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Evaluated spectroscopic data and level schemes from radioactive decay and nuclear reaction studies are presented for {sup 22}C, {sup 22}N, {sup 22}O, {sup 22}F, {sup 22}Ne, {sup 22}Na, {sup 22}Mg, {sup 22}Al, and {sup 22}Si. This evaluation for A = 22 supersedes the earlier one by R. B. Firestone (2005Fi16)
Massive stars at solar metallicity contribute to the production of heavy elements with atomic masses between A = 60 and A = 90 via the so-called weak s-process (which takes place during core He and shell C burning phases). Furthermore, recent studies have shown that rotation boosts the s-process production in massive stars at low metallicities, with a production that may reach the barium neutron-magic peak. These results are very sensitive to neutron source and neutron poison reaction rates. For the weak s-process, the main neutron source is the reaction {sup 22}Ne(α,n){sup 25}Mg, which is in competition with {sup 22}Ne(α,γ){sup 26}Mg. The uncertainty of both rates strongly affects the nucleosynthesis predictions from stellar model calculations. In this study, we investigate the impact of the uncertainty in α-captures on {sup 22}Ne on the s-process nucleosynthesis in massive stars both at solar and at very low metallicity. For this purpose, we post-process, with the Nugrid mppnp code, non-rotating and rotating evolutionary models 25M{sub ⊙} stars at two different metallicities: Z = Z{sub ⊙} and Z = 10{sup −5}Z{sub ⊙}, respectively. Our results show that uncertainty of {sup 22}Ne(α,n){sup 25}Mg and {sup 22}Ne(α,γ){sup 26}Mg rates have a significant impact on the final elemental production especially for metal poor rotating models. Beside uncertainties in the neutron source reactions, for fast rotating massive stars at low metallicity we revisit the impact of the neutron poisoning effect by the reaction chain {sup 16}O(n,γ){sup 17}O(α,γ){sup 21}Ne, in competition with the {sup 17}O(α,n){sup 20}Ne, recycling the neutrons captured by {sup 16}O.
We report the first tungsten isotopic measurements in stardust silicon carbide (SiC) grains recovered from the Murchison carbonaceous chondrite. The isotopes {sup 182,183,184,186}W and {sup 179,180}Hf were measured on both an aggregate (KJB fraction) and single stardust SiC grains (LS+LU fraction) believed to have condensed in the outflows of low-mass carbon-rich asymptotic giant branch (AGB) stars with close-to-solar metallicity. The SiC aggregate shows small deviations from terrestrial (= solar) composition in the {sup 182}W/{sup 184}W and {sup 183}W/{sup 184}W ratios, with deficits in {sup 182}W and {sup 183}W with respect to {sup 184}W. The {sup 186}W/{sup 184}W ratio, however, shows no apparent deviation from the solar value. Tungsten isotopic measurements in single mainstream stardust SiC grains revealed lower than solar {sup 182}W/{sup 184}W, {sup 183}W/{sup 184}W, and {sup 186}W/{sup 184}W ratios. We have compared the SiC data with theoretical predictions of the evolution of W isotopic ratios in the envelopes of AGB stars. These ratios are affected by the slow neutron-capture process and match the SiC data regarding their {sup 182}W/{sup 184}W, {sup 183}W/{sup 184}W, and {sup 179}Hf/{sup 180}Hf isotopic compositions, although a small adjustment in the s-process production of {sup 183}W is needed in order to have a better agreement between the SiC data and model predictions. The models cannot explain the {sup 186}W/{sup 184}W ratios observed in the SiC grains, even when the current {sup 185}W neutron-capture cross section is increased by a factor of two. Further study is required to better assess how model uncertainties (e.g., the formation of the {sup 13}C neutron source, the mass-loss law, the modeling of the third dredge-up, and the efficiency of the {sup 22}Ne neutron source) may affect current s-process predictions.
