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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. 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). The abundances were derived from spectra obtained with the HRS echelle spectrograph at the SALT, using both LTE and NLTE approaches. 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. 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 metallicity 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 borderline r-process enrichment between limited-r and r-I star., Comment: 27 pages, 15 figures, accepted in A&A 23.04.2024

We present the state-of-the-art single-zone nuclear reaction network WinNet that is capable of calculating the nucleosynthetic yields of a large variety of astrophysical environments and conditions. This ranges from the calculation of the primordial nucleosynthesis, where only a few nuclei are considered, to the ejecta of neutron star mergers with several thousands of involved nuclei. Here we describe the underlying physics and implementation details of the reaction network. We additionally present the numerical implementation of two different integration methods, the implicit Euler method and Gears method along with their advantages and disadvantages. We furthermore describe basic example cases of thermodynamic conditions that we provide together with the network and demonstrate the reliability of the code by using simple test cases. With this publication, WinNet is publicly available and open source at GitHub and Zenodo.

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 A (170 stars). For about 70% of the stars in our sample Gd and Dy are measured for the first time, and Th for 95% 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 behavior 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 increasing of [Fe/H]., Comment: 18 pages, 21 figures, accepted in MNRAS, stac2361

Rapid neutron capture process (r-process) elements have been detected in a large fraction of metal-poor halo stars, with abundances relative to iron (Fe) that vary by over two orders of magnitude. This scatter is reduced to less than a factor of 3 in younger Galactic disc stars. The large scatter of r-process elements in the early Galaxy suggests that the r-process is made by rare events, like compact binary mergers and rare sub-classes of supernovae. Although being rare, neutron star mergers alone have difficulties to explain the observed enhancement of r-process elements in the lowest metallicity stars compared to Fe. The supernovae producing the two neutron stars already provide a substantial Fe abundance where the r-process ejecta from the merger would be injected. In this work we investigate another complementary scenario, where the r-process occurs in neutron star-black hole mergers in addition to neutron star mergers. Neutron star-black hole mergers would eject similar amounts of r-process matter as neutron star mergers, but only the neutron star progenitor would have produced Fe. Furthermore, a reduced efficiency of Fe production from single stars significantly alters the age-metallicity relation, which shifts the onset of r-process production to lower metallicities. We use the high-resolution [(20 pc)3/cell] inhomogeneous chemical evolution tool `ICE' to study the outcomes of these effects. In our simulations, an adequate combination of neutron star mergers and neutron star-black hole mergers qualitatively reproduces the observed r-process abundances in the Galaxy.

The recent detection of 60Fe in the cosmic rays provides conclusive evidence that there is a recently synthesized component (few MY) in the GCRs (Binns et al. 2016). In addition, these nuclei must have been synthesized and accelerated in supernovae near the solar system, probably in the Sco-Cen OB association subgroups, which are about 100 pc distant from the Sun. Recent theoretical work on the production of r-process nuclei appears to indicate that it is difficult for SNe to produce the solar system abundances relative to iron of r-process elements with high atomic number (Z), including the actinides (Th, U, Np, Pu, and Cm). Instead, it is believed by many that the heaviest r-process nuclei, or perhaps even all r-process nuclei, are produced in binary neutron star mergers. Since we now know that there is at least a component of the GCRs that has been recently synthesized and accelerated, models of r-process production by SNe and BNSM can be tested by measuring the relative abundances of these ultra-heavy r-process nuclei, and especially the actinides, since they are radioactive and provide clocks that give the time interval from nucleosynthesis to detection at Earth. Since BNSM are believed to be much less frequent in our galaxy than SNe (roughly 1000 times less frequent, the ratios of the actinides, each with their own half-life, will enable a clear determination of whether the heaviest r-process nuclei are synthesized in SNe or in BNSM. In addition, the r-process nuclei for the charge range from 34 to 82 can be used to constrain models of r-process production in BNSM and SNe. Thus, GCRs become a multi-messenger component in the study of BNSM and SNe., Comment: Astro2020 Science White Paper

The enrichment history of heavy neutron-capture elements in the Milky Way disc provides fundamental information about the chemical evolution of our Galaxy and about the stellar sources that made those elements. In this work we give new observational data for Sr, the element at the first neutron-shell closure beyond iron, N=50, based on the analysis of the high resolution spectra of 276 Galactic disc stars. The Sr abundance was derived by comparing the observed and synthetic spectra in the region of the SrI 4607 A line, making use of the LTE approximation. NLTE corrections lead to an increase of the abundance estimates obtained under LTE, but for these lines they are minor near solar metallicity. The average correction that we find is 0.151 dex. The star that is mostly affected is HD 6582, with a 0.244 dex correction. The behavior of the Sr abundance as a function of metallicity is discussed within a stellar nucleosynthesis context, in comparison with the abundance of the heavy neutron-capture elements Ba (Z=56) and Eu (Z=63). The comparison of the observational data with the current GCE models confirm that the s-process contributions from Asymptotic Giant Branch stars and from massive stars are the main sources of Sr in the Galactic disc and in the Sun, while different nucleosynthesis sources can explain the high [Sr/Ba] and [Sr/Eu] ratios observed in the early Galaxy., Comment: 16 pages, 14 figures, accepted for publication in MNRAS

