S-process In Massive Stars Research Articles

The Stellar 72Ge(n, γ) Cross Section for weak s-process: A First Measurement at n_TOF

The slow neutron capture process (s-process) is responsible for producing about half of the elemental abundances heavier than iron in the universe. Neutron capture cross sections on stable isotopes are a key nuclear physics input for s-process studies. The 72Ge(n, γ) Maxwellian-Averaged Cross Section (MACS) has an important influence on the production of isotopes between Ge and Zr in the weak s-process in massive stars and so far only theoretical estimations are available. An experiment was carried out at the neutron time-of-flight facility n_TOF at CERN to measure the 72Ge(n, γ) reaction for the first time at stellar neutron energies. The capture measurement was performed using an enriched 72GeO2 sample at a flight path length of 184 m, which provided high neutron energy resolution. The prompt gamma rays produced after neutron capture were detected with a set of liquid scintillation detectors (C6D6). The neutron capture yield is derived from the counting spectra taking into account the neutron flux and the gamma-ray detection efficiency using the Pulse Height Weighting Technique. Over 70 new neutron resonances were identified, providing an improved resolved reaction cross section to calculate experimental MACS values for the first time. The experiment, data analysis and the new MACS results will be presented including their impact on stellar nucleosynthesis, which was investigated using the post-processing nucleosynthesis code mppnp for a 25 solar mass model.

Thermonuclear 17O(n, γ)18O Reaction Rate and Its Astrophysical Implications

Abstract A new thermonuclear 17O(n,γ)18O rate is derived based on a complete calculation of the direct-capture (DC) and resonant-capture contributions, for a temperature region up to 2 GK of astrophysical interest. We have first calculated the DC and subthreshold contributions in the energy region up to 1 MeV, and estimated the associated uncertainties by a Monte Carlo approach. It shows that the present rate is remarkably larger than that adopted in the JINA REACLIB in the temperature region of 0.01 ∼ 2 GK, by up to a factor of ∼80. The astrophysical impacts of our rate have been examined in both s-process and r-process models. In our main s-process model, which simulates flash-driven convective mixing in metal-deficient asymptotic giant branch stars, both 18O and 19F abundances in interpulse phases are enhanced dramatically by factors of ∼20–40 due to the new larger 17O(n,γ)18O rate. It shows, however, that this reaction hardly affects the weak s-process in massive stars since the 17O abundance never becomes significantly large in the massive stars. For the r-process nucleosynthesis, we have studied impacts of our rate in both the collapsar and neutron burst models, and found that the effect can be neglected, although an interesting loophole effect is found owing to the enhanced new rate, which significantly changes the final nuclear abundances if fission recycling is not involved in the model; however, these significant differences are almost completely eliminated if the fission recycling is considered.

Constraints on stellar rotation from the evolution of Sr and Ba in the Galactic halo

ABSTRACT Recent studies show that the chemical evolution of Sr and Ba in the Galaxy can be explained if different production sites, hosting r- and s-processes, are taken into account. However, the question of unambiguously identifying these sites is still unsolved. Massive stars are shown to play an important role in the production of s-material if rotation is considered. In this work, we study in detail the contribution of rotating massive stars to the production of Sr and Ba, in order to explain their chemical evolution, but also to constrain the rotational behaviour of massive stars. A stochastic chemical evolution model was employed to reproduce the enrichment of the Galactic halo. We developed new methods for model-data comparison which help to objectively compare the stochastic results to the observations. We employed these methods to estimate the value of free parameters which describe the rotation of massive stars, assumed to be dependent on the stellar metallicity. We constrain the parameters using the observations for Sr and Ba. Employing these parameters for rotating massive stars in our stochastic model, we are able to correctly reproduce the chemical evolution of Sr and Ba, but also Y, Zr, and La. The data supports a decrease of both the mean rotational velocities and their dispersion with increasing metallicity. Our results show that a metallicity-dependent rotation is a necessary assumption to explain the s-process in massive stars. Our novel methods of model-data comparison represent a promising tool for future galactic chemical evolution studies.

