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47-444: In the s -process, a seed nucleus undergoes neutron capture to form an isotope with one higher atomic mass . If the new isotope is stable , a series of increases in mass can occur, but if it is unstable , then beta decay will occur, producing an element of the next higher atomic number . The process is slow (hence the name) in the sense that there is sufficient time for this radioactive decay to occur before another neutron

94-621: A ledge-precipice structure . A series of papers in the 1970s by Donald D. Clayton utilizing an exponentially declining neutron flux as a function of the number of iron seed exposed became the standard model of the s -process and remained so until the details of AGB-star nucleosynthesis became sufficiently advanced that they became a standard model for s -process element formation based on stellar structure models. Important series of measurements of neutron-capture cross sections were reported from Oak Ridge National Lab in 1965 and by Karlsruhe Nuclear Physics Center in 1982 and subsequently, these placed

141-519: A bright red giant with a luminosity ranging up to thousands of times greater than the Sun. Its interior structure is characterized by a central and largely inert core of carbon and oxygen, a shell where helium is undergoing fusion to form carbon (known as helium burning ), another shell where hydrogen is undergoing fusion forming helium (known as hydrogen burning ), and a very large envelope of material of composition similar to main-sequence stars (except in

188-418: A few years. The shell flash causes the star to expand and cool which shuts off the hydrogen shell burning and causes strong convection in the zone between the two shells. When the helium shell burning nears the base of the hydrogen shell, the increased temperature reignites hydrogen fusion and the cycle begins again. The large but brief increase in luminosity from the helium shell flash produces an increase in

235-654: A larger seed-to-neutron or seed-to-proton ratio will tend to produce comparatively lighter masses. This nuclear physics or atomic physics –related article is a stub . You can help Misplaced Pages by expanding it . Asymptotic Giant Branch The asymptotic giant branch (AGB) is a region of the Hertzsprung–Russell diagram populated by evolved cool luminous stars . This is a period of stellar evolution undertaken by all low- to intermediate-mass stars (about 0.5 to 8 solar masses ) late in their lives. Observationally, an asymptotic-giant-branch star will appear as

282-476: A thermal pulse occurs and the star quickly returns to the AGB, becoming a helium-burning, hydrogen-deficient stellar object. If the star still has a hydrogen-burning shell when this thermal pulse occurs, it is termed a "late thermal pulse". Otherwise it is called a "very late thermal pulse". The outer atmosphere of the born-again star develops a stellar wind and the star once more follows an evolutionary track across

329-445: A transition to the more massive supergiant stars that undergo full fusion of elements heavier than helium. During the triple-alpha process , some elements heavier than carbon are also produced: mostly oxygen, but also some magnesium, neon, and even heavier elements. Super-AGB stars develop partially degenerate carbon–oxygen cores that are large enough to ignite carbon in a flash analogous to the earlier helium flash. The second dredge-up

376-424: Is a maximum value since the wind material will start to mix with the interstellar medium at very large radii, and it also assumes that there is no velocity difference between the star and the interstellar gas . These envelopes have a dynamic and interesting chemistry , much of which is difficult to reproduce in a laboratory environment because of the low densities involved. The nature of the chemical reactions in

423-473: Is almost aligned with its previous red-giant track, hence the name asymptotic giant branch , although the star will become more luminous on the AGB than it did at the tip of the red-giant branch. Stars at this stage of stellar evolution are known as AGB stars. The AGB phase is divided into two parts, the early AGB (E-AGB) and the thermally pulsing AGB (TP-AGB). During the E-AGB phase, the main source of energy

470-465: Is an isotope that is the starting point for any of a variety of fusion chain reactions . The mix of nuclei produced at the conclusion of the chain reaction generally depends strongly on the relative availability of the seed nucleus or nuclei and the component being fused—whether neutrons as in the r-process and s-process or protons as in the rp-process . A smaller proportion of seed nuclei will generally result in products of larger mass , whereas

517-450: Is captured. A series of these reactions produces stable isotopes by moving along the valley of beta-decay stable isobars in the table of nuclides . A range of elements and isotopes can be produced by the s -process, because of the intervention of alpha decay steps along the reaction chain. The relative abundances of elements and isotopes produced depends on the source of the neutrons and how their flux changes over time. Each branch of