In this White Paper we present the potential of the enhanced X-ray Timing and Polarimetry (eXTP) mission for studies related to Observatory Science targets. These include flaring stars, supernova remnants, accreting white dwarfs, low and high mass X-ray binaries, radio quiet and radio loud active galactic nuclei, tidal disruption events, and gamma-ray bursts. eXTP will be excellently suited to study one common aspect of these objects: their often transient nature. Developed by an international Consortium led by the Institute of High Energy Physics of the Chinese Academy of Science, the eXTP mission is expected to be launched in the mid 2020s., Comment: Accepted for publication on Sci. China Phys. Mech. Astron. (2019)

Female role models reduce the impact on women of stereotype threat, i.e., of being at risk of conforming to a negative stereotype about one's social, gender, or racial group. This can lead women scientists to underperform or to leave their scientific career because of negative stereotypes such as, not being as talented or as interested in science as men. Sadly, history rarely provides role models for women scientists; instead, it often renders these women invisible. In response to this situation, we present a selection of twelve outstanding women who helped to develop nuclear astrophysics., Comment: 5 pages; to appear in Springer Proceedings in Physics (Proc. of Intl. Conf. "Nuclei in the Cosmos XV", LNGS Assergi, Italy, June 2018)

Using isochronous mass spectrometry at the experimental storage ring CSRe in Lanzhou, the masses of $^{82}$Zr and $^{84}$Nb were measured for the first time with an uncertainty of $\sim 10$ keV, and the masses of $^{79}$Y, $^{81}$Zr, and $^{83}$Nb were re-determined with a higher precision. %The latter differ significantly from their literature values. The latter are significantly less bound than their literature values. Our new and accurate masses remove the irregularities of the mass surface in this region of the nuclear chart. Our results do not support the predicted island of pronounced low $\alpha$ separation energies for neutron-deficient Mo and Tc isotopes, making the formation of Zr-Nb cycle in the $rp$-process unlikely. The new proton separation energy of $^{83}$Nb was determined to be 490(400)~keV smaller than that in the Atomic Mass Evaluation 2012. This partly removes the overproduction of the $p$-nucleus $^{84}$Sr relative to the neutron-deficient molybdenum isotopes in the previous $\nu p$-process simulations.

The existence of neutron star mergers has been supported since the discovery of the binary pulsar and the observation of its orbital energy loss, consistent with General Relativity. They are considered nucleosynthesis sites of the rapid neutron-capture process (r-process), which is responsible for creating approximately half of all heavy elements beyond Fe and is the only source of elements beyond Pb and Bi. Detailed nucleosynthesis calculations based on the decompression of neutron star matter are consistent with solar r-process abundances of heavy nuclei. Neutron star mergers have also been identified with short-duration {\gamma}-ray bursts via their IR afterglow. The high neutron densities in ejected matter permit a violent r-process, leading to fission cycling of the heaviest nuclei in regions far from (nuclear) stability. Uncertainties in several nuclear properties affect the abundance distributions. The modeling of astrophysical events also depends on the hydrodynamic treatment, the occurrence of a neutrino wind after the merger and before the possible emergence of a black hole, and the properties of black hole accretion disks. We discuss the effect of nuclear and modeling uncertainties and conclude that binary compact mergers are probably a (or the) dominant site of the production of r-process nuclei in our Galaxy., Comment: preprint version of a review to appear in Ann. Rev. Nucl. Part. Sci. 67 (2017), literature review up to February 2017

Core-collapse supernovae are the first polluters of heavy elements in the galactic history. As such, it is important to study the nuclear compositions of their ejecta, and understand their dependence on the progenitor structure (e.g., mass, compactness, metallicity). Here, we present a detailed nucleosynthesis study based on two long-term, two-dimensional core-collapse supernova simulations of a 11.2 M$_{\odot}$ and a 17.0 M$_{\odot}$ star. We find that in both models nuclei well beyond the iron group (up to $Z \approx 44$) can be produced, and discuss in detail also the nucleosynthesis of the p-nuclei $^{92,94}$Mo and $^{96,98}$Ru. While we observe the production of $^{92}$Mo and $^{94}$Mo in slightly neutron-rich conditions in both simulations, $^{96,98}$Ru can only be produced efficiently via the $\nu$p-process. Furthermore, the production of Ru in the $\nu$p-process heavily depends on the presence of very proton-rich material in the ejecta. This disentanglement of production mechanisms has interesting consequences when comparing to the abundance ratios between these isotopes in the solar system and in presolar grains., Comment: 48 pages, 19 figures, accepted for publication in: J. Phys. G: Nucl. Part. Phys