The contribution from rotating massive stars to the enrichment in Sr and Ba of the Milky Way

Abstract Most neutron capture elements have a double production by r- and s-processes, but the question of production sites is complex and still open. Recent studies show that including stellar rotation can have a deep impact on nucleosynthesis. We studied the evolution of Sr and Ba in the Milky Way. A chemical evolution model was employed to reproduce the Galactic enrichment. We tested two different nucleosynthesis prescriptions for s-process in massive stars, adopted from the Geneva group and the Rome group. Rotation was taken into account, studying the effects of stars without rotation or rotating with different velocities. We also tested different production sites for the r-process: magneto rotational driven supernovae and neutron star mergers. The evolution of the abundances of Sr and Ba is well reproduced. The comparison with the the most recent observations shows that stellar rotation is a good assumption, but excessive velocities result in overproduction of these elements. In particular, the predicted evolution of the [Sr/Ba] ratio at low metallicity does not explain the data at best if rotation is not included. Adopting different rotational velocities for different stellar mass and metallicity better explains the observed trends. Despite the differences between the two sets of adopted stellar models, both show a better agreement with the data assuming an increase of rotational velocity toward low metallicity. Assuming different r-process sources does not alter this conclusion.

Non-standard s-process in massive rotating stars

Context. Recent studies show that rotation significantly affects the s-process in massive stars. Aims. We provide tables of yields for non-rotating and rotating massive stars between 10 and 150 M⊙ at Z = 10−3 ([Fe/H] = −1.8). Tables for different mass cuts are provided. The complete s-process is followed during the whole evolution with a network of 737 isotopes, from hydrogen to polonium. Methods. A grid of stellar models with initial masses of 10, 15, 20, 25, 40, 60, 85, 120, and 150 M⊙ and with an initial rotation rate of both 0% or 40% of the critical velocity was computed. Three extra models were computed in order to investigate the effect of faster rotation (70% of the critical velocity) and of a lower 17O(α, γ) reaction rate. Results. At the considered metallicity, rotation has a strong impact on the production of s-elements for initial masses between 20 and 60 M⊙. In this range, the first s-process peak is boosted by 2−3 dex if rotation is included. Above 60 M⊙, s-element yields of rotating and non-rotating models are similar. Increasing the initial rotation from 40% to 70% of the critical velocity enhances the production of 40 ≲ Z ≲ 60 elements by ∼0.5−1 dex. Adopting a reasonably lower 17O(α, γ) rate in the fast-rotating model (70% of the critical velocity) boosts again the yields of s-elements with 55 ≲ Z ≲ 82 by about 1 dex. In particular, a modest amount of Pb is produced. Together with s-elements, some light elements (particularly fluorine) are strongly overproduced in rotating models.

Sensitivity to neutron captures and β-decays of the enhanced s-process in rotating massive stars at low metallicities

The s-process in massive stars, producing nuclei up to A ≈ 90, has a different behaviour at low metallicity if stellar rotation is significant. This enhanced s-process is distinct from the s-process in massive stars around solar metallicity, and details of the nucleosynthesis are poorly known. We investigated nuclear physics uncertainties in the enhanced s-process in metalpoor stars within a Monte-Carlo framework. We applied temperature-dependent uncertainties of reaction rates, distinguishing contributions from the ground state and from excited states. We found that the final abundance of several isotopes shows uncertainties larger than a factor of 2, mostly due to the neutron capture uncertainties. A few nuclei around branching points are affected by uncertainties in the β-decay.

First Measurement of 72Ge(n, γ) at n_TOF

The slow neutron capture process (s-process) is responsible for producing about half of the elemental abundances heavier than iron in the universe. Neutron capture cross sections on stable isotopes are a key nuclear physics input for s-process studies. The 72Ge(n, γ) cross section has an important influence on production of isotopes between Ge and Zr during s-process in massive stars and therefore experimental data are urgently required. 72Ge(n, γ) was measured at the neutron time-of-flight facility n_TOF (CERN) for the first time at stellar energies. The measurement was performed using an enriched 72GeO2 sample at a flight path of 185m with a set of liquid scintillation detectors (C6D6). The motivation, experiment and current status of the data analysis are reported.