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564-474: Is helium fusion in a shell around a core consisting mostly of carbon and oxygen . During this phase, the star swells up to giant proportions to become a red giant again. The star's radius may become as large as one astronomical unit (~215  R ☉ ). After the helium shell runs out of fuel, the TP-AGB starts. Now the star derives its energy from fusion of hydrogen in a thin shell, which restricts

611-470: Is somewhat different from that which was previously assumed. It has also been shown with trapped isotopes of krypton and xenon that the s -process abundances in the AGB-star atmospheres changed with time or from star to star, presumably with the strength of neutron flux in that star or perhaps the temperature. This is a frontier of s -process studies in the 2000s. Seed nucleus A seed nucleus

658-506: Is very strong in this mass range and that keeps the core size below the level required for burning of neon as occurs in higher-mass supergiants. The size of the thermal pulses and third dredge-ups are reduced compared to lower-mass stars, while the frequency of the thermal pulses increases dramatically. Some super-AGB stars may explode as an electron capture supernova, but most will end as oxygen–neon white dwarfs. Since these stars are much more common than higher-mass supergiants, they could form

705-423: The Hertzsprung–Russell diagram . However, this phase is very brief, lasting only about 200 years before the star again heads toward the white dwarf stage. Observationally, this late thermal pulse phase appears almost identical to a Wolf–Rayet star in the midst of its own planetary nebula . Stars such as Sakurai's Object and FG Sagittae are being observed as they rapidly evolve through this phase. Mapping

752-434: The nuclear shell model , are particularly stable nuclei, much like the noble gases are chemically inert . This implied that some abundant nuclei must be created by slow neutron capture , and it was only a matter of determining how other nuclei could be accounted for by such a process. A table apportioning the heavy isotopes between s -process and r -process was published in the famous BFH review paper in 1957. There it

799-475: The photosphere of the stars which are 2,000 – 3,000 K . Chemical peculiarities of an AGB CSE outwards include: The dichotomy between oxygen -rich and carbon -rich stars has an initial role in determining whether the first condensates are oxides or carbides, since the least abundant of these two elements will likely remain in the gas phase as CO x . In the dust formation zone, refractory elements and compounds ( Fe , Si , MgO , etc.) are removed from

846-488: The s -process moves up the elements in the chart of isotopes to higher mass numbers is essentially determined by the degree to which the star in question is able to produce neutrons . The quantitative yield is also proportional to the amount of iron in the star's initial abundance distribution. Iron is the "starting material" (or seed) for this neutron capture-beta minus decay sequence of synthesizing new elements. The main neutron source reactions are: One distinguishes

893-483: The s -process on the firm quantitative basis that it enjoys today. The s -process is believed to occur mostly in asymptotic giant branch stars, seeded by iron nuclei left by a supernova during a previous generation of stars. In contrast to the r -process which is believed to occur over time scales of seconds in explosive environments, the s -process is believed to occur over time scales of thousands of years, passing decades between neutron captures. The extent to which

940-418: The s -process path. This approximation is – as the name indicates – only valid locally, meaning for isotopes of nearby mass numbers, but it is invalid at magic numbers where the ledge-precipice structure dominates. Because of the relatively low neutron fluxes expected to occur during the s -process (on the order of 10 to 10 neutrons per cm per second), this process does not have the ability to produce any of

987-412: The s -process reaction chain eventually terminates at a cycle involving lead , bismuth , and polonium . The s -process contrasts with the r -process , in which successive neutron captures are rapid : they happen more quickly than the beta decay can occur. The r -process dominates in environments with higher fluxes of free neutrons ; it produces heavier elements and more neutron-rich isotopes than

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1034-456: The s -process. Together the two processes account for most of the relative abundance of chemical elements heavier than iron. The s -process was seen to be needed from the relative abundances of isotopes of heavy elements and from a newly published table of abundances by Hans Suess and Harold Urey in 1956. Among other things, these data showed abundance peaks for strontium , barium , and lead , which, according to quantum mechanics and

1081-493: The AGB phase. The mass-loss rates typically range between 10 and 10 M ⊙ year , and can even reach as high as 10 M ⊙ year ; while wind velocities are typically between 5 and 30 km/s. The extensive mass loss of AGB stars means that they are surrounded by an extended circumstellar envelope (CSE). Given a mean AGB lifetime of one Myr and an outer velocity of 10  km/s , its maximum radius can be estimated to be roughly 3 × 10   km (30 light years ). This

