National honors

• Fellow, American Academy of Arts and Sciences
• NASA Exceptional Scientific Achievement Medal (1992)
• Leonard Medal of the Meteoritical Society (1991)
• NASA Public Service Group Achievement Award for the Oriented Scintillation Spectrometer Team on NASA's Compton Gamma Ray Observatory (1992)
• Jesse Beams Medal of the American Physical Society (1998)
• South Carolina Governor's Award for Excellence in Science (1994)
• Alexander von Humboldt Award (1977 and 1982) sponsored by Max Planck Institut für Kernphysik, Heidelberg
• Selection for one of the 50 most influential research papers of the 20th century by American Astronomical Society and inclusion in the AAS Centennial Volume

Clayton was elected to Phi Beta Kappa during his third year as a student at Southern Methodist University. He was awarded many supporting fellowships: National Science Foundation Predoctoral Fellow (1956-58); Alfred P. Sloan Foundation Fellow (1966-68); Fulbright Fellow (1979-80); Fellow of St. Mary's College, Durham University (1987); SERC Senior Visiting Fellow, The Open University, Milton Keynes, U.K. (1993). In 1993 Clayton was named Distinguished Alumnus of Southern Methodist University, 37 years after his BS degree there.

Early life and education

Donald D. Clayton was born on March 18, 1935 in a modest rented duplex on Walnut Street in Shenandoah, Iowa while his parents were there seeking work during the Great Depression. They were temporarily away from both family farms near Fontanelle in Adair County in southwestern Iowa, farms to which they often returned. Clayton spent much childhood on those farms and has rhapsodized over his love of the farm. Young Clayton attended public school in Texas, however, after his father's new job moved the family to Dallas in 1940 as co-pilot for Braniff Airlines. Fortunately, his parents obtained a home in the already renowned Highland Park School System, providing him an excellent education. His description of his advantage from High School is revealing. He graduated third in his 1953 class of 92 students from Highland Park High School. The first among his entire Iowa relations, including his father, to seek any post-high-school education, Clayton matriculated and excelled in physics and mathematics, graduating from Southern Methodist University summa cum laude in 1956.

At his professor's urging he was accepted as a physics research student by California Institute of Technology (Caltech), which he attended bearing a National Science Foundation Predoctoral Fellowship. In the 1957 nuclear physics course at Caltech Clayton learned from William Alfred Fowler about a new theory that the chemical elements had been assembled within the stars by nuclear reactions occurring there. He was captivated for life by that idea Clayton completed his Ph.D. Thesis in 1961 on the evolution of the abundances of the elements owing to their slow capture of free neutrons (the s process) in stars. Clayton and his wife Mary Lou played a small role in producing the celebrated Feynman Lectures on Physics by converting the taped audio of Richard Feynman's lectures to prose. Caltech also afforded Clayton the chance to meet and later become a lifelong friend of Fred Hoyle, British cosmologist and creator of the nuclear theory of nucleosynthesis in stars. Collaborations with Fowler (1983 Nobel Laureate in Physics) as Fowler's research student (1957-60) and as Fowler's post-doctoral research associate (1961-63) launched Clayton's scientific career.

He established himself as a post-Hoyle leader of nucleosynthesis in stars by calculating the first time-dependent models of both the s-process and the fast neutron-capture chains (r-process) of heavy-element nucleosynthesis and of the nuclear abundance quasiequilibrium that establishes the highly radioactive abundances between silicon and nickel during silicon burning in stars. He came onto the field early, at a time when nucleosynthesis was a vibrant, modern frontier. Citations are in the Nucleosynthesis section below.