High-precision abundances of Sc, Mn, Cu, and Ba in solar twins

A previous study of correlations between element abundance ratios, [X/Fe], and ages of solar twin stars is extended to include Sc, Mn, Cu, and Ba. HARPS spectra with S/N > 600 are used to derive very precise (+/- 0.01 dex) differential abundances, and stellar ages with internal errors less than 1 Gyr are obtained by interpolation in the logg - Teff diagram between isochrones calculated with the Aarhus Stellar Evolution Code. For stars younger than 6 Gyr, [X/Fe] is tightly correlated with stellar age for all elements. For ages between 6 and 9 Gyr, the [X/Fe] - age correlations break down and the stars split up into two groups having respectively high and low [X/Fe] for the odd-Z elements. It is concluded that while stars in the solar neighborhood younger than about 6 Gyr were formed from interstellar gas with a smooth chemical evolution, older stars have originated from regions enriched by supernovae with different neutron excesses. Furthermore, the correlations between abundance ratios and stellar age suggest that: i) Sc is made in Type II supernovae along with the alpha-capture elements, ii) the Type II to Ia SNe yield ratio is about the same for Mn and Fe, iii) Cu is mainly made by the weak s-process in massive stars, iv) the Ba/Y yield ratio for AGB stars increases with decreasing stellar mass, v) [Y/Mg] and [Y/Al] can be used as "chemical clocks" when determining ages of solar metallicity stars.

SYNTHESIS OF THE S-ONLY 122,123,124TE ISOTOPES AND THE SELECTIVE DEPLETION OF 123TE BY ELECTRON CAPTURE PROCESS IN MASSIVE STARS

ABSTRACT We investigate the production ratios of the naturally occurring s-only nuclides 122 − 124 ?> Te by solving a nuclear network in the mass number 120 ≤ A ≤ 124 ?> region under some conditions representing those in the helium–hydrogen inter-shell in low-mass AGB stars as well as those in the helium burning and the carbon burning phases in massive stars. We show that the electron-capture process on 123Te (the natural half-life > 10 16 ?> years) could proceed decisively fast during the late evolutionary phases of massive stars. We thus re-iterate and re-enforce the decades-old notion that detailed analysis of the relative 122 − 124 ?> Te abundances in due consideration of the selective 123Te depletion in massive stars would set a stringent constraint on the s-process modelings. We stress that the relative abundance ratios of 122 − 124 ?> Te in the solar system become quite difficult to decipher if the s-process in massive stars contributes significantly to the synthesis of nuclides in this intermediate A region.

CHEMISTRY OF THE SAGITTARIUS DWARF GALAXY: A TOP-LIGHT INITIAL MASS FUNCTION, OUTFLOWS, AND THER-PROCESS

From chemical abundance analysis of stars in the Sagittarius dwarf spheroidal galaxy (Sgr), we conclude that the alpha-element deficiencies cannot be due to the Type Ia supernova (SNIa) time-delay scenario of Tinsley (1979). Instead, the evidence points to low [alpha/Fe] ratios resulting from an initial mass function (IMF) deficient in the highest mass stars. The critical evidence is the 0.4 dex deficiency of [O/Fe], [Mg/Fe] and other hydrostatic elements, contrasting with the normal trend of r-process [Eu/Fe]r with [Fe/H]. Supporting evidence comes from the hydrostatic element (O, Mg, Na, Al, Cu) [X/Fe] ratios, which are inconsistent with iron added to the Milky Way (MW) disk trends. Also, the ratio of hydrostatic to explosive (Si, Ca, Ti) element abundances suggests a relatively top-light IMF. Abundance similarities with the LMC, Fornax and IC 1613, suggest that their alpha-element deficiencies also resulted from IMFs lacking the most massive SNII. For such a top-light IMF, the normal trend of r-process [Eu/Fe]r with [Fe/H], as seen in Sgr, indicates that massive Type II supernovae (>30Msun) cannot be major sources of r-process elements. High [La/Y] ratios, consistent with leaky-box chemical evolution, are confirmed but ~0.3 dex larger than theoretical AGB predictions. This may be due to the 13C pocket mass, or a difference between MW and Sgr AGB stars. Sgr has the lowest [Rb/Zr] ratios known, consistent with low-mass (~2Msun) AGB stars near [Fe/H]=-0.6, likely resulting from leaky-box chemical evolution. The [Cu/O] trend in Sgr and the MW suggest that Cu yields increase with both metallicity and stellar mass, as expected from Cu production by the weak s-process in massive stars. Finally, we present an updated hfs line list, an abundance analysis of Arcturus, and further develop our error analysis formalism.

