et al. report the detection of highly blue-shifted absorption lines of the singly ionized radioactive isotope of 7Be, 7Be II, in the near-ultraviolet spectra of the CO classical nova V339 Del, between 38 and 52 days after the explosion. The spectra were obtained using the Subaru Telescope of the National Astronomical Observatory of Japan, which delivers high spectral resolution (about 0.0052 nanometres) and so allowed the authors to tease apart the lines of 7Be II from those of 9Be II, both of which occur at wavelengths around 312–313 nm.
The finding lends support to the hypothesis that the Cameron–Fowler 7Be transport mechanism is at work in novae, as predicted theoretically 40 years ago6. The observations indicate that nova V339 Del produced at least as much 7Be and 7Li as predicted by theory.
The implications of these results are manifold. First, they mean that novae may play a larger part in lithium production than previously thought. Second, they may increase the probability of detecting the 478-keV γ-ray photons emitted in the 7Be-to-7Li reaction9, which have remained elusive despite observational efforts made by γ-ray missions10,11. Third, and perhaps most importantly, they suggest that measurements of 7Be lines in the near-ultraviolet range and within the lifetime of the element may well provide a way of estimating the contribution of novae to the lithium abundance in the Milky Way and in the Universe in its entirety.Footnote 1
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Deciphering the Local Interstellar Spectra of Secondary Nuclei with the Galprop/Helmod Framework and a Hint for Primary Lithium in Cosmic Rays
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