Perspectives

Abstract

It is more than a century now since the phenomenon of radioactivity has been discovered. As noticed by D. D. Clayton in his introduction to Chap. 2, E. Rutherford was the first to point out that radioactive elements were produced in the Cosmos: In 1929 he had suggested that the long lived actinides uranium and thorium were synthesised in the Sun about 100 Myr ago and were somehow transported to Earth. It took another 20 years before another scientist suggested that radioactivity occurs now in the Universe and has observable effects: in 1950, L. Borst noticed that the exponential decrease of the luminosity of Type I supernovae (with a characteristic timescale of 55 days) is a strong indication for a radioactive source. But he misidentified that source with ⁷Be-decay. Two years later, P. Merrill identified absorption lines of short-lived technetium in the spectra of S-stars, a clear indication that radioactivity (and nucleosynthesis) occurs in the Universe today.

It is more than a century now since the phenomenon of radioactivity has been discovered. Since then, astronomical observations unravelled a fascinating trace of compositional evolution of cosmic matter, as seen in stellar atmospheres, interstellar gas, meteorites, and in the composition of matter here on Earth. Astrophysical studies established models for sources of new atomic nuclei, such as stars and their explosions, where nuclear reactions can occur. Isotopes are the fundamental sources of information about the cosmic compositional evolution, and radioactive isotopes add a natural clock. A basic understanding has been set up about the cosmic cycle of matter and nucleosynthesis in cosmic places and on cosmic time scales in the later half of the past century. Now, multiple disciplines of astronomies and astrophysics need to play together and help, so that the striking deficits we still have can be worked on. This will neither be cheap nor easy, but rewarding. Nuclear astrophysics and astronomy will have to be part of this.

Since then, the phenomenon of radioactivity evolved to a major astrophysical tool, allowing astronomers, among many other things, to

• determine the ages of stars and of our Galaxy

• probe the physical processes occurring deep inside supernova envelopes

• infer the current rate of nucleosynthesis in the Milky Way

• understand (or, at least, constrain) the timescales of the acceleration of Galactic cosmic rays and of their propagation through the interstellar medium

• infer the physical conditions inside the stars producing some of the extinct radioactivities found in meteoritic dust grains

• constrain the environment of the proto-solar nebula and the activity of the young Sun

Radioactivity is the decay of unstable nuclei, therefore it is intimately related to nuclear astrophysics and stellar nucleosynthesis, i.e. the production of such nuclei in various cosmic environments. Obviously, the steady progress in our understanding of stellar structure, evolution and explosions, and of nuclear reactions in stellar conditions, was necessary in order to turn radioactivity into a powerful astrophysical tool.

The current state of the art in modelling nucleosynthesis in single stars of low and high mass, as well as in stellar explosions occurring in binary systems, is presented in Chaps. 3, 4 and 5 of this book, respectively. Present day models are much more sophisticated (including, e.g., mass loss, stellar rotation, neutrino interactions for all flavors, general-relativistic treatment) than their earlier counterparts. Then we believe that their results are, presumably, closer to reality. Despite their sophistication, however, such models can hardly be considered as realistic, since key ingredients, such as convection, mass loss and stellar rotation, neutrino directional interactions, and magnetic field effects, can only be treated in a parametrised way at present; this is also the case for models of both core collapse and thermonuclear supernovae, and even more so in rare transients such as the collisions of neutron stars or even black holes. The exploration of geometrical (3D) effects has only recently been established for pioneering studies, and will without doubt bring more surprises; dealing with the large variety of possible initial conditions and sub-grid model approximations will challenge computing power, and leave a role for simpler model implementations. Interesting results have also been reported on the effects of the stellar magnetic field. As one example, at least, for positron escape from explosions such as SNIa, or from relativistic jets such as microquasars, such effects are known to play a critical role.

The book’s Chap. 6 briefly describes the exciting aspects of our Sun and the solar system, which have been connected to measurements and insights on radioactive isotopes all along. The Sun has taught us to retain modesty and critical views: Neutrino measurements were in striking conflict with solar nuclear reaction models, and could only be reconciled through the newly-established phenomena of neutrino flavors and neutrinos with non-zero mass. Refined (3D) models of the solar photosphere have significantly reduced metallicity in our cosmic standard for elemental abundances. Chemical evolution descriptions have explored numerically the chances for a special nearby nucleosynthesis event making our Solar System special, with a tendency to favor a broader cosmic variance rather than very special conditions for the Sun. These recent insights tell us that it is prudent scientific endeavor to question and consolidate seemingly-obvious details, before taking giant leaps and declare physics and astrophysics “understood”. The early history of the solar system is now again debated with great enthusiasm, making use of more precise observations and theoretical models of radioactivity signatures in our solar system bodies, the only place where we can study cosmic materials directly. We open the curtain to cosmic chemical evolution with measurements of ⁶⁰Fe and ²⁴⁴Pu on Earth, and with ²⁶Al from nearby stars helping us to understand the local interstellar environment. Detail matters.