Atmospheric parameters and chemical compositions for ten stars with metallicities in the region of -2.2< [Fe/H] <-0.6 were precisely determined using high resolution, high signal to noise, spectra. For each star the abundances, for 14 to 27 elements, were derived using both LTE and NLTE approaches. In particular, differences by assuming LTE or NLTE are about 0.10 dex; depending on [Fe/H], Teff, gravity and element lines used in the analysis. We find that the O abundance has the largest error, ranging from 0.10 and 0.2 dex. The best measured elements are Cr, Fe, and Mn; with errors etween 0.03 and 0.11 dex. The stars in our sample were included in previous different observational work. We provide a consistent data analysis. The data dispersion introduced in the literature by different techniques and assumptions used by the different authors is within the observational errors, excepting for HD103095. We compare these results with stellar observations from different data sets and a number of theoretical galactic chemical evolution (GCE) simulations. We find a large scatter in the GCE results, used to study the origin of the elements. Within this scatter as found in previous GCE simulations, we cannot reproduce the evolution of the elemental ratios [Sc/Fe], [Ti/Fe], and [V/Fe] at different metallicities. The stellar yields from core collapse supernovae (CCSN) are likely primarily responsible for this discrepancy. Possible solutions and open problems are discussed., Comment: 25 pages, 7 figures

In this work we present and discuss the observations of the Mn abundances for 247 FGK dwarfs, located in the Galactic disc with metallicity -1

Nowadays the number of models aimed at explaining the Type Ia supernova phenomenon is high and discriminating between them is a must-do. In this work we explore the influence of rotation in the evolution of the nuclear flame which drives the explosion in the so called gravitational confined detonation models. Assuming that the flame starts in a point-like region slightly above the center of the white dwarf (WD) and adding a moderate amount of angular velocity to the star we follow the evolution of the deflagration using a smoothed particle hydrodynamics code. We find that the results are very dependent on the angle between the rotational axis and the line connecting the initial bubble of burned material with the center of the white dwarf at the moment of the ignition. The impact of rotation is larger for angles close to 90{\deg} because the Coriolis force on a floating element of fluid is maximum, and its principal effect is to break the symmetry of the deflagration. Such symmetry breaking weakens the convergence of the nuclear flame at the antipodes of the initial ignition volume, changing the environmental conditions around the convergence region with respect to non-rotating models. These changes seem to disfavor the emergence of a detonation in the compressed volume at the antipodes, thus compromising the viability of the so called gravitational confined detonation mechanism., Comment: 14 pages, 12 figures, submitted to ApJ

Baryon density inhomogeneities in the early universe can give rise to a floor of heavy elements (up to $A\approx 270$) produced in a primordial r-process with fission cycling. A parameter study with variation of the global baryon to photon ratio $\eta$, and under inclusion of neutron diffusion effects was performed. New results concerning the dependence of the results on nuclear physics parameters are presented., Comment: 6 pages, 1 table, 2 figures; talk at the European Workshop on Heavy Element Nucleosynthesis, Budapest 1994; appeared in the proceedings, uploaded for easy access. in Proc. European Workshop on Heavy Element Nucleosynthesis, Budapest 1994, eds. E. Somorjai und Z. F\"ul\"op (Institute of Nuclear Research of the Hungarian Academy of Sciences, Debrecen 1994), p. 121

A number of possible mechanisms have been suggested to generate density inhomogeneities in the early Universe which could survive until the onset of primordial nucleosynthesis and generate neutron-rich regions. In this work we are not concerned with how the inhomogeneities were generated but we want to focus on the effect of such inhomogeneities on primordial nucleosynthesis. One of the proposed signatures of inhomogeneity, the synthesis of very heavy elements by neutron capture in a primordial r-process, was analyzed for varying baryon to photon ratios $\eta$ and fluctuation length scales $L$., Comment: 4 pages, 1 figure; talk at the ESO/EIPC Workshop on "Light Element Abundances", Marciana Marina, Isola d'Elba 21-26 May 1994; appeared in the workshop proceedings, uploaded for easy access. in Proceedings of the ESO/EIPC Workshop "The Light Element Abundances", ed. P. Crane (Springer, Berlin Heidelberg 1995), p. 31

For the origin of heavy r-process elements, different sources have been proposed, e.g., core-collapse supernovae or neutron star mergers. Old metal-poor stars carry the signature of the astrophysical source(s). Among the elements dominantly made by the r-process, europium (Eu) is relatively easy to observe. In this work we simulate the evolution of europium in our galaxy with the inhomogeneous chemical evolution model 'ICE', and compare our results with spectroscopic observations. We test the most important parameters affecting the chemical evolution of Eu: (a) for neutron star mergers the coalescence time scale of the merger ($t_{\mathrm{coal}}$) and the probability to experience a neutron star merger event after two supernova explosions occurred and formed a double neutron star system ($P_{\mathrm{NSM}}$) and (b) for the sub-class of magneto-rotationally driven supernovae ("Jet-SNe"), their occurrence rate compared to standard supernovae ($P_{\mathrm{Jet-SN}}$). We find that the observed [Eu/Fe] pattern in the galaxy can be reproduced by a combination of neutron star mergers and magneto-rotationally driven supernovae as r-process sources. While neutron star mergers alone seem to set in at too high metallicities, Jet-SNe provide a cure for this deficiency at low metallicities. Furthermore, we confirm that local inhomogeneities can explain the observed large spread in the europium abundances at low metallicities. We also predict the evolution of [O/Fe] to test whether the spread in $\alpha$-elements for inhomogeneous models agrees with observations and whether this provides constraints on supernova explosion models and their nucleosynthesis., Comment: 11 Figures, Submitted to MNRAS