Because of the large neutron excess of {sup 22}Ne, sedimentation of this isotope occurs rapidly in the interior of white dwarfs. This process releases an additional amount of energy, thus delaying the cooling times of the white dwarf. This influences the ages of different stellar populations derived using white dwarf cosmochronology. Furthermore, the overabundance of {sup 22}Ne in the inner regions of the star modifies the Brunt–Väisälä frequency, thus altering the pulsational properties of these stars. In this work we discuss the impact of {sup 22}Ne sedimentation in white dwarfs resulting from solar metallicity progenitors (Z = 0.02). We performed evolutionary calculations of white dwarfs with masses of 0.528, 0.576, 0.657, and 0.833 M{sub ⊙} derived from full evolutionary computations of their progenitor stars, starting at the zero-age main sequence all the way through the central hydrogen and helium burning, the thermally pulsing asymptotic giant branch (AGB), and post-AGB phases. Our computations show that at low luminosities (log(L/L{sub ⊙})≲−4.25), {sup 22}Ne sedimentation delays the cooling of white dwarfs with solar metallicity progenitors by about 1 Gyr. Additionally, we studied the consequences of {sup 22}Ne sedimentation on the pulsational properties of ZZ Ceti white dwarfs. We find that {sup 22}Ne sedimentation induces differences in the periods of these stars larger than the present observational uncertainties, particularly in more massive white dwarfs.
The {sup 22}Ne(p,{gamma}){sup 23}Na reaction is a part of the hydrogen burning NeNa cycle. In second-generation stars hydrogen burning may proceed via this cycle. The rate of the {sup 22}Ne(p,{gamma}){sup 23}Na reaction depends on the strength of several resonances in the energy range of the LUNA 400 kV accelerator which have never been observed in direct experiments. A related study is under preparation at LUNA.
At the National Metrology Institute of Japan (NMIJ), the triple points of {sup 20}Ne and {sup 22}Ne were realized using modular sealed cells, Ec3Ne20 and Ec8Ne22, made by the Istituto Nazionale di Ricerca Metrologica (INRiM) in Italy. The difference of the triple-point temperatures of {sup 20}Ne and {sup 22}Ne was estimated by using the sub-range of standard platinum resistance thermometers (SPRTs) calibrated by NMIJ on the International Temperature Scale of 1990 (ITS-90). The melting curves obtained with the Ec3Ne20 and Ec8Ne22 cells show narrow widths (0.1 mK) over a wide range of the inverse of the melted fraction (1/F) from 1/F=1 to 1/F=10. The liquidus point T{sub tp} estimated by the melting curves from F∼0.5 to F∼0.85 using the Ec8Ne22 is 0.146 29 (4) K higher than that using the Ec3Ne20 cell, which is in good agreement with that observed by INRiM using the same cells. After correction of the effect of impurities and other isotopes for Ec3Ne20 and Ec8Ne22 cells, the difference of T{sub tp} between pure {sup 20}Ne and pure {sup 22}Ne is estimated to be 0.146 61 (4) K, which is consistent with the recent results reported elsewhere. The sub-ranges of SPRTs computed by using the triple point of {sup 20}Ne or {sup 22}Ne realized by the Ec3Ne20 cell or the Ec8Ne22 cell in place of the triple point of Ne for the defining fixed point of the ITS-90 are in good agreement with those realized on the basis of the ITS-90 at NMIJ within 0.03 mK, which is much smaller than the non-uniqueness and the sub-range inconsistency of SPRTs.