Academic history

Donald Clayton's academic history is lengthy. The year 2016 concludes his sixth decade of postgraduate research. It includes seminal contributions to five fields of astrophysics (detailed below). Its international breadth is exemplified by his seven-year affiliations both in Cambridge and later in Heidelberg and by visiting summer positions in Cardiff UK (1976, 1977) and by sabbatical leave in Durham University UK (1987). Following a two-year (1961-63) postdoctoral research fellowship at Caltech, Clayton claimed in 1963 an Assistant Professorship as one of the four founding faculty members in Rice University's newly created Department of Space Science (later renamed Space Physics and Astronomy). There he initiated a graduate-student course explaining nuclear reactions in stars as the mechanism for the creation of the atoms of the chemical elements. His textbook based on that course (Principles of Stellar Evolution and Nucleosynthesis, McGraw-Hill 1968) earned ongoing praise. In 2016, 48 years after its first publication, it is still in common usage in graduate education throughout the world. Clayton was awarded the newly endowed Andrew Hays Buchanan Professorship of Astrophysics at Rice University in 1968 and held that endowed professorship for twenty years until transferring to Clemson University in 1989. At Rice University in the 1970s, Clayton guided Ph.D. theses of many students who achieved renown, especially Stanford E. Woosley, William Michael Howard, H. C. Goldwire, Richard A. Ward, Michael J. Newman, Eliahu Dwek, Mark Leising and Kurt Liffman. Senior thesis students at Rice University included Bradley S. Meyer and Lucy Ziurys, both of whom forged distinguished careers in the subjects of those senior theses. Historical photos of several students can be seen on Clayton' s photo archive for the history of nuclear astrophysics.

1966 letters from W.A. Fowler unexpectedly invited Clayton to return to Caltech in order to coauthor a book on nucleosynthesis with Fowler and Fred Hoyle. In his autobiography Clayton quotes these letters. He accepted; but while resident at Caltech Clayton was invited by Hoyle to Cambridge University (UK) in spring 1967 to advise a research program in nucleosynthesis at Hoyle's newly created Institute of Astronomy. The award to Clayton of a prestigious Alfred P. Sloan Foundation Fellowship (1966-68) facilitated leaves of absence from Rice University for this purpose. Clayton exerted that research leadership in Cambridge during 1967-72 by bringing his research students from Rice University with him. That prolific period ended abruptly by Hoyle's unexpected resignation in 1972. Clayton was during these years a Visiting Fellow of Clare Hall. At Rice University W.D. Arnett, S.E. Woosley, and W.M. Howard published jointly numerous innovative studies with Clayton on the topic of explosive supernova nucleosynthesis. During his Cambridge years, Clayton proposed radioactive gamma-ray-emitting nuclei as nucleosynthesis targets for the field of gamma-ray astronomy of line transitions from radioactive nuclei with coauthors (Stirling Colgate, Gerald J. Fishman, and Joseph Silk). Detection of these gamma-ray lines two decades later provided the decisive proof that iron had been synthesized explosively in supernovae in the form of radioactive nickel isotopes rather than as iron itself as Fowler and Hoyle had both advocated. That paper was designated one of the fifty most influential papers in astronomy during the twentieth century by the American Astronomical Society.

During a subsequent seven-year period (1977-84) Clayton resided about 1/3 time sponsored by Till Kirsten at the Max Planck Institute for Nuclear Physics in Heidelberg as Humboldt Prize awardee. He did so with annual academic leaves from Rice University. There he joined the Meteoritical Society seeking audience for his newly published theoretical picture of a new type of isotopic astronomy based on the relative abundances of the isotopes of the chemical elements within interstellar dust grains. He hoped that such interstellar grains could be discovered within meteorites; and he also advanced a related theory that he called cosmic chemical memory by which the effects of stardust can be measured in meteoritic minerals even if stardust itself no longer exists there. Clayton designated the crystalline component of interstellar dust that had condensed thermally from hot and cooling stellar gases by a new scientific name, stardust. Stardust became an important component of cosmic dust. Clayton has described the stiff resistance encountered from meteoriticist referees of his early papers advancing this new theory. He nonetheless established that research program at Rice University, where he continued guiding graduate-student research on that topic. He and student Kurt Liffman computed a pathbreaking history of survival rates of refractory stardust in the interstellar medium; and with student Mark D. Leising computed a propagation model of positron annihilation lines emitted from nova explosions and of the angular distribution of gamma ray lines from radioactive 26Al in the galaxy. Following laboratory discovery in 1987 of meteoritic stardust bearing unequivocal isotopic markers of stars, Clayton was awarded the 1991 Leonard Medal, the highest honor of the Meteoritical Society, sixteen years after his refereeing battles over his papers on stardust. Feeling vindicated, Clayton exulted in Nature "the human race holds solid samples of supernovae in its hands and studies them in terrestrial laboratories".