1128-454: The AGB stars are the main site of the s -process in the galaxy, the heavy elements in the SiC grains contain almost pure s -process isotopes in elements heavier than iron. This fact has been demonstrated repeatedly by sputtering-ion mass spectrometer studies of these stardust presolar grains . Several surprising results have shown that within them the ratio of s -process and r -process abundances

1175-557: The case of carbon stars ). When a star exhausts the supply of hydrogen by nuclear fusion processes in its core, the core contracts and its temperature increases, causing the outer layers of the star to expand and cool. The star becomes a red giant, following a track towards the upper-right hand corner of the HR diagram. Eventually, once the temperature in the core has reached approximately 3 × 10   K , helium burning (fusion of helium nuclei) begins. The onset of helium burning in

1222-399: The circumstellar magnetic fields of thermal-pulsating (TP-) AGB stars has recently been reported using the so-called Goldreich-Kylafis effect . Stars close to the upper mass limit to still qualify as AGB stars show some peculiar properties and have been dubbed super-AGB stars. They have masses above 7  M ☉ and up to 9 or 10  M ☉ (or more ). They represent

1269-442: The core halts the star's cooling and increase in luminosity, and the star instead moves down and leftwards in the HR diagram. This is the horizontal branch (for population II stars ) or a blue loop for stars more massive than about 2.3  M ☉ . After the completion of helium burning in the core, the star again moves to the right and upwards on the diagram, cooling and expanding as its luminosity increases. Its path

1316-415: The core regions remain, they evolve further into short-lived protoplanetary nebula . The final fate of the AGB envelopes are represented by planetary nebulae (PNe). Physical samples, known as presolar grains, of mineral grains from AGB stars are available for laboratory analysis in the form of individual refractory presolar grains . These formed in the circumstellar dust envelopes and were transported to

1363-405: The cycle: The net result of this cycle therefore is that 4 neutrons are converted into one alpha particle , two electrons , two anti-electron neutrinos and gamma radiation : The process thus terminates in bismuth, the heaviest "stable" element, and polonium, the first non-primordial element after bismuth. Bismuth is actually slightly radioactive, but with a half-life so long—a billion times

1410-496: The dust no longer completely shields the envelope from interstellar UV radiation and the gas becomes partially ionized. These ions then participate in reactions with neutral atoms and molecules. Finally as the envelope merges with the interstellar medium, most of the molecules are destroyed by UV radiation. The temperature of the CSE is determined by heating and cooling properties of the gas and dust, but drops with radial distance from

1457-409: The early Solar System by stellar wind . A majority of presolar silicon carbide grains have their origin in 1–3 M ☉ carbon stars in the late thermally-pulsing AGB phase of their stellar evolution. As many as a quarter of all post-AGB stars undergo what is dubbed a "born-again" episode. The carbon–oxygen core is now surrounded by helium with an outer shell of hydrogen. If the helium is re-ignited

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1504-502: The end of helium - and carbon-burning in massive stars. It employs primarily the Ne neutron source. These stars will become supernovae at their demise and spew those s -process isotopes into interstellar gas. The s -process is sometimes approximated over a small mass region using the so-called "local approximation", by which the ratio of abundances is inversely proportional to the ratio of neutron-capture cross-sections for nearby isotopes on

1551-418: The envelope changes as the material moves away from the star, expands and cools. Near the star the envelope density is high enough that reactions approach thermodynamic equilibrium. As the material passes beyond about 5 × 10   km the density falls to the point where kinetics , rather than thermodynamics, becomes the dominant feature. Some energetically favorable reactions can no longer take place in

1598-484: The first few, so third dredge-ups are generally the deepest and most likely to circulate core material to the surface. AGB stars are typically long-period variables , and suffer mass loss in the form of a stellar wind . For M-type AGB stars, the stellar winds are most efficiently driven by micron-sized grains. Thermal pulses produce periods of even higher mass loss and may result in detached shells of circumstellar material. A star may lose 50 to 70% of its mass during

1645-414: The formation of carbon stars . All dredge-ups following thermal pulses are referred to as third dredge-ups, after the first dredge-up, which occurs on the red-giant branch, and the second dredge up, which occurs during the E-AGB. In some cases there may not be a second dredge-up but dredge-ups following thermal pulses will still be called a third dredge-up. Thermal pulses increase rapidly in strength after