We present updated calculations of stellar evolutionary sequences and detailed nucleosynthesis predictions for the brightest asymptotic giant branch (AGB) stars in the Galaxy with masses between 5 M{sub Sun} and 9 M{sub Sun }, with an initial metallicity of Z = 0.02 ([Fe/H] = 0.14). In our previous studies we used the Vassiliadis and Wood mass-loss rate, which stays low until the pulsation period reaches 500 days after which point a superwind begins. Vassiliadis and Wood noted that for stars over 2.5 M{sub Sun} the superwind should be delayed until P Almost-Equal-To 750 days at 5 M{sub Sun }. We calculate evolutionary sequences where we delay the onset of the superwind to pulsation periods of P Almost-Equal-To 700-800 days in models of M = 5, 6, and 7 M{sub Sun }. Post-processing nucleosynthesis calculations show that the 6 and 7 M{sub Sun} models produce the most Rb, with [Rb/Fe] Almost-Equal-To 1 dex, close to the average of most of the Galactic Rb-rich stars ([Rb/Fe] Almost-Equal-To 1.4 {+-} 0.8 dex). Changing the rate of the {sup 22}Ne +{alpha} reactions results in variations of [Rb/Fe] as large as 0.5 dex in models with a delayed superwind. The largest enrichment in heavy elements is found for models that adopt the NACRE rate of the {sup 22}Ne({alpha}, n){sup 25}Mg reaction. Using this rate allows us to best match the composition of most of the Rb-rich stars. A synthetic evolution algorithm is then used to remove the remaining envelope resulting in final [Rb/Fe] of Almost-Equal-To 1.4 dex although with C/O ratios >1. We conclude that delaying the superwind may account for the large Rb overabundances observed in the brightest metal-rich AGB stars.
The {sup 95}Zr( n , γ ){sup 96}Zr reaction cross section is crucial in the modeling of s -process nucleosynthesis in asymptotic giant branch stars because it controls the operation of the branching point at the unstable {sup 95}Zr and the subsequent production of {sup 96}Zr. We have carried out the measurement of the {sup 94}Zr({sup 18}O, {sup 16}O) and {sup 90}Zr({sup 18}O, {sup 16}O) reactions and obtained the γ -decay probability ratio of {sup 96}Zr* and {sup 92}Zr* to determine the {sup 95}Zr( n , γ ){sup 96}Zr reaction cross sections with the surrogate ratio method. Our deduced Maxwellian-averaged cross section of 66 ± 16 mb at 30 keV is close to the value recommended by Bao et al., but 30% and more than a factor of two larger than the values proposed by Toukan and Käppeler and Lugaro et al., respectively, and routinely used in s -process models. We tested the new rate in stellar models with masses between 2 and 6 M {sub ⊙} and metallicities of 0.014 and 0.03. The largest changes—up to 80% variations in {sup 96}Zr—are seen in models of mass 3–4 M {sub ⊙}, where the {sup 22}Ne neutron source is mildly activated. The new rate can still provide a match to data from meteoritic stardust silicon carbide grains, provided that the maximum mass of the parent stars is below 4 M {sub ⊙}, for a metallicity of 0.03.
The {sup 22}Ne(p,γ){sup 23}Na reaction is involved in the hydrogen burning NeNa cycle. This determines the nucleosynthesis of the Ne and Na isotopes in the Red Giant Branch and Asymptotic Giant Branch phases of stellar evolution. In the energy range relevant for astrophysics (20 keV < E < 600 keV), the {sup 22}Ne(p,γ){sup 23}Na reaction rate is highly uncertain because of the contribution of a large number of resonances never measured directly. A related study is under preparation at the Laboratory for Underground Nuclear Astrophysics (LUNA), in the Gran Sasso National Laboratory, and it will cover the energy range 100 keV < E < 400 keV. Meanwhile, a measurement at higher energies (i.e. 436 keV) has been carried out at the Tandetron accelerator of the HZDR (Helmholtz Zentrum Dresden Rossendorf) in Germany. Some preliminary results will be presented.