This is a White Paper in support of the mission concept of the Large Observatory for X-ray Timing (LOFT), proposed as a medium-sized ESA mission. We discuss the potential of LOFT for the study of thermonuclear X-ray bursts on accreting neutron stars. For a summary, we refer to the paper., Comment: White Paper in Support of the Mission Concept of the Large Observatory for X-ray Timing

We report on a method, PUSH, for triggering core-collapse supernova explosions of massive stars in spherical symmetry. We explore basic explosion properties and calibrate PUSH such that the observables of SN1987A are reproduced. Our simulations are based on the general relativistic hydrodynamics code AGILE combined with the detailed neutrino transport scheme IDSA for electron neutrinos and ALS for the muon and tau neutrinos. To trigger explosions in the otherwise non-exploding simulations, we rely on the neutrino-driven mechanism. The PUSH method locally increases the energy deposition in the gain region through energy deposition by the heavy neutrino flavors. Our setup allows us to model the explosion for several seconds after core bounce. We explore the progenitor range 18-21M$_{\odot}$. Our studies reveal a distinction between high compactness (HC) and low compactness (LC) progenitor models, where LC models tend to explore earlier, with a lower explosion energy, and with a lower remnant mass. HC models are needed to obtain explosion energies around 1 Bethe, as observed for SN1987A. However, all the models with sufficiently high explosion energy overproduce $^{56}$Ni. We conclude that fallback is needed to reproduce the observed nucleosynthesis yields. The nucleosynthesis yields of $^{57-58}$Ni depend sensitively on the electron fraction and on the location of the mass cut with respect to the initial shell structure of the progenitor star. We identify a progenitor and a suitable set of PUSH parameters that fit the explosion properties of SN1987A when assuming 0.1M$_{\odot}$ of fallback. We predict a neutron star with a gravitational mass of 1.50M$_{\odot}$. We find correlations between explosion properties and the compactness of the progenitor model in the explored progenitors. However, a more complete analysis will require the exploration of a larger set of progenitors with PUSH., Comment: revised version as accepted by ApJ (results unchanged, text modified for clarification, a few references added); 26 pages, 20 figures

The direct impact of white dwarfs has been suggested as a plausible channel for type Ia supernovae. In spite of their (a priori) rareness, in highly populated globular clusters and in galactic centers, where the amount of white dwarfs is considerable, the rate of violent collisions between two of them might be non-negligible. Even more, there are indications that binary white dwarf systems orbited by a third stellar-mass body have an important chance to induce a clean head-on collision. Therefore, this scenario represents a source of contamination for the supernova light-curves sample that it is used as standard candles in cosmology, and it deserves further investigation. Some groups have conducted numerical simulations of this scenario, but their results show several differences. In this paper we address some of the possible sources of these differences, presenting the results of high resolution hydrodynamical simulations jointly with a detailed nuclear post-processing of the nuclear abundances, to check the viability of white dwarf collisions to produce significant amounts of 56Ni. To that purpose, we use a 2D-axial symmetric smoothed particle hydrodynamic code to obtain a resolution considerably higher than in previous studies. In this work, we also study how the initial mass and nuclear composition affect the results. The gravitational wave emission is also calculated, as this is a unique signature of this kind of events. All calculated models produce a significant amount of 56Ni, ranging from 0.1 Msun to 1.1 Msun, compatible not only with normal-Branch type Ia supernova but also with the subluminous and super-Chandrasekhar subset. Nevertheless, the distribution mass-function of white dwarfs favors collisions among 0.6-0.7 Msun objects, leading to subluminous events., Comment: 24 pages, 12 figures, accepted for publication in MNRAS

We follow the longterm evolution of the dynamic ejecta of neutron star mergers for up to 100 years and over a density range of roughly 40 orders of magnitude. We include the nuclear energy input from the freshly synthesized, radioactively decaying nuclei in our simulations and study its effects on the remnant dynamics. Although the nuclear heating substantially alters the longterm evolution, we find that running nuclear networks over purely hydrodynamic simulations (i.e. without heating) yields actually acceptable nucleosynthesis results. The main dynamic effect of the radioactive heating is to quickly smooth out inhomogeneities in the initial mass distribution, subsequently the evolution proceeds self-similarly and after 100 years the remnant still carries the memory of the initial binary mass ratio. We also explore the nucleosynthetic yields for two mass ejection channels. The dynamic ejecta very robustly produce "strong" r-process elements with $A > 130$ with a pattern that is essentially independent of the details of the merging system. From a simple model we find that neutrino-driven winds yield "weak" r-process contributions with $50 < A < 130$ whose abundance patterns vary substantially between different merger cases. This is because their electron fraction, set by the ratio of neutrino luminosities, varies considerably from case to case. Such winds do not produce any $^{56}{\rm Ni}$, but a range of radioactive isotopes that are long-lived enough to produce a second, radioactively powered electromagnetic transient in addition to the "macronova" from the dynamic ejecta. While our wind model is very simple, it nevertheless demonstrates the potential of such neutrino-driven winds for electromagnetic transients and it motivates further, more detailed neutrino-hydrodynamic studies. The properties of the mentioned transients are discussed in more detail in a companion paper., Comment: 13 pages, 11 figures, accepted to MNRAS