1692-788: The gas phase and end up in dust grains . The newly formed dust will immediately assist in surface catalyzed reactions . The stellar winds from AGB stars are sites of cosmic dust formation, and are believed to be the main production sites of dust in the universe. The stellar winds of AGB stars ( Mira variables and OH/IR stars ) are also often the site of maser emission . The molecules that account for this are SiO , H 2 O , OH , HCN , and SiS . SiO, H 2 O, and OH masers are typically found in oxygen-rich M-type AGB stars such as R Cassiopeiae and U Orionis , while HCN and SiS masers are generally found in carbon stars such as IRC +10216 . S-type stars with masers are uncommon. After these stars have lost nearly all of their envelopes, and only

1739-409: The gas, because the reaction mechanism requires a third body to remove the energy released when a chemical bond is formed. In this region many of the reactions that do take place involve radicals such as OH (in oxygen rich envelopes) or CN (in the envelopes surrounding carbon stars). In the outermost region of the envelope, beyond about 5 × 10   km , the density drops to the point where

1786-533: The grain. First experimental detection of s -process xenon isotopes was made in 1978, confirming earlier predictions that s -process isotopes would be enriched, nearly pure, in stardust from red giant stars. These discoveries launched new insight into astrophysics and into the origin of meteorites in the Solar System. Silicon carbide (SiC) grains condense in the atmospheres of AGB stars and thus trap isotopic abundance ratios as they existed in that star. Because

1833-469: The heavy radioactive isotopes such as thorium or uranium . The cycle that terminates the s -process is: Bi captures a neutron, producing Bi , which decays to Po by β decay . Po in turn decays to Pb by α decay : Pb then captures three neutrons, producing Pb , which decays to Bi by β decay, restarting

1880-405: The inner helium shell to a very thin layer and prevents it fusing stably. However, over periods of 10,000 to 100,000 years, helium from the hydrogen shell burning builds up and eventually the helium shell ignites explosively, a process known as a helium shell flash . The power of the shell flash peaks at thousands of times the observed luminosity of the star, but decreases exponentially over just

1927-475: The main and the weak s -process component. The main component produces heavy elements beyond Sr and Y , and up to Pb in the lowest metallicity stars. The production sites of the main component are low-mass asymptotic giant branch stars. The main component relies on the C neutron source above. The weak component of the s -process, on the other hand, synthesizes s -process isotopes of elements from iron group seed nuclei to Fe on up to Sr and Y, and takes place at

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1974-415: The nuclear fusion in the deep interior of the star that provides its power. A calculable model for creating the heavy isotopes from iron seed nuclei in a time-dependent manner was not provided until 1961. That work showed that the large overabundances of barium observed by astronomers in certain red-giant stars could be created from iron seed nuclei if the total neutron flux (number of neutrons per unit area)

2021-414: The present age of the universe—that it is effectively stable over the lifetime of any existing star. Polonium-210 , however, decays with a half-life of 138 days to stable lead-206 . Stardust is one component of cosmic dust . Stardust is individual solid grains that condensed during mass loss from various long-dead stars. Stardust existed throughout interstellar gas before the birth of the Solar System and

2068-553: The visible brightness of the star of a few tenths of a magnitude for several hundred years. These changes are unrelated to the brightness variations on periods of tens to hundreds of days that are common in this type of star. During the thermal pulses, which last only a few hundred years, material from the core region may be mixed into the outer layers, changing the surface composition, in a process referred to as dredge-up . Because of this dredge-up, AGB stars may show S-process elements in their spectra and strong dredge-ups can lead to

2115-444: Was also argued that the s -process occurs in red giant stars. In a particularly illustrative case, the element technetium , whose longest half-life is 4.2 million years, had been discovered in s-, M-, and N-type stars in 1952 by Paul W. Merrill . Since these stars were thought to be billions of years old, the presence of technetium in their outer atmospheres was taken as evidence of its recent creation there, probably unconnected with

2162-445: Was appropriate. It also showed that no one single value for neutron flux could account for the observed s -process abundances, but that a wide range is required. The numbers of iron seed nuclei that were exposed to a given flux must decrease as the flux becomes stronger. This work also showed that the curve of the product of neutron-capture cross section times abundance is not a smoothly falling curve, as BFH had sketched, but rather has

2209-457: Was trapped in meteorites when they assembled from interstellar matter contained in the planetary accretion disk in early Solar System. Today they are found in meteorites, where they have been preserved. Meteoriticists habitually refer to them as presolar grains . The s -process enriched grains are mostly silicon carbide (SiC). The origin of these grains is demonstrated by laboratory measurements of extremely unusual isotopic abundance ratios within

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