We present a new set of models for intermediate-mass asymptotic giant branch (AGB) stars (4.0, 5.0, and 6.0 M{sub ⊙}) at different metallicities (−2.15 ≤ [Fe/H] ≤ +0.15). This set integrates the existing models for low-mass AGB stars (1.3 ≤ M/M{sub ⊙} ≤ 3.0) already included in the FRUITY database. We describe the physical and chemical evolution of the computed models from the main sequence up to the end of the AGB phase. Due to less efficient third dredge up episodes, models with large core masses show modest surface enhancements. This effect is due to the fact that the interpulse phases are short and, therefore, thermal pulses (TPs) are weak. Moreover, the high temperature at the base of the convective envelope prevents it from deeply penetrating the underlying radiative layers. Depending on the initial stellar mass, the heavy element nucleosynthesis is dominated by different neutron sources. In particular, the s-process distributions of the more massive models are dominated by the {sup 22}Ne(α,n){sup 25}Mg reaction, which is efficiently activated during TPs. At low metallicities, our models undergo hot bottom burning and hot third dredge up. We compare our theoretical final core masses to available white dwarf observations. Moreover, we quantify the influence intermediate-mass models have on the carbon star luminosity function. Finally, we present the upgrade of the FRUITY web interface, which now also includes the physical quantities of the TP-AGB phase for all of the models included in the database (ph-FRUITY)
We explore the sensitivity of nucleosynthesis in massive stars to the truncation of supernova explosions above a certain mass. It is assumed that stars of all masses contribute to nucleosynthesis by their pre-explosive winds, but above a certain limiting main sequence mass, M{sub BH}, the presupernova star becomes a black hole and ejects nothing more. The solar abundances from oxygen to atomic mass 90 are fit quite well assuming no cutoff at all, i.e., by assuming all stars up to 120 M{sub Sun} make successful supernovae. Little degradation in the fit occurs if M{sub BH} is reduced to 25 M{sub Sun }. If this limit is reduced further however, the nucleosynthesis of the s-process declines precipitously and the production of species made in the winds, e.g., carbon, becomes unacceptably large compared with elements made in the explosion, e.g., silicon and oxygen. By varying uncertain physics, especially the mass loss rate for massive stars and the rate for the {sup 22}Ne({alpha}, n){sup 25}Mg reaction rate, acceptable nucleosynthesis might still be achieved with a cutoff as low as 18 M{sub Sun }. This would require, however, a supernova frequency three times greater than the fiducial value obtained when all stars explode in order to produce the required {sup 16}O. The effects of varying M{sub BH} on the nucleosynthesis of {sup 60}Fe and {sup 26}Al, the production of helium as measured by {Delta}Y/{Delta}Z, and the average masses of compact remnants are also examined.
The observational {sup 19}F abundance in stellar environments systematically exceeds the predicted one, thus representing one of the unsolved challenges for stellar modeling. It is therefore clear that further investigation is needed in this field. In this work, we focus our attention on the measurement of the {sup 19}F(α,p){sup 22}Ne reaction in the astrophysical energy range, between 0.2 and 0.8 MeV (far below the Coulomb barrier, 3.8 MeV), as it represents the main destruction channel in He-rich environments. The lowest energy at which this reaction has been studied with direct measurements is ∼0.66 MeV, covering only the upper tail of the Gamow window, causing the reaction-rate evaluation to be based on extrapolation. To investigate lower energies, the {sup 19}F(α,p){sup 22}Ne reaction has been studied by means of the Trojan horse method, applied to the quasi-free {sup 6}Li({sup 19}F,p{sup 22}Ne){sup 2}H reaction at E {sub beam} = 6 MeV. The indirect cross section of the {sup 19}F(α,p){sup 22}Ne reaction at energies ≲1 MeV was extracted, fully covering the astrophysical region of interest and overlapping existing direct data for normalization. Several resonances have been detected for the first time inside the Gamow window. The reaction rate has been calculated, showing an increase up to a factor of 4 with respect to the literature at astrophysical temperatures. This might lead to potential major astrophysical implications.