Carbon-rich grains are observed to condense in the ejecta of recent core-collapse supernovae, within a year after the explosion. Silicon carbide grains of type X are C-rich grains with isotpic signatures of explosive supernova nucleosynthesis have been found in primitive meteorites. Much rarer silicon carbide grains of type C are a special sub-group of SiC grains from supernovae. They show peculiar abundance signatures for Si and S, isotopically heavy Si and isotopically light S, which appear to to be in disagreement with model predictions. We propose that C grains are formed mostly from C-rich stellar material exposed to lower SN shock temperatures than the more common type X grains. In this scenario, extreme $^{32}$S enrichments observed in C grains may be explained by the presence of short-lived $^{32}$Si ($\tau$$_{1/2}$ = 153 years) in the ejecta, produced by neutron capture processes starting from the stable Si isotopes. No mixing from deeper Si-rich material and/or fractionation of Si from S due to molecular chemistry is needed to explain the $^{32}$S enrichments. The abundance of $^{32}$Si in the grains can provide constraints on the neutron density reached during the supernova explosion in the C-rich He shell material. The impact of the large uncertainty of the neutron capture cross sections in the $^{32}$Si region is discussed., Comment: 13 pages, 5 figures, The Astrophysical Journal Letters

About a year after core collapse supernova, dust starts to condense in the ejecta. In meteorites, a fraction of C-rich presolar grains (e.g., silicon carbide (SiC) grains of Type-X and low density graphites) are identified as relics of these events, according to the anomalous isotopic abundances. Several features of these abundances remain unexplained and challenge the understanding of core-collapse supernovae explosions and nucleosynthesis. We show, for the first time, that most of the measured C-rich grain abundances can be accounted for in the C-rich material from explosive He burning in core-collapse supernovae with high shock velocities and consequent high temperatures. The inefficiency of the $^{12}$C($\alpha$,$\gamma$)$^{16}$O reaction relative to the rest of the $\alpha$-capture chain at $T > 3.5\times10^8 \mathrm{K}$ causes the deepest He-shell material to be carbon rich and silicon rich, and depleted in oxygen. The isotopic ratio predictions in part of this material, defined here as the C/Si zone, are in agreement with the grain data. The high-temperature explosive conditions that our models reach at the bottom of the He shell, can also be representative of the nucleosynthesis in hypernovae or in the high-temperature tail of a distribution of conditions in asymmetric supernovae. Finally, our predictions are consistent with the observation of large $^{44}$Ca/$^{40}$Ca observed in the grains. This is due to the production of $^{44}$Ti together with $^{40}$Ca in the C/Si zone, and/or to the strong depletion of $^{40}$Ca by neutron captures., Comment: 13 pages, 4 figures, The Astrophysical Journal Letters, accepted

The aim of this work is to present and discuss the observations of the iron peak (Fe, Ni) and neutron-capture element (Y, Zr, Ba, La, Ce, Nd, Sm, and Eu) abundances for 276 FGK dwarfs, located in the galactic disk with metallicity -1 < [Fe/H] < +0.3. Atmospheric parameters and chemical composition of the studied stars were determined from an high resolution, high signal-to-noise echelle spectra obtained with the echelle spectrograph ELODIE at the Observatoire de Haute-Provence (France). Effective temperatures were estimated by the line depth ratio method and from the H_{\alpha} line-wing fitting. Surface gravities (log g) were determined by parallaxes and the ionization balance of iron. Abundance determinations were carried out using the LTE approach, taking the hyperfine structure for Eu into account, and the abundance of Ba was computed under the NLTE approximation. We are able to assign most of the stars in our sample to the substructures of the Galaxy thick disk, thin disk, or Hercules stream according to their kinematics. The classification of 27 stars is uncertain. For most of the stars in the sample, the abundances of neutron-capture elements have not been measured earlier. For all of them, we provide the chemical composition and discuss the contribution from different nucleosynthesis processes.

We compare three different statistical models for the equation of state (EOS) of stellar matter at subnuclear densities and temperatures (0.5-10 MeV) expected to occur during the collapse of massive stars and supernova explosions. The models introduce the distributions of various nuclear species in nuclear statistical equilibrium, but use somewhat different nuclear physics inputs. It is demonstrated that the basic thermodynamical quantities of stellar matter under these conditions are similar, except in the region of high densities and low temperatures. We demonstrate that mass and isotopic distributions have considerable differences related to the different assumptions of the models on properties of nuclei at these stellar conditions. Overall, the three models give similar trends, but the details reflect the uncertainties related to the modeling of medium effects, such as the temperature and density dependence of surface and bulk energies of heavy nuclei, and the nuclear shell structure effects. We discuss importance of new physics inputs for astrophysical calculations from experimental data obtained in intermediate energy heavy-ion collisions, in particular, the similarities of the conditions reached during supernova explosions and multifragmentation reactions., Comment: 52 pages including 24 figures, accepted to Nucl.Phys. A