In 1989 Clayton surprised academia by accepting a professorship at Clemson University to develop a graduate research program in astrophysics there. This academic segment of his career (1989-present) began by hiring three young astrophysicists to vitalize joint research with the Compton Gamma Ray Observatory (launched in 1991 after several delays), whose instruments successfully detected gamma-ray lines identifying several of the radioactive nuclei that Clayton had predicted to be present in supernova remnants. At Rice University Clayton had been named ten years earlier Co-Investigator on the NASA proposal submitted by James Kurfess for the Oriented Scintillation Spectrometer Experiment OSSE, one of the four successful instruments carried into orbit by Space Shuttle Atlantis, and he carried that research contract to Clemson. Simultaneously Clayton developed at Clemson his stardust research, introducing annual workshops for its researchers. The initial NASA sponsored workshop at Clemson University in 1990 was so lively that it was repeated the following year jointly with Washington University (St. Louis) cosponsorship, and in later years cosponsored also by the University of Chicago and by the Carnegie Institution of Washington. These workshops featured the excitement of new isotopic discoveries, but also helped participants focus their ideas for submission of abstracts to NASA's Lunar and Planetary Science Conference. Otherwise participants' workshop discussions were not shared or publicized. Eventually a unique new goal became to assemble from Clayton's large personal collection of photographs a web-based archive for the history of nuclear astrophysics and to donate the originals to the Center for the History of Physics. The thrusts of Clayton's career at Clemson University are well represented on that Photo Archive by photos between 1990 and 2014. Following his retirement from academic duties in 2007, Clayton remained quite active in research problems involving condensation of dust within supernovae and has also published a scientific autobiography, Catch a Falling Star. Clayton's published refereed research papers are listed at http://claytonstarcatcher.com/files/documents/JournalPub.pdf

Personal

Clayton married three times, first in 1954 in Dallas to Mary Lou Keesee (deceased 1981, Houston) while they were students at SMU; second, in 1972 in St. Blasien he married in Germany a young German woman, Annette Hildebrand, while residing in Heidelberg (divorced 1981, Houston). Clayton remarried in 1983 in the Rice University Chapel the former Nancy Eileen McBride, who was trained in art and in architecture and is today an artist.

Clayton's life as Assistant Professor at Rice University 1963-66 was devoted to buying a family home, schooling for his children, and to building his academic experience and credentials. He was promoted through the academic ranks at Rice, until 1989 when he transferred to a professorship at Clemson University in South Carolina, residing today with Nancy in historic G. W. Gignilliat House (1898) in Seneca, South Carolina (pop. 8,000), seven miles from the city of Clemson. They jointly have one son (Andrew), born in 1987 in Houston. Clayton's three previous children arose from his earlier marriages. A son (Donald Douglas Clayton b.1960, Pasadena CA) lives in Houston and a daughter (Alia Clayton Fisher b.1977, Houston) lives with her husband and four children in Longmont, Colorado. Another son, Devon Clayton (b. 1961 Pasadena), died in 1996 in Seneca SC. Clayton has one brother and two sisters, each still resident in Texas, two of them also born in Iowa. Clayton's mother was born on a farm in Fontanelle IA to parents (Kembery and Keisel) that had lived their entire lives on Fontanelle farms. Their own parents had immigrated to Iowa near 1850, one from England (Thomas Kembery) and one from Germany (William Keisel). Clayton's father was also born on a Fontanelle farm to English parents (Paul Clayton and Verna Porter) having one Dutch grandparent (Yerkes). Two of Clayton's great grandfathers (Kembery and Clayton) fought in the Civil War (North). Robert M. Clayton fought in Sherman's Army at the battle of Atlanta.

In 1969 at Rice University Clayton was introduced by patron of the arts Dominique de Menil to Italian filmmaker Roberto Rosselini, and they conceived of a film about one scientist's deepening realizations during a cosmological life, a sequence of experiences which Clayton proposed to provide for that project. In summer 1970 he spent two weeks in Rome working daily with Rosselini on that effort, which failed owing to insufficient financial support or to insufficiently theatrical plan. Clayton's published memoir "The Dark Night Sky: a personal adventure in cosmology" laid out his plan for that film.