Presolar grains are small particles that condensed in the vicinity of dying stars. Some of these grains survived the voyage through the interstellar medium (ISM) and were incorporated into meteorite parent bodies at the formation of the Solar System. An important question is when these stellar processes happened, i.e., how long presolar grains were drifting through the ISM. While conventional radiometric dating of such small grains is very difficult, presolar grains are irradiated with galactic cosmic rays (GCRs) in the ISM, which induce the production of cosmogenic nuclides. This opens the possibility to determine cosmic-ray exposure (CRE) ages, i.e., how long presolar grains were irradiated in the ISM. Here, we present a new model for the production and loss of cosmogenic {sup 3}He, {sup 6,7}Li, and {sup 21,22}Ne in presolar SiC grains. The cosmogenic production rates are calculated using a state-of-the-art nuclear cross-section database and a GCR spectrum in the ISM consistent with recent Voyager data. Our findings are that previously measured {sup 3}He and {sup 21}Ne CRE ages agree within the (sometimes large) 2 σ uncertainties and that the CRE ages for most presolar grains are smaller than the predicted survival times. The obtained results are relatively robust since interferences from implanted low-energy GCRs into the presolar SiC grains and/or from cosmogenic production within the meteoroid can be neglected.
We investigate the enrichment in elements produced by the slow neutron-capture process (s-process) in the globular clusters M4 (NGC 6121) and M22 (NGC 6656). Stars in M4 have homogeneous abundances of Fe and neutron-capture elements, but the entire cluster is enhanced in s-process elements (Sr, Y, Ba, Pb) relative to other clusters with a similar metallicity. In M22, two stellar groups exhibit different abundances of Fe and s-process elements. By subtracting the mean abundances of s-poor from s-rich stars, we derive s-process residuals or empirical s-process distributions for M4 and M22. We find that the s-process distribution in M22 is more weighted toward the heavy s-peak (Ba, La, Ce) and Pb than M4, which has been enriched mostly with light s-peak elements (Sr, Y, Zr). We construct simple chemical evolution models using yields from massive star models that include rotation, which dramatically increases s-process production at low metallicity. We show that our massive star models with rotation rates of up to 50% of the critical (break-up) velocity and changes to the preferred {sup 17}O(α, γ){sup 21}Ne rate produce insufficient heavy s-elements and Pb to match the empirical distributions. For models that incorporate asymptotic giant branch yields, we find that intermediate-mass yields (with a {sup 22}Ne neutron source) alone do not reproduce the light-to-heavy s-element ratios for M4 and M22, and that a small contribution from models with a {sup 13}C pocket is required. With our assumption that {sup 13}C pockets form for initial masses below a transition range between 3.0 and 3.5 M {sub ☉}, we match the light-to-heavy s-element ratio in the s-process residual of M22 and predict a minimum enrichment timescale of between 240 and 360 Myr. Our predicted value is consistent with the 300 Myr upper limit age difference between the two groups derived from isochrone fitting.
Carbon-rich grains with isotopic anomalies compared to the Sun are found in primitive meteorites. They were made by stars, and carry the original stellar nucleosynthesis signature. Silicon carbide grains of Type X and C and low-density (LD) graphites condensed in the ejecta of core-collapse supernovae. We present a new set of models for the explosive He shell and compare them with the grains showing {sup 12}C/{sup 13}C and {sup 14}N/{sup 15}N ratios lower than solar. In the stellar progenitor H was ingested into the He shell and not fully destroyed before the explosion. Different explosion energies and H concentrations are considered. If the supernova shock hits the He-shell region with some H still present, the models can reproduce the C and N isotopic signatures in C-rich grains. Hot-CNO cycle isotopic signatures are obtained, including a large production of {sup 13}C and {sup 15}N. The short-lived radionuclides {sup 22}Na and {sup 26}Al are increased by orders of magnitude. The production of radiogenic {sup 22}Ne from the decay of {sup 22}Na in the He shell might solve the puzzle of the Ne-E(L) component in LD graphite grains. This scenario is attractive for the SiC grains of type AB with {sup 14}N/{sup 15}N ratios lower than solar, and provides an alternative solution for SiC grains originally classified as nova grains. Finally, this process may contribute to the production of {sup 14}N and {sup 15}N in the Galaxy, helping to produce the {sup 14}N/{sup 15}N ratio in the solar system.