We discuss the possibility whether superheavy elements can be produced in Nature by the astrophysical rapid neutron capture process. To this end we have performed fully dynamical network r-process calculations assuming an environment with neutron-to-seed ratio large enough to produce superheavy nuclei. Our calculations include two sets of nuclear masses and fission barriers and include all possible fission channels and the associated fission yield distributions. Our calculations produce superheavy nuclei with A ~ 300 that however decay on timescales of days., Comment: 12 pages, 11 figures

Neutrino-driven winds, which follow core-collapse supernova explosions, present a fascinating nuclear astrophysics problem that requires understanding advanced astrophysics simulations, the properties of matter and neutrino interactions under extreme conditions, the structure and reactions of exotic nuclei, and comparisons against forefront astronomical observations. The neutrino-driven wind has attracted vast attention over the last 20 years as it was suggested to be a candidate for the astrophysics site where half of the heavy elements are produced via the r-process. In this review, we summarize our present understanding of neutrino-driven winds from the dynamical and nucleosynthesis perspectives. Rapid progress has been made during recent years in understanding the wind with improved simulations and better micro physics. The current status of the fields is that hydrodynamical simulations do not reach the extreme conditions necessary for the r-process and the proton or neutron richness of the wind remains to be investigated in more detail. However, nucleosynthesis studies and observations point already to neutrino-driven winds to explain the origin of lighter heavy elements, such as Sr, Y, Zr., Comment: Submitted to: J. Phys. G: Nucl. Phys

We discuss the possible impact of strange quark matter on the evolution of core-collapse supernovae with emphasis on low critical densities for the quark-hadron phase transition. For such cases the hot proto-neutron star can collapse to a more compact hybrid star configuration hundreds of milliseconds after core-bounce. The collapse triggers the formation of a second shock wave. The latter leads to a successful supernova explosion and leaves an imprint on the neutrino signal. These dynamical features are discussed with respect to their compatibility with recent neutron star mass measurements which indicate a stiff high density nuclear matter equation of state., Comment: 8 pages, 3 figures, Invited talk at the "Strangeness in Quark Matter" conference, 18-24 September 2011, Polish Academy of Arts and Sciences, Cracow, Poland

We examine simulations of core-collapse supernovae in spherical symmetry. Our model is based on general relativistic radiation hydrodynamics with three-flavor Boltzmann neutrino transport. We discuss the different supernova phases, including the long-term evolution up to 20 seconds after the onset of explosion during which the neutrino fluxes and mean energies decrease continuously. In addition, the spectra of all flavors become increasingly similar, indicating the change from charged- to neutral-current dominance. Furthermore, it has been shown recently by several groups independently, based on sophisticated supernova models, that collective neutrino flavor oscillations are suppressed during the early mass-accretion dominated post-bounce evolution. Here we focus on the possibility of collective flavor flips between electron and non-electron flavors during the later, on the order of seconds, evolution after the onset of an explosion with possible application for the nucleosynthesis of heavy elements., Comment: 12 pages, 7 figures, conference proceeding, HANSE 2011 workshop

Context. Rotation is known to affect the nucleosynthesis of light elements in massive stars, mainly by rotation-induced mixing. In particular, rotation boosts the primary nitrogen production. Models of rotating stars are able to reproduce the nitrogen observed in low-Z halo stars. Aims. Here we present the first grid of stellar models for rotating massive stars at low Z, where a full s-process network is used to study the impact of rotation-induced mixing on the nucleosynthesis of heavy elements. Methods. We used the Geneva stellar evolution code that includes an enlarged reaction network with nuclear species up to bismuth to calculate 25 M$_\odot$ models at three different Z and with different initial rotation rates. Results. First, we confirm that rotation-induced mixing leads to a production of primary $^{22}$Ne, which is the main neutron source for the s process in massive stars. Therefore rotation boosts the s process in massive stars at all Z. Second, the neutron-to-seed ratio increases with decreasing Z in models including rotation, which leads to the complete consumption of all iron seeds at Z < 1e-3 by the end of core He-burning. Thus at low Z, the iron seeds are the main limitation for this boosted s process. Third, as Z decreases, the production of elements up to the Ba peak increases at the expense of the elements of the Sr peak. We studied the impact of the initial rotation rate and of the uncertain $^{17}$O$(\alpha,\gamma)$ rate (which strongly affects the neutron poison strength of $^{16}$O) on our results. This study shows that rotating models can produce significant amounts of elements up to Ba over a wide range of Z. Fourth, compared to the He-core, the primary $^{22}$Ne production in the He-shell is even higher (> 1% in mass fraction at all Z), which could open the door for an explosive neutron capture nucleosynthesis in the He-shell, with a primary neutron source., Comment: 4 pages, 1 figure, accepted by A&A, language edited