This book’s Chap. 7 then is a mixture of complex astrophysical problems and observational constraints which come into play once single objects are put into a greater context of a galaxy. The interstellar medium connects stellar sources across cosmic time intervals, in its transport properties for kinetic energy and matter. Our models for these processes are first-order and simple, still. They help us to explain the coarse aspects of galaxy evolution. But again, more detailed astrophysical understanding will be required for a realistic model of how stars and supernovae feed back their newly-produced isotopes and their violence from stellar winds and explosions into next generations of stars to form, and to carry on the cosmic cycle of matter. For this reason, a new Chap. 11 now addresses our contextual knowledge of our own Galaxy, and our description of chemical evolution within a galaxy and across cosmic times, as an educational chapter (see below). All great themes of current astrophysics necessarily are involved here: Nucleosynthesis yields for entire populations of source types, their occurrence rates over a galaxy’s evolution, the feedback, mixing, and transport processes across the many degrees of freedom for energy. Observing electromagnetic radiation from cosmic objects is a great tool to study distant physical processes. Its limitations become obvious in view of the complexities of a galaxy’s evolution.

The book’s Chaps. 8–12 provide a glimpse of the variety of tools which are involved in today’s astrophysical work; we concentrate here on tools which most-directly relate to cosmic radioactivities. Nuclear physics made great strides to establish the concepts of nature as atomic nuclei are held together and interact; in recent years, much attention turned towards nuclear-structure details which matter in cosmic environments, and involves experiments with radioactive beams or targets. This field evolves, and returns again its focus more towards fundamental science, away from the power and weapons applications of earlier years, as demonstrated in Chap. 9. Supernovae still challenge even most-advanced modern computing facilities, from their huge dynamic ranges in space and time domains. Chapter 8 gives a snapshot of how physics and mathematics ideas are employed to obtain computer simulations which are approaching reality, within such technical limitations; this is a prominent example of complex-systems study. The instruments to obtain measurements from cosmic radioactivities then are highlighted in Chap. 10. We face very different categories of equipment, from space telescopes for penetrating gamma-rays through cosmic-ray probes and to sophisticated laboratory mass spectroscopy of minute samples of cosmic material extracted from meteorites. Each of these experimental fields is in the hands of small groups of experts, only a handful of laboratories world-wide working in each of these fields. Advances are very sensitive to specific conditions under which such small laboratories operate in their countries. Neither large science communities nor large industrial applications support technological evolutions here, rather the passion of experimental physicists is the driver of progress. Then, we add a chapter on chemical evolution, describing how best we can formulate an evolution with time of the composition of cosmic matter, putting into this all the complex models described in Part II of the book, and linking it to the observations described in Part III.

While the theoretical insights of D.D. Clayton laid the foundations of astronomy with radioactivities in the 1960s, the field of γ-ray line astronomy was mainly driven by observations. The discovery of the 1.8 MeV γ-ray line of ²⁶Al came somewhat as a surprise (as discussed in Chap. 2), and the one of the 511 keV line (the first γ-ray line originating from outside the solar system that was ever detected) was also unexpected. Equally unexpected was the detection of the ⁵⁶Co lines from SN1987A about 6 months earlier than predicted from spherically symmetric models of the supernova explosion. It was the improved angular resolution of the COMPTEL imaging gamma-ray telescope within a large field of view and the multi-year CGRO mission that made possible the identification of massive stars as major ²⁶Al sources in the 1990s. Similarly, the improved spectral capabilities of SPI/INTREGRAL (and its energy range, including the few-hundred keV range) set the stage for a first clear measurement of the decay γ rays from the ⁵⁶Ni decay chain in a thermonuclear supernova in 2014, with 2014J being sufficiently nearby. INTEGRAL’s extended mission also allowed astronomers to perform the first reliable all-sky map of the Galactic 511 keV emission, which revealed and consolidated its surprisingly large intensity ratio of the Galaxy’s bulge to its disk—the current puzzle both for cosmic-ray propagation and for Galactic positron sources. Detecting fainter emission requires mastering an inherent large instrumental background, and building up experience over a longer mission helps to discriminate systematic effects from the statistical uncertainty that always accompany signals at an instrument’s sensitivity threshold. Finally, an X-ray telescope played a major role for advancing supernova knowledge: NuSTAR’s mirrors work well up to 80 keV, and so include the low-energy lines from ⁴⁴Ti decay. So, we obtained a unique image of radioactivity emission in a 350-year-old supernova remnant, Cas A. Together with tracking the evolution of SN1987A, these two objects likely will drive our understanding of the final explosions from massive-star evolution.