Citations of research leadership

Clayton's research innovations in astrophysics and planetary science lay in the following five disciplines. Clayton's own history of each within his autobiography, Catch a Falling Star, is given at the ends of the following sections. The references in each are to noteworthy seminal published papers by Clayton (names of his co-authors can be read from his publication list). Clayton's independent style can be seen from his publication list, being author of an unusual 120 single-author research papers, the latest in 2013. Sole-author research papers are relatively rare in astrophysics.

Nuclear physics origin of the chemical elements (Nucleosynthesis)

Trained as a nuclear physicist by Wm. A. Fowler at Caltech, Clayton was well positioned to consider the interactions of heavy nuclei with neutrons which were believed by Fowler to govern the nucleosynthesis of nuclei heavier than iron. He calculated the isotopic abundances produced by neutron irradiation of iron in stars for both the S-process and the R-process of heavy-element stellar nucleosynthesis (processes first defined by B2FH). Clayton's two papers (1961 and 1965) demonstrated that solar-system abundances had been created not by a single neutron irradiation but as superpositions of abundance patterns established in multiple presolar stars by differing neutron irradiations. His 1961 calculations, achieved prior to adequate computers by mathematical analysis, established Clayton as a nucleosynthesis theorist. They provided standard models that guided four decades of progress on the s process. In 1967 Clayton turned to the supernova origin of the abundances of elements that can be created in stars from only their hydrogen and helium. Those so-called primary nucleosynthesis nuclei having atomic weights between silicon and nickel (A=28-62) are very abundant. To understand their dramatic alternating abundances he tested a new conceptual tool that he named nuclear quasiequilibrium during silicon burning. That concept explained the observed numbers of isotopes in the A=28-62 mass range, which had previously been unsolved. Nuclear quasiequilibrium was the grandest advance in theory of primary nucleosynthesis in supernovae since Hoyle's 1954 paper, whose focus it validated. Of extreme importance was its demonstration that supernova silicon burning should become profoundly radioactive because rapid quasiequilibrium between atomic weights A=44-62 is overwhelmingly of radioactive nuclei. Clayton's 2016 new description in terms of a secondary supernova machine of this important process with B. S. Meyer clarified that the intense radioactivity resulted from loading excess Coulomb energy into those nuclei.

Abundant radioactivity was Clayton's most important discovery for astronomy because it defined the appearance of supernovae. Quasiequilibrium demanded that even the mountain-like abundance peak at iron was synthesized as radioactive nickel parents 56Ni and 57Ni in the supernovae explosions rather than as iron directly. This discovery intensified Clayton's long and productive focus with radioactive isotopes ejected from supernovae, leading to his predictions of both gamma-ray line astronomy and of radioactive supernova grains condensed from hot supernova gases. Experimental confirmation two decades later of both predictions spurred those new fields of astronomy and brought Clayton high honors. At Rice University a prolific 1970-74 with colleagues W. David Arnett, Stanford E. Woosley and W.Michael Howard explored other explosive nucleosynthesis caused by the radially outgoing supernova shock wave. Leadership of nucleosynthesis had shifted by 1975 from Hoyle in Cambridge and Fowler at Caltech to Rice University.

During 1967-72 Clayton resided half time in Cambridge U.K. at Hoyle's invitation to import and advise nucleosynthesis research at Hoyle's newly constructed Institute of Theoretical Astronomy. Clayton did this by bringing his graduate students at Rice with him to Cambridge. Afterwards, Hoyle made three research visits with Clayton at Rice University. After Clayton's 1989 move to Clemson University, his research with Bradley S. Meyer showed how the uniquely puzzling 48Ca isotope of calcium had become so abundant in the Galaxy owing to a relatively rare form of Type Ia supernovae in which the appropriate neutron-enriched quasiiequilibrium nucleosynthesis occurs. They later explained why the minor 95Mo and 97Mo isotopes of the element molybdenum had become dominant in supernovae stardust, explaining an experimental riddle in stardust isotopic abundances.