The s-process nucleosynthesis in Asymptotic giant branch (AGB) stars depends on the modeling of convective boundaries. We present models and s-process simulations that adopt a treatment of convective boundaries based on the results of hydrodynamic simulations and on the theory of mixing due to gravity waves in the vicinity of convective boundaries. Hydrodynamics simulations suggest the presence of convective boundary mixing (CBM) at the bottom of the thermal pulse-driven convective zone. Similarly, convection-induced mixing processes are proposed for the mixing below the convective envelope during third dredge-up (TDU), where the {sup 13}C pocket for the s process in AGB stars forms. In this work, we apply a CBM model motivated by simulations and theory to models with initial mass M = 2 and M=3 M{sub ⊙}, and with initial metal content Z = 0.01 and Z = 0.02. As reported previously, the He-intershell abundances of {sup 12}C and {sup 16}O are increased by CBM at the bottom of the pulse-driven convection zone. This mixing is affecting the {sup 22}Ne(α, n){sup 25}Mg activation and the s-process efficiency in the {sup 13}C-pocket. In our model, CBM at the bottom of the convective envelope during the TDU represents gravity wave mixing. Furthermore, we take into account the fact that hydrodynamic simulations indicate a declining mixing efficiency that is already about a pressure scale height from the convective boundaries, compared to mixing-length theory. We obtain the formation of the {sup 13}C-pocket with a mass of ≈10{sup −4} M{sub ⊙}. The final s-process abundances are characterized by 0.36<[s/Fe]<0.78 and the heavy-to-light s-process ratio is −0.23<[hs/ls]<0.45. Finally, we compare our results with stellar observations, presolar grain measurements and previous work.
Recent observations of heavy elements in globular clusters reveal intriguing deviations from the standard paradigm of the early galactic nucleosynthesis. If the r-process contamination is a common feature of halo stars, s-process enhancements are found in a few globular clusters only. We show that the combined pollution of asymptotic giant branch (AGB) stars with a mass ranging between 3 to 6 M {sub ☉} may account for most of the features of the s-process overabundance in M4 and M22. In these stars, the s process is a mixture of two very different neutron-capture nucleosynthesis episodes. The first is due to the {sup 13}C(α, n){sup 16}O reaction and takes place during the interpulse periods. The second is due to the {sup 22}Ne(α, n){sup 25}Mg reaction and takes place in the convective zones generated by thermal pulses. The production of the heaviest s elements (from Ba to Pb) requires the first neutron burst, while the second produces large overabundances of light s (Rb, Sr, Y, Zr). The first mainly operates in the less massive AGB stars, while the second dominates in the more massive. From the heavy-s/light-s ratio, we derive that the pollution phase should last for 150 ± 50 Myr, a period short enough compared to the formation timescale of the globular cluster system, but long enough to explain why the s-process pollution is observed in a few cases only. With few exceptions, our theoretical prediction provides a reasonable reproduction of the observed s-process abundances, from Sr to Hf. However, Ce is probably underproduced by our models, while Rb and Pb are overproduced. Possible solutions are discussed.
Mass and energy distributions of binary reaction products obtained in the reactions {sup 22}Ne+{sup 249}Cf,{sup 26}Mg+{sup 248}Cm,{sup 36}S+{sup 238}U and {sup 58}Fe+{sup 208}Pb leading to Hs isotopes have been measured. At energies below the Coulomb barrier the bimodal fission of Hs*, formed in the reaction {sup 26}Mg+{sup 248}Cm, is observed. In the reaction {sup 36}S+{sup 238}U the considerable part of the symmetric fragments arises from the quasifission process. At energies above the Coulomb barrier the symmetric fragments originate mainly from fusion-fission process for both reactions with Mg and S ions. In the case of the {sup 58}Fe+{sup 208}Pb reaction the quasifission process dominates at all measured energies. The pre- and post-scission neutron multiplicities as a function of the fragment mass have been obtained for the reactions studied.