Effects of a New Triple-α Reaction on the s-Process in Massive Stars

Effects of a new triple-{\alpha} reaction rate on the s-process during the evolution of a massive star of 25 M\odot are investigated for the first time. Although the s-process in massive stars has been believed to be established with only minor change, we find that the s-process with use of the new rate during the core He-burning is very inefficient compared to the case with the previous triple-{\alpha} rate. However, the difference of the overproduction is found to be largely compensated by the subsequent C-burning. Since the s-process in massive stars has been attributed so far to the neutron irradiation during core He-burning, our finding reveals the importance of C-burning for the s-process during the evolution of massive stars.

THE WEAKs-PROCESS IN MASSIVE STARS AND ITS DEPENDENCE ON THE NEUTRON CAPTURE CROSS SECTIONS

The slow neutron capture process in massive stars (weak s process) produces most of the s-process isotopes between iron and strontium. Neutrons are provided by the 22Ne(?,n)25Mg reaction, which is activated at the end of the convective He-burning core and in the subsequent convective C-burning shell. The s-process-rich material in the supernova ejecta carries the signature of these two phases. In the past years, new measurements of neutron capture cross sections of isotopes beyond iron significantly changed the predicted weak s-process distribution. The reason is that the variation of the Maxwellian-averaged cross sections (MACS) is propagated to heavier isotopes along the s path. In the light of these results, we present updated nucleosynthesis calculations for a 25 M ? star of Population I (solar metallicity) in convective He-burning core and convective C-burning shell conditions. In comparison with previous simulations based on the Bao et?al. compilation, the new measurement of neutron capture cross sections leads to an increase of s-process yields from nickel up to selenium. The variation of the cross section of one isotope along the s-process path is propagated to heavier isotopes, where the propagation efficiency is higher for low cross sections. New 74Ge, 75As, and 78Se MACS result in a higher production of germanium, arsenic, and selenium, thereby reducing the s-process yields of heavier elements by propagation. Results are reported for the He core and for the C shell. In shell C-burning, the s-process nucleosynthesis is more uncertain than in the He core, due to higher MACS uncertainties at higher temperatures. We also analyze the impact of using the new lower solar abundances for CNO isotopes on the s-process predictions, where CNO is the source of 22Ne, and we show that beyond Zn this is affecting the s-process yields more than nuclear or stellar model uncertainties considered in this paper. In particular, using the new updated initial composition, we obtain a high s-process production (overproduction higher than 16O, ~100) for Cu, Ga, Ge, and As. Using the older abundances by Anders & Grevesse, also Se, Br, Kr, and Rb are efficiently produced. Our results have important implications in explaining the origin of copper in the solar abundance distribution, pointing to a prevailing contribution from the weak s-process in agreement with spectroscopic observations and Galactic chemical evolution calculations. Because of the improvement due to the new MACS for nickel and copper isotopes, the nucleosynthesis of copper is less affected by nuclear uncertainties compared to heavier s-process elements. An experimental determination of the 63Ni MACS is required for a further improvement of the abundance prediction of copper. The available spectroscopic observations of germanium and gallium in stars are also discussed, where most of the cosmic abundances of these elements derives from the s-process in massive stars.

S‐Process Nucleosynthesis in Advanced Burning Phases of Massive Stars

We present a detailed study of s-process nucleosynthesis in massive stars of solar-like initial composition and masses 15, 20, 25, and 30 M☉. We update our previous results of s-process nucleosynthesis during the core He burning of these stars and then focus on an analysis of the s-process under the physical conditions encountered during the shell carbon burning. We show that the recent compilation of the 22Ne(α,n)25Mg rate leads to a remarkable reduction of the efficiency of the s-process during core He burning. In particular, this rate leads to the lowest overproduction factor of 80Kr found to date during core He burning in massive stars. The s-process yields resulting from shell carbon burning turn out to be very sensitive to the structural evolution of the carbon shell. This structure is influenced by the mass fraction of 12C attained at the end of core helium burning, which in turn is mainly determined by the 12C(α,γ)16O reaction. The still-present uncertainty in the rate for this reaction implies that the s-process in massive stars is also subject to this uncertainty. We identify some isotopes like 70Zn and 87Rb as the signatures of the s-process during shell carbon burning in massive stars. In determining the relative contribution of our s-only stellar yields to the solar abundances, we find it is important to take into account the neutron exposure of shell carbon burning. When we analyze our yields with a Salpeter initial mass function, we find that massive stars contribute at least 40% to s-only nuclei with mass A ≤ 87. For s-only nuclei with mass A > 90, massive stars contribute on average ~7%, except for 152Gd, 187Os, and 198Hg, which contribute ~14%, ~13%, and ~11%, respectively.

