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Context.Theγprocess in core-collapse supernovae (CCSNe) can produce a number of neutron-deficient stable isotopes heavier than iron (pnuclei). However, current model predictions do not fully reproduce solar abundances, especially for^92, 94Mo and^96, 98Ru.

Aims.We investigate the impact of different explosion energies and parametrizations on the nucleosynthesis ofpnuclei, by studying stellar models with different initial masses and different CCSN explosions.

Methods.We compared thep-nucleus yields obtained using a semi-analytical method to simulate the supernova to those obtained using hydrodynamic models. We explored the effect of varying the explosion parameters on thep-nucleus production in two sets of CCSN models with initial masses of 15, 20, and 25M_⊙at solar metallicity. We calculated a new set of 24 CCSN models (eight for each stellar progenitor mass) and compared our results with another recently published set of 80 CCSN models that includes a wide range of explosion parameters: explosion energy or initial shock velocity, energy injection time, and mass location of the injection.

Results.We find that the totalp-nucleus yields are only marginally affected by the CCSN explosion prescriptions if theγ-process production is already efficient in the stellar progenitors due to a C−O shell merger. In most CCSN explosions from progenitors without a C−O shell merger, theγ-process yields increase with the explosion energy by up to an order of magnitude, depending on the progenitor structure and the CCSN prescriptions. The general trend of thep-nucleus production with the explosion energy is more complicated if we look at the production of singlepnuclei. The lightp-nuclei tend to be the most enhanced with increasing explosion energy. In particular, for the CCSN models where theα-rich freeze-out component is ejected, the yields of the lightestpnuclei (including^92, 94Mo and⁹⁶Ru) increase by up to three orders of magnitude.

Conclusions.We provide the first extensive study using different sets of massive stars of the impact of varying CCSN explosion prescriptions on the production ofpnuclei. Unlike previous expectations and recent results in the literature, we find that the average production ofpnuclei tends to increase with the explosion energy. We also confirm that the pre-explosion production ofpnuclei in C−O shell mergers is a robust result, independent of the subsequent explosive nucleosynthesis. More generally, a realistic range of variations in the evolution of stellar progenitors and in the CCSN explosions might boost the CCSN contribution to the galactic chemical evolution ofpnuclei.

 

Context. The oldest stars in the Milky Way are metal-poor with [Fe/H] < −1.0, displaying peculiar elemental abundances compared to solar values. The relative variations in the chemical compositions among stars is also increasing with decreasing stellar metallicity, allowing for the pure signature of unique nucleosynthesis processes to be revealed. The study of ther-process is, for instance, one of the main goals of stellar archaeology and metal-poor stars exhibit an unexpected complexity in the stellar production of ther-process elements in the early Galaxy.

Aims. In this work, we report the atmospheric parameters, main dynamic properties, and the abundances of four metal-poor stars: HE 1523-0901, HD 6268, HD 121135, and HD 195636 (−1.5 > [Fe/H] > −3.0).

Methods. The abundances were derived from spectra obtained with the HRS echelle spectrograph at the Southern African Large Telescope, using both local and non-local thermodynamic equilibrium (LTE and NLTE) approaches, with the average error between 0.10 and 0.20 dex.

Results. Based on their kinematical properties, we show that HE 1523-0901 and HD 195636 are halo stars with typical high velocities. In particular, HD 121135 displays a peculiar kinematical behaviour, making it unclear whether it is a halo or an accreted star. Furthermore, HD 6268 is possibly a rare prototype of very metal-poor thick disk stars. The abundances derived for our stars are compared with theoretical stellar models and with other stars with similar metallicity values from the literature.

Conclusions. HD 121135 is Al-poor and Sc-poor, compared to stars observed in the same metallicity range (−1.62 > [Fe/H] > −1.12). The most metal-poor stars in our sample, HE 1523-0901, HD 6268, and HD 195636, exhibit anomalies that are better explained by supernova models from fast-rotating stellar progenitors for elements up to the Fe group. Compared to other stars in the same metal-licity range, their common biggest anomaly is represented by the low Sc abundances. If we consider the elements beyond Zn, HE 1523-0901 can be classified as an r-II star, HD 6268 as an r-I candidate, and HD 195636 and HD 121135 exhibiting a borderliner-process enrichment between limited-r and r-I star. Significant relative differences are observed between the r-process signatures in these stars.