Clayton began in 2000 a spirited prose description of isotopic nucleosynthesis in order to increase its accessibility to public and to scientists conducting isotopic analyses of stardust. Becoming increasingly disappointed that Hoyle's theory of primary nucleosynthesis in massive stars was being overlooked and forgotten after he fell into science disfavor over his views on interstellar biology, Clayton published two historical papers reestablishing community consciousness of Hoyle's pioneering achievement. See chapters 7, 9 and 18 of Clayton's life in Catch a Falling Star.

Gamma-ray-line astronomy of radioactive nuclei in supernovae

Clayton, Colgate and Fishman's 1969 prediction that motivated pursuing gamma-ray-line astronomy as an empirical test of supernova nucleosynthesis was recognized in the American Astronomical Society Centennial Volume as one of the 50 most influential astrophysics papers of the 20th century. Observational discovery of those gamma rays would later confirm explosive nucleosynthesis theory and cement mankind's understanding of the profoundly radioactive nature of supernovae. It is the innovation for which Clayton is best known. His NASA-funded research at Rice University during the 1970s sought after additional nuclear prospects for that high-energy spectroscopic astronomy, which is based on the recognizable energies of gamma rays emitted by individual radioactive nuclei that had recently been ejected from supernovae. Today it has blossomed with many observational results after quickly becoming a goal for future space astronomy missions, especially at a time when Compton Gamma Ray Observatory was being proposed to NASA in 1977 (launched by Space Shuttle Atlantis in 1991). Hopes were suddenly raised for a detectable source when in 1987 optical astronomers discovered a nearby supernova called SN1987A in the Large Magellanic Cloud. Clayton described those hopes from his 1987 sabbatical-year office at Durham University UK as a mounting excitement generated by observed X-ray emission from its supernova surface. His research with L-S The augmented understanding of those hard X-rays and their derivation from the radioactivity gamma rays permeating supernova interiors. Supernova 1987A gamma-ray-line emission did yield exciting first detections of those gamma-ray lines from 56Co and from 57Co (by OSSE with Clayton a coauthor) thereby establishing this field of astronomy. CGRO, the space gamma-ray telescope mission that detected several predicted gamma-ray lines, was the second mission of NASA’s Great Observatories program.

In 1977 at Rice University Clayton had been named Co-Investigator for the NASA-approved proposal for the OSSE spectrometer on CGRO, and in 1982 he summarized physical expectations for several gamma-ray-line emitting young nuclei. Key to the intense supernova radioactivity had been Clayton's 1967 discovery that rapid-silicon-burning was dominated by abundances of radioactive alpha-particle nuclei (those having equal numbers of protons and neutrons). Clayton has quipped that SN explosions are "the largest nuclear accidents of all time". Supernova 1987A ejected 20,000 times the mass of the earth as pure radioactive 56Ni nuclei! Abundant iron of our world was demonstrated to be a daughter of radioactive nickel, the most important of the radioactive nuclei. Modern studies of supernovae are dominated by their intensely radioactive natures. Spacetime data for cosmology relies on 56Ni radioactivity providing the energy for the optical brightness of supernovae of Type Ia, which are the "standard candles" of cosmology but whose diagnostic 847keV and 1238keV gamma rays were first detected only in 2014, fully 47 years after Clayton's prediction of their emission by supernovae. Clayton's work earned for him NASA's 1992 Exceptional Scientific Achievement Award and in the same year the NASA Public Service Group Achievement Award for the OSSE Spectrometer on CGRO. Both the OSSE instrument and the Comptel instrument confirmed predictions. Clayton had previously attempted to establish gamma-ray-line astronomy from r process radioactive nuclei; but r-process nuclei are much less abundant in supernovae than are the nuclei fused during silicon-burning. So it was the latter that became the demonstrated source of radioactive nuclei. Chapters 8, 11, 17 and 18 in Catch a Falling Star, whose title Clayton has said he chose as an allusion to the gravitational core collapse that triggers these supernovae.