Proton-activation reactions on natural and enriched palladium samples were investigated via the activation technique in the energy range of E_p=2.75 MeV to 9 MeV, close to the upper end of the respective Gamow window of the gamma process. We have determined cross sections for 102Pd(p,gamma)103Ag, 104Pd(p,gamma)105Ag, and 105Pd(p,n)105Ag, as well as partial cross sections of 104Pd(p,n)104Ag^g, 105Pd(p,gamma)106Ag^m, 106Pd(p,n)106Ag^m, and 110Pd(p,n)110Ag^m with uncertainties between 3% and 15% for constraining theoretical Hauser-Feshbach rates and for direct use in gamma-process calculations., Comment: 12 pages, accepted for publication in Phys. Rev. C (2011)

We propose to perform a continuously scanning all-sky survey from 200 keV to 80 MeV achieving a sensitivity which is better by a factor of 40 or more compared to the previous missions in this energy range. The Gamma-Ray Imaging, Polarimetry and Spectroscopy (GRIPS) mission addresses fundamental questions in ESA's Cosmic Vision plan. Among the major themes of the strategic plan, GRIPS has its focus on the evolving, violent Universe, exploring a unique energy window. We propose to investigate $\gamma$-ray bursts and blazars, the mechanisms behind supernova explosions, nucleosynthesis and spallation, the enigmatic origin of positrons in our Galaxy, and the nature of radiation processes and particle acceleration in extreme cosmic sources including pulsars and magnetars. The natural energy scale for these non-thermal processes is of the order of MeV. Although they can be partially and indirectly studied using other methods, only the proposed GRIPS measurements will provide direct access to their primary photons. GRIPS will be a driver for the study of transient sources in the era of neutrino and gravitational wave observatories such as IceCUBE and LISA, establishing a new type of diagnostics in relativistic and nuclear astrophysics. This will support extrapolations to investigate star formation, galaxy evolution, and black hole formation at high redshifts., Comment: to appear in Exp. Astron., special vol. on M3-Call of ESA's Cosmic Vision 2010; 25 p., 25 figs; see also www.grips-mission.eu

We explore explosions of massive stars, which are triggered via the quark-hadron phase transition during the early post bounce phase of core-collapse supernovae. We construct a quark equation of state, based on the bag model for strange quark matter. The transition between the hadronic and the quark phases is constructed applying Gibbs conditions. The resulting quark-hadron hybrid equations of state are used in core-collapse supernova simulations, based on general relativistic radiation hydrodynamics and three flavor Boltzmann neutrino transport in spherical symmetry. The formation of a mixed phase reduces the adiabatic index, which induces the gravitational collapse of the central protoneutron star. The collapse halts in the pure quark phase, where the adiabatic index increases. A strong accretion shock forms, which propagates towards the protoneutron star surface. Due to the density decrease of several orders of magnitude, the accretion shock turns into a dynamic shock with matter outflow. This moment defines the onset of the explosion in supernova models that allow for a quark-hadron phase transition, where otherwise no explosions could be obtained. The shock propagation across the neutrinospheres releases a burst of neutrinos. This serves as a strong observable identification for the structural reconfiguration of the stellar core. The ejected matter expands on a short timescale and remains neutron-rich. These conditions might be suitable for the production of heavy elements via the r-process. The neutron-rich material is followed by proton-rich neutrino-driven ejecta in the later cooling phase of the protoneutron star where the vp-process might occur., Comment: 29 pages, 24 figures, submitted to ApJ

For finite chemical potential effective models of QCD predict a first order phase transition. In favour for the search of such a phase transition in nature, we construct an equation of state for strange quark matter based on the MIT bag model. We apply this equation of state to highly asymmetric core collapse supernova matter with finite temperatures and large baryon densities. The phase transition is constructed using the general Gibbs conditions, which results in an extended coexistence region between the pure hadronic and pure quark phases in the phase diagram, i.e. the mixed phase. The supernovae are simulated via general relativistic radiation hydrodynamics based on three flavor Boltzmann neutrino transport in spherical symmetry. During the dynamical evolution temperatures above 10 MeV, baryon densities above nuclear saturation density and a proton-to-baryon ratio below 0.2 are obtained. At these conditions the phase transition is triggered which leads to a significant softening of the EoS for matter in the mixed phase. As a direct consequence of the stiffening of the EoS again for matter in the pure quark phase, a shock wave forms at the boundary between the mixed and the pure hadronic phases. This shock is accelerated and propagates outward which releases a burst of neutrinos dominated by electron anti-neutrinos due to the lifted degeneracy of the shock-heated hadronic material. We discuss the radiation-hydrodynamic evolution of the phase transition at the example of several low and intermediate mass Fe-core progenitor stars and illustrate the expected neutrino signal from the phase transition., Comment: 14 pages, 4 figures, 1 table. To appear in Proceedings for "Compact stars in the QCD phase diagram II (CSQCD II), May 20-24, 2009, KIAA at Peking University, Beijing - P. R. China [http://vega.bac.pku.edu.cn/rxxu/csqcd.htm]