It is somewhat risky, though unavoidable, to rely on few but well-observed events towards understanding an astrophysical source. Variety often reveals interesting physical processes that were overlooked at first. The gravitational-wave event/gamma-ray burst GW170817 again illustrates this clearly: For the first time, a neutron star binary collision has been witnessed, and observed in great detail due to its proximity. But, still too distant for nuclear-line observations from radioactivity, assumptions need to be made on the nucleosynthesis and the ejection of isotopes that we have not had a chance to measure at all, far from the nuclear stability regime. Such events will remain rare, and a lively discussion of what that means for the enhancement of cosmic matter through the r-process is ongoing, and will remain with us for a while.

All these discoveries boosted an intense activity both in theoretical astrophysics and in nuclear laboratory experiments in the past four decades, generating hundreds of papers on various aspects of cosmic radioactivities. Also the cosmic environment of the Sun and the important diagnostics of radioactivities received a boost in the past decade, from more precise measurements, and not least from the discovery of ⁶⁰Fe radioactivity on Earth and in cosmic rays near Earth.

Compared to other fields in astrophysics, the ratio of theoretical implications to observational data has been extremely high in the case of γ-ray line astronomy. It should be noticed that some of these implications were totally unforseen by the pioneers of the field. For instance, long-lived radioactivities, such as ²⁶Al, can be used not only to probe the interior physics of their stellar sources, but also the physics of the Galaxy at large: star formation rate, locations of high-mass star forming sites, distribution of supernova ejecta out of the Galactic plane, etc. Similarly, the properties of positron annihilation emission can help to probe the physics of cosmic ray propagation and the interstellar medium in a new way, or the configuration of the galactic magnetic field (see Chap. 7).

One might think then that such a prolific field can only have a bright future. Laboratory equipment for the analysis of meteoritic inclusions and their isotopic abundances proceeds to ever smaller grains and precision. However, the scarcity of the astronomical data from gamma-ray studies tells a different story. Ideas and concepts promise to dig deeper into the nuclear universe, yet by modest increments, compared to other disciplines of astronomy. It will take a considerable effort to increase the number of sources where our theories can be challenged and expanded; even an increase by a modest factor of a few will require a multi-national space mission. No mission dedicated to γ-ray line astronomy is on the horizon at present, although technically, advances in sensitivity by almost two orders of magnitude have been demonstrated in lab studies (see Chap. 10). Current excitement in the astrophysical community focuses on questions of cosmology and dark components of the universe, and on consolidations of the grounds for new astronomy through gravitational waves, neutrinos, and highest-energy γ-rays and cosmic rays. Proposed nuclear-gamma-ray telescope projects may stimulate more excitement and support in new space programs only beyond the next decade, given a 10–15 year lead time for any such space mission. Other isotopic information may become available from superb resolutions now reaching into isotopic line shifts for molecular lines in the radio regime with ALMA, and even for atomic lines in the optical. X-ray lines may help to constrain elemental abundances in hot gas of the intergalactic medium, in addition to the already fruitful explorations of hot ISM in supernova remnants, and, for very few cases, even from nuclear transitions. It is clear that a considerable effort will be required in all those fields before robust theoretical predictions become available on the yields of various radioactivities. These will remain to be verified most-directly by the intensities of the corresponding γ-ray lines from radioactive decay. Valuable new insights on cosmic radioactivities may derive, as spin-offs from other fields, such as high-resolution spectroscopy resolving isotopic information, or astro-particle advances on cosmic ray details, or solid-state research connected to materials science.

Similarly, significant effort is required in theoretical and associated fields of physics: Refined descriptions of chemical evolution accounting for various galaxy components, a better understanding of nuclear structure derived from nuclear reaction experiments and theories, and more realistic models of stellar explosions. All these elements are needed in order to improve our view and theoretical understanding of the cosmic sources of new isotopes which have been detected up to now. This book aims to help students of astrophysics better understand the role of cosmic radioactivities in relation to their specific interests. It will hopefully also help build up resources and excitement to further advance our understanding of the origins of the cosmic elements—one of the big questions agreed upon in the science community.