Astronomy of Stardust

Clayton introduced the idea that the relative abundances of the isotopes in tiny solid grains embedded in gases leaving stars may be observable. He named these solids stardust, a component of interstellar Cosmic dust. Stardust inherits its unusual isotopic compositions from the evolved nuclear compositions of the host stars within which they condensed. Clayton's disclosures began with isotopic excesses in supernova dust owing to decays of abundant short-lived radioactive nuclei, but were generalized to all stellar mass loss in 1978. Clayton predicted novel isotopic abundance ratios to be expected in stardust, which he described as being ubiquitous among the interstellar dust grains. These papers initially encountered such incredulity in the field of cosmochemistry that most were rejected first and published later; nonetheless, R.W. Walker and E. Zinner at Washington University undertook instrumental development that might prove capable of measuring isotope ratios in such tiny solids. Almost two decades of experimental search were required before intact stardust grains, (also called presolar grains by some meteoriticists), were successfully isolated from the vast remainder of other presolar dust particles. These tiny grains were successfully extracted from meteorites and their isotopes counted by precision laboratory technique of secondary ion mass spectrometry (SIMS). Those dramatic experimental discoveries in the 1990s, led primarily by Ernst Zinner (1937-2015) and his colleagues at Washington University (St. Louis) confirmed the stunning reality of this new astronomy; namely, solid particles that condensed within stellar gases long before the earth was created are today handled in laboratories on earth. These tiny stones are quite literally solid pieces of long dead stars. This was revolutionary. The discovery experiments dispelled skepticism surrounding Clayton's predictions, causing him to be awarded the 1991 Leonard Medal of the Meteoritical Society. Main modern themes of this solid-state astronomical science have been summarized in 2004 by Clayton & Nittler. To debate the meanings of the frequent new discoveries, Clayton initiated in 1990 at Clemson University an annual series of workshops cosponsored by NASA and planned jointly with Ernst Zinner and his colleagues at Washington University (St. Louis), where presolar stardust particles were being documented in the laboratory by SIMS Clayton continued in the interpretation of stardust for three decades after his founding ideas. Noterworthy was his interpretation of the puzzling silicon isotope ratios found in the presolar Asymptotic giant branch stars, which demonstrably have been the donor stars of the known presolar mainstream silicon carbide stardust grains to the interstellar solar birth cloud, as having arisen from a galactic merger of Milky Way interstellar gas with the interstellar gas from a smaller captured satellite galaxy possessing a lower gaseous isotopic abundance ratio for 30Si28Si owing to its lesser degree of galactic abundance evolution. That picture audaciously claimed that the merger of a small satellite galaxy with the Milky Way (a large-scale event) can be seen within microscopic interstellar grains of sand. Chapters 14 and 15 and pages 504-508 in Catch a Falling Star