Explosive astrophysical systems, such as supernovae or compact star binary mergers, provide conditions where strange quark matter can appear. The high degree of isospin asymmetry and temperatures of several MeV in such systems may cause a transition to the quark phase already around saturation density. Observable signals from the appearance of quark matter can be predicted and studied in astrophysical simulations. As input in such simulations, an equation of state with an integrated quark matter phase transition for a large temperature, density and proton fraction range is required. Additionally, restrictions from heavy ion data and pulsar observation must be considered. In this work we present such an approach. We implement a quark matter phase transition in a hadronic equation of state widely used for astrophysical simulations and discuss its compatibility with heavy ion collisions and pulsar data. Furthermore, we review the recently studied implications of the QCD phase transition during the early post-bounce evolution of core-collapse supernovae and introduce the effects from strong interactions to increase the maximum mass of hybrid stars. In the MIT bag model, together with the strange quark mass and the bag constant, the strong coupling constant $\alpha_s$ provides a parameter to set the beginning and extension of the quark phase and with this the mass and radius of hybrid stars., Comment: 6 pages, 5 figures, talk given at the International Conference on Strangeness in Quark Matter (SQM2009), Buzios, Brasil, September 28 - October 2, 2009, to be published in Journal Phys. G

The nucleosynthetic characteristics of various explosion mechanisms of Type Ia supernovae (SNe Ia) is explored based on three two-dimensional explosion simulations representing extreme cases: a pure turbulent deflagration, a delayed detonation following an approximately spherical ignition of the initial deflagration, and a delayed detonation arising from a highly asymmetric deflagration ignition. Apart from this initial condition, the deflagration stage is treated in a parameter-free approach. The detonation is initiated when the turbulent burning enters the distributed burning regime. This occurs at densities around $10^{7}$ g cm$^{-3}$ -- relatively low as compared to existing nucleosynthesis studies for one-dimensional spherically symmetric models. The burning in these multidimensional models is different from that in one-dimensional simulations as the detonation wave propagates both into unburned material in the high density region near the center of a white dwarf and into the low density region near the surface. Thus, the resulting yield is a mixture of different explosive burning products, from carbon-burning products at low densities to complete silicon-burning products at the highest densities, as well as electron-capture products synthesized at the deflagration stage. In contrast to the deflagration model, the delayed detonations produce a characteristic layered structure and the yields largely satisfy constraints from Galactic chemical evolution. In the asymmetric delayed detonation model, the region filled with electron capture species (e.g., $^{58}$Ni, $^{54}$Fe) is within a shell, showing a large off-set, above the bulk of $^{56}$Ni distribution, while species produced by the detonation are distributed more spherically (abridged)., Comment: Accepted by the Astrophysical Journal. 15 pages, 14 figures, 4 tables

We have investigated the (n,gamma) cross sections of p-process isotopes with the activation technique. The measurements were carried out at the Karlsruhe Van de Graaff accelerator using the 7Li(p,n)7Be source for simulating a Maxwellian neutron distribution of kT = 25 keV. Stellar cross section measurements are reported for the light p-process isotopes 102Pd, 120Te, 130,132Ba, and 156Dy. In a following paper the cross sections of 168Yb, 180W, 184Os, 190Pt, and 196Hg will be discussed. The data are extrapolated to p-process energies by including information from evaluated nuclear data libraries. The results are compared to standard Hauser-Feshbach models frequently used in astrophysics., Comment: 13 pages, 4 figures

Neutron-induced reaction rates, including fission, are calculated in the temperature range 1.d8

Massive stars end their life in an explosion event with kinetic energies of the order 1 Bethe. Immediately after the explosion has been launched, a region of low density and high entropy forms behind the ejecta which is continuously subject to neutrino heating. The neutrinos emitted from the remnant at the center, the protoneutron star (PNS), heat the material above the PNS surface. This heat is partly converted into kinetic energy and the material accelerates to an outflow that is known as the neutrino driven wind. For the first time, we simulate the collapse, bounce, explosion and the neutrino driven wind phases consistently over more than 20 seconds. Our numerical model is based on spherically symmetric general relativistic radiation hydrodynamics using spectral three flavor Boltzmann neutrino transport. In simulations where no explosions are obtained naturally, we model neutrino driven explosions for low and intermediate mass Fe-core progenitor stars by enhancing the charged current reaction rates. In the case of a special progenitor star, the O-Ne-Mg-core, the explosion in spherical symmetry was obtained without enhanced opacities. The post explosion evolution is in qualitative agreement with static steady-state and parametrized dynamic models of the neutrino driven wind. On the other hand, we find generally smaller neutrino luminosities and mean neutrino energies as well as a different evolutionary behavior of the neutrino luminosities and mean neutrino energies. The neutrino driven wind is proton-rich for more than 10 seconds and the contraction of the PNS differs from the assumptions made for the conditions at the inner boundary in previous neutrino driven wind studies. Despite the moderately large entropies per baryon of about 100 and the fast expansion timescale, the conditions found in our model are unlikely to favor..., Comment: 26 pages, 19 figures

We have computed dipole strength distributions for nickel and tin isotopes within the Relativistic Quasiparticle Time Blocking approximation (RQTBA). These calculations provide a good description of data, including the neutron-rich tin isotopes $^{130,132