The s-process Nucleosynthesis

Theoretical as well as observational aspects of the s-process nucleosynthesis are reviewed. The classical site-independent s-process model as well as the s-process in massive stars are shortly described. A special attention is paid to the nucleosynthesis taking place in AGB stars and the extra-mixing invoked to explain the production of neutrons in the C-rich layers during the interpulse. We also discuss the nucleosynthesis found in hot AGB stars for which the s-process during the interpulse phase is inhibited, but the one resulting from the large temperatures in the thermal pulse is boosted. We comment on the uncertainties affecting our understanding of the physical mechanisms responsible for a successful s-process. Finally, various types of spectroscopic observations of s-process elements are discussed.

Massive star evolution: nucleosynthesis and nuclear reaction rate uncertainties

We present a nucleosynthesis calculation of a 25 M ⊙ star of solar composition that includes all relevant isotopes up to polonium. We follow the stellar evolution from hydrogen burning till iron core collapse and simulate the explosion using a ‘piston’ approach. We discuss the influence of two key nuclear reaction rates, 12C( α, γ) 16O and 22Ne( α, n) 25Mg, on stellar evolution and nucleosynthesis. The former significantly influences the resulting core sizes (iron, silicon, oxygen) and the overall presupernova structure of the star. It thus has significant consequences for the supernova explosion itself and the compact remnant formed. The later rate considerably affects the s-process in massive stars and we demonstrate the changes that different currently suggested values for this rate cause.

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The (n, γ) cross section of 34S has been measured by means of the activation technique using a quasi-stellar neutron spectrum corresponding to a thermal energy of kT = 25 keV. In spite of the very small cross section of this neutron-magic nucleus and a number of additional experimental complications, the stellar value of σv/vT = 226 ± 10 μb could be reliably determined. Because of this small cross section, 34S acts as a bottleneck for the reaction flow to 36S. Consequently, the suggested interpretation of the 36S abundance as a neutron monitor for the s-process in massive stars must be ruled out. This implies, however, that the origin of 36S remains an open problem.

S-processing in massive stars as a function of metallicity and interpretation of observational trends

The s-process in massive stars is analyzed as a function of metallicity. The nucleosynthesis occurring in both core helium and shell carbon burning is investigated by numerically modeling nuclear reaction networks, subject to the conditions provided by stellar models. The efficiency of the neutron-capture mechanism depends on the amount of enhancement that is assumed for the α-rich nuclei, that are observed to be overabundant with respect to iron in stars of metallicity lower than solar

S-process nucleosynthesis in massive stars and the weak component. II - Carbon burning and galactic enrichment

The s-process that occurs in the shell carbon-burning phase of a typical massive star of 25 solar masses is examined. It is shown that neutron captures during shell C-burning can significantly change the s-abundances. The composition of the s-processed material ejected by a 25 solar mass model is evaluated, and the contributions of massive stars to the chemical enrichment of the Galaxy in s-isotopes are estimated. The s-contributions to the solar composition from low-mass stars are then taken into account, and possibility of matching the solar distribution of the s-isotopes produced by the weak component is investigated. It is found that the s-process in massive stars can completely account for the weak component observed in the solar system and can put important constraints on the scenario of the chemical evolution of the Galaxy. 47 refs.

Study of the 22Ne(α, n)25Mg reaction: a possible source of stellar neutronS

Abstract The 22 Ne(α, n) 25 Mg reaction was studied at bombarding energies 1.9 to 4.7 MeV, using pulsed beam time-of-flight spectroscopy. Angular distributions of the neutrons were taken at energy intervals of 60–100 keV. Simultaneously the gamma yield from the excited states in 25 Mg was detected. The cross sections for the various final states in 25 Mg, as well as the total cross section for the reaction, were obtained. The reaction is shown to proceed mainly through resonances in the compound system. Twelve resonances are identified, of which seven are also assigned spins and parities. The reaction is shown to be a possible major neutron source for the sprocess in massive stars.