Galactic abundance evolution of radioactive nuclei

Clayton created new tools for calculating the interstellar abundances of radioactive nuclei in the Galaxy. In 1964 he discovered a new method for measuring the age of interstellar nuclei based on the larger than expected observed abundances of stable daughters of radioactive nuclei. The decays of rhenium-187 to osmium-187 and of uranium and thorium to three differing isotopes of lead (Pb) defined the cosmoradiogenic chronologies. Merging his method with an earlier method based only on the abundances of uranium and thorium themselves still did not yield a precise galactic age, however. Clayton wrote that the discord arose from inadequate treatments of both the history of star formation in the Galaxy and of the rate of infall of pristine metal-free gas onto the young Milky Way, compounded by a prevailing but erroneous technique for computation of the radioactive abundances within interstellar gas. Clayton reasoned that interstellar gas contains higher concentrations of shorter-lived radioactive nuclei than do the stars. With that insight he invented in 1985 new mathematical solutions for the simplified differential equation of galactic abundance evolution that for the first time rendered these relationships transparent, ending decades of confusion. Clayton calculated an age of 13-15 billion years for the oldest galactic nuclei, which would necessarily be approximately equal to the age of our galaxy. More recently radioactive cosmochronology has diminished in importance because more accurate techniques for determining the age of the Milky Way have been discovered in the microwave background; but agreement confirmed correctness of his treatment of radioactivity in astronomy. His analytic mathematical models demonstrated that the concentration of short-lived radioactive nuclei in interstellar gas had routinely been underestimated by the factor 1/(k+1), where k is an integer near 2 or 3 that measures the steepness of the rate of decline of the infall of pristine gas onto our galaxy. In a parallel scientific development the identities and initial abundances of shorter-lived radioactive nuclei that remained alive at variable levels within the interstellar gas cloud that formed the early solar system, but which are now extinct, has grown in importance with more experimental discoveries of such nuclei within the meteorites. These are called extinct radioactivities because none remain on earth today. Simultaneous solution for their abundances became the guiding principle for a new discipline of galactic abundance evolution that focuses on nucleosynthesis near the solar interstellar cloud during the billion years preceding solar birth. In 1983, at a time when astrophysicists were relying on only a uniform model of a well mixed interstellar gas, Clayton introduced a new aspect of the ISM that has proven to be essential for understanding the abundances of the extinct radioactivities; namely the time required for isotopic mixing between freshly synthesized atoms ejected from supernovae with distinct physical phases of interstellar gas. He showed that owing to those time delays allowing more interstellar decay of radioactive nuclei, each phase of interstellar gas contains a distinctly different concentration of each of the extinct radioactive nuclides; but that the early solar system radioactivities measured specifically only abundances in the dense molecular-cloud phase in which the solar system had been born. In the 21st century many researchers have begun to present their own calculations of the effect of interstellar inter-phase mixing, often unaware of Clayton's (1983) paper owing to intervening decades. These aspects of interstellar phase mixing will remain important for decades to come while astronomers probe the circumstances of solar birth using accurate meteoritic data revealing the abundances of the extinct radioactive nuclei. Clayton gave emphasis to extinct radioactivity in the Glossary of his 2003 book on isotopes in the cosmos. Chapters 16 and 17 of "Catch a Falling Star".

Condensation of carbon solids from oxygen-rich supernova gas

Clayton leaped on a crossroad for supernova chemistry that occurred in 1995 when Liu and Dalgarno showed that radioactivity would prevent the total oxidation of carbon atoms during expansion and cooling of the post-nucleosynthesis supernova remnants. Using that clue Clayton began in 1998 an energetic crusade to argue that the vast reservoir of carbon in core-collapse supernovae must then condense as carbon dust despite its nucleosynthesis bathed in more-abundant oxygen gas. He advocated that supernova carbon stardust (which in 1977 he had named SUNOCONs (acronym for SUperNOva CONdensates) could have assembled within hot supernova C+O gases containing more oxygen than carbon. Meteoritic chemists to whom his 1998-99 LPSC papers were addressed strenuously doubted that possibility on intuitive but erroneous chemical grounds, believing that abundant hot oxygen gas would oxidize all carbon atoms leaving them trapped within chemically inert CO molecules. Clayton asserted in his papers that this incorrect chemical rule-of-thumb was dominating interpretive studies of carbon SUNOCONs (primarily SiC grains and graphite grains). He spearheaded that new research field for eighteen years (1998-2016) by emphasizing how copius energetic electrons produced by scattering of gamma rays emitted by radioactive cobalt continuously replenish the abundance of free carbon atoms in the supernova interior by breaking apart those abundant CO molecules. Those free carbon atoms enable carbon-chain molecules to continuously capture carbon atoms until they become macroscopic grains of carbon. He summarized his new picture in a 2011 review paper, advancing new rules for carbon condensation in oxygen-rich supernovae gases. The kinetic-chemical-reaction model underlying all of these works was initially devised by Clayton, Weihong Liu and Alexander Dalgarno and later expanded by Clayton and his colleagues at Clemson. Their works showed that very large dust grains (micrometers in radius) in comparison with average interstellar-medium dust sizes can grow within the expanding oxygen-rich supernova interior owing to the principle of Population Control. According to that principle rapid oxidation abets large-grain carbon condensation by keeping the population of carbon solids small so that those few can grow large by accreting the continuously replenished free carbon. This topic establishes another new aspect of carbon's uniquely versatile chemistry. Chapter 18 of Catch a Falling Star

[ Source: Wikipedia ]