 

Context.An accurate²⁸P(p,γ)²⁹S reaction rate is crucial to defining the nucleosynthesis products of explosive hydrogen burning in ONe novae. Using the recently released nuclear mass of²⁹S, together with a shell model and a direct capture calculation, we reanalyzed the²⁸P(p,γ)²⁹S thermonuclear reaction rate and its astrophysical implication.

Aims.We focus on improving the astrophysical rate for²⁸P(p,γ)²⁹S based on the newest nuclear mass data. Our goal is to explore the impact of the new rate and associated uncertainties on the nova nucleosynthesis.

Methods.We evaluated this reaction rate via the sum of the isolated resonance contribution instead of the previously used Hauser-Feshbach statistical model. The corresponding rate uncertainty at different energies was derived using a Monte Carlo method. Nova nucleosynthesis is computed with the 1D hydrodynamic code SHIVA.

Results.The contribution from the capture on the first excited state at 105.64 keV in²⁸P is taken into account for the first time. We find that the capture rate on the first excited state in²⁸P is up to more than 12 times larger than the ground-state capture rate in the temperature region of 2.5 × 10⁷K to 4 × 10⁸K, resulting in the total²⁸P(p,γ)²⁹S reaction rate being enhanced by a factor of up to 1.4 at ~1 × 10⁹K. In addition, the rate uncertainty has been quantified for the first time. It is found that the new rate is smaller than the previous statistical model rates, but it still agrees with them within uncertainties for nova temperatures. The statistical model appears to be roughly valid for the rate estimation of this reaction in the nova nucleosynthesis scenario. Using the 1D hydrodynamic code SHIVA, we performed the nucleosynthesis calculations in a nova explosion to investigate the impact of the new rates of²⁸P(p,γ)²⁹S. Our calculations show that the nova abundance pattern is only marginally affected if we use our new rates with respect to the same simulations but statistical model rates. Finally, the isotopes whose abundance is most influenced by the present²⁸P(p,γ)²⁹S uncertainty are²⁸Si,^33,34S,^35,37Cl, and³⁶Ar, with relative abundance changes at the level of only 3% to 4%.

 

Context. The γ -process nucleosynthesis in core-collapse supernovae is generally accepted as a feasible process for the synthesis of neutron-deficient isotopes beyond iron. However, crucial discrepancies between theory and observations still exist: the average yields of γ -process nucleosynthesis from massive stars are still insufficient to reproduce the solar distribution in galactic chemical evolution calculations, and the yields of the Mo and Ru isotopes are a factor of ten lower than the yields of the other γ -process nuclei. Aims. We investigate the γ -process in five sets of core-collapse supernova models published in the literature with initial masses of 15, 20, and 25 M ⊙ at solar metallicity. Methods. We compared the γ -process overproduction factors from the different models. To highlight the possible effect of nuclear physics input, we also considered 23 ratios of two isotopes close to each other in mass relative to their solar values. Further, we investigated the contribution of C–O shell mergers in the supernova progenitors as an additional site of the γ -process. Results. Our analysis shows that a large scatter among the different models exists for both the γ -process integrated yields and the isotopic ratios. We find only ten ratios that agree with their solar values, all the others differ by at least a factor of three from the solar values in all the considered sets of models. The γ -process within C–O shell mergers mostly influences the isotopic ratios that involve intermediate and heavy proton-rich isotopes with A  > 100. Conclusions. We conclude that there are large discrepancies both among the different data sets and between the model predictions and the solar abundance distribution. More calculations are needed; particularly updating the nuclear network, because the majority of the models considered in this work do not use the latest reaction rates for the γ -process nucleosynthesis. Moreover, the role of C–O shell mergers requires further investigation. 

Context.Accurate⁴²Ti(p,γ)⁴³V reaction rates are crucial for understanding the nucleosynthesis path of the rapid capture process (rpprocess) that occurs in X-ray bursts.

Aims.We aim to improve the thermonuclear rates of⁴²Ti(p,γ)⁴³V based on more complete resonance information and a more accurate direct component, together with the recently released nuclear masses data. We also explore the impact of the newly obtained rates on therpprocess.

Methods.We reevaluated the reaction rate of⁴²Ti(p,γ)⁴³V by the sum of the isolated resonance contribution instead of the Hauser-Feshbach statistical model. We used a Monte Carlo method to derive the associated uncertainties of new rates. The nucleosynthesis simulations were performed via the NuGrid post-processing code ppn.

Results.The new rates differ from previous estimations due to the use of a series of updated resonance parameters and a direct S factor. Compared with the previous results from the Hauser-Feshbach statistical model, which assumes compound nucleus⁴³V with a sufficiently high-level density in the energy region of astrophysical interest, large differences exist over the entire temperature region ofrp-process interest, up to two orders of magnitude. We consistently calculated the photodisintegration rate using our new nuclear masses via the detailed balance principle, and found the discrepancies among the different reverse rates are much larger than those for the forward rate, up to ten orders of magnitude at the temperature of 10⁸K. Using a trajectory with a peak temperature of 1.95×10⁹K, we performed therp-process nucleosynthesis simulations to investigate the impact of the new rates. Our calculations show that the adoption of the new forward and reverse rates result in abundance variations for Sc and Ca of 128% and 49%, respectively, compared to the variations for the statistical model rates. On the other hand, the overall abundance pattern is not significantly affected. The results of using new rates also confirm that therp-process path does not bypass the isotope⁴³V.

Conclusions.Our study found that the Hauser-Feshbach statistical model is inappropriate to the reaction rate evaluation for⁴²Ti(p,γ)⁴³V. The adoption of the new rates confirms that the reaction path of⁴²Ti(p,γ)⁴³V(p,γ)⁴⁴Cr(β⁺)⁴⁴V is a key branch of therpprocess in X-ray bursts.

 

Accurate nuclear reaction rates for²⁶P(p,γ)²⁷S are pivotal for a comprehensive understanding of therp-process nucleosynthesis path in the region of proton-rich sulfur and phosphorus isotopes. However, large uncertainties still exist in the current rate of²⁶P(p,γ)²⁷S because of the lack of nuclear mass and energy level structure information for²⁷S. We reevaluate this reaction rate using the experimentally constrained²⁷S mass, together with the shell model predicted level structure. It is found that the²⁶P(p,γ)²⁷S reaction rate is dominated by a direct capture reaction mechanism despite the presence of three resonances atE= 1.104, 1.597, and 1.777 MeV above the proton threshold in²⁷S. The new rate is overall smaller than the other previous rates from the Hauser–Feshbach statistical model by at least 1 order of magnitude in the temperature range of X-ray burst interest. In addition, we consistently update the photodisintegration rate using the new²⁷S mass. The influence of new rates of forward and reverse reaction in the abundances of isotopes produced in therp-process is explored by postprocessing nucleosynthesis calculations. The final abundance ratio of²⁷S/²⁶P obtained using the new rates is only 10% of that from the old rate. The abundance flow calculations show that the reaction path²⁶P(p,γ)²⁷S(β⁺,ν)²⁷P is not as important as previously thought for producing²⁷P. The adoption of the new reaction rates for²⁶P(p,γ)²⁷S only reduces the final production of aluminum by 7.1% and has no discernible impact on the yield of other elements.

 

Context. Barium (Ba) stars are characterised by an abundance of heavy elements made by the slow neutron capture process ( s -process). This peculiar observed signature is due to the mass transfer from a stellar companion, bound in a binary stellar system, to the Ba star observed today. The signature is created when the stellar companion is an asymptotic giant branch (AGB) star. Aims. We aim to analyse the abundance pattern of 169 Ba stars using machine learning techniques and the AGB final surface abundances predicted by the F RUITY and Monash stellar models. Methods. We developed machine learning algorithms that use the abundance pattern of Ba stars as input to classify the initial mass and metallicity of each Ba star’s companion star using stellar model predictions. We used two algorithms. The first exploits neural networks to recognise patterns, and the second is a nearest-neighbour algorithm that focuses on finding the AGB model that predicts the final surface abundances closest to the observed Ba star values. In the second algorithm, we included the error bars and observational uncertainties in order to find the best-fit model. The classification process was based on the abundances of Fe, Rb, Sr, Zr, Ru, Nd, Ce, Sm, and Eu. We selected these elements by systematically removing s -process elements from our AGB model abundance distributions and identifying the elements whose removal had the biggest positive effect on the classification. We excluded Nb, Y, Mo, and La. Our final classification combined the output of both algorithms to identify an initial mass and metallicity range for each Ba star companion. Results. With our analysis tools, we identified the main properties for 166 of the 169 Ba stars in the stellar sample. The classifications based on both stellar sets of AGB final abundances show similar distributions, with an average initial mass of M = 2.23 M ⊙ and 2.34 M ⊙ and an average [Fe/H] = −0.21 and −0.11, respectively. We investigated why the removal of Nb, Y, Mo, and La improves our classification and identified 43 stars for which the exclusion had the biggest effect. We found that these stars have statistically significant and different abundances for these elements compared to the other Ba stars in our sample. We discuss the possible reasons for these differences in the abundance patterns. 

The study of the origin of heavy elements is one of the main goals of nuclear astrophysics. In this paper, we present new observational data for the heavy r-process elements gadolinium (Gd, Z= 64), dysprosium (Dy, Z= 66), and thorium (Th, Z= 90) in a sample of 276 Galactic disc stars (–1.0 < [Fe/H] < + 0.3). The stellar spectra have a high resolution of 42 000 and 75 000, and the signal-to-noise ratio higher than 100. The LTE abundances of Gd, Dy, and Th have been determined by comparing the observed and synthetic spectra for three Gd lines (149 stars), four Dy lines (152 stars), and the Th line at 4019.13 Å (170 stars). For about 70 per cent of the stars in our sample, Gd and Dy are measured for the first time, and Th for 95 per cent of the stars. Typical errors vary from 0.07 to 0.16 dex. This paper provides the first extended set of Th observations in the Milky Way disc. Together with europium (Eu, Z= 63) data from our previous studies, we have compared these new observations with nucleosynthesis predictions and Galactic Chemical Evolution simulations. We confirm that [Gd/Fe] and [Dy/Fe] show the same behaviour of Eu. We study with GCE simulations the evolution of [Th/Fe] in comparison with [Eu/Fe], showing that unlike Eu, either the Th production is metallicity dependent in case of a unique source of the r-process in the Galaxy, or the frequency of the Th-rich r-process source is decreasing with the increase in [Fe/H].

 

The Istituto Nazionale di Fisica Nucleare—Laboratori Nazionali del Gran Sasso (LNGS) is one of the largest underground physics laboratory, a very peculiar environment suited for experiments in Astroparticle Physics, Nuclear Physics and Fundamental Symmetries. The newly established Bellotti Ion Beam facility represents a major advance in the possibilities of studying nuclear processes in an underground environment. A workshop was organized at LNGS in the framework of the Nuclear Physics Mid Term Plan in Italy, an initiative of the Nuclear Physics Division of the Instituto Nazionale di Fisica Nucleare to discuss the opportunities that will be possible to study in the near future by employing state-of-the-art detection systems. In this report, a detailed discussion of the outcome of the workshop is presented.

 

Abstract Radioactive nuclei were present in the early solar system (ESS), as inferred from analysis of meteorites. Many are produced in massive stars, either during their lives or their final explosions. In the first paper of this series (Brinkman et al. 2019), we focused on the production of 26 Al in massive binaries. Here, we focus on the production of another two short-lived radioactive nuclei, 36 Cl and 41 Ca, and the comparison to the ESS data. We used the MESA stellar evolution code with an extended nuclear network and computed massive (10–80 M ⊙ ), rotating (with initial velocities of 150 and 300 km s −1 ) and nonrotating single stars at solar metallicity ( Z = 0.014) up to the onset of core collapse. We present the wind yields for the radioactive isotopes 26 Al, 36 Cl, and 41 Ca, and the stable isotopes 19 F and 22 Ne. In relation to the stable isotopes, we find that only the most massive models, ≥60 and ≥40 M ⊙ give positive 19 F and 22 Ne yields, respectively, depending on the initial rotation rate. In relation to the radioactive isotopes, we find that the ESS abundances of 26 Al and 41 Ca can be matched with by models with initial masses ≥40 M ⊙ , while 36 Cl is matched only by our most massive models, ≥60 M ⊙ . 60 Fe is not significantly produced by any wind model, as required by the observations. Therefore, massive star winds are a favored candidate for the origin of the very short-lived 26 Al, 36 Cl, and 41 Ca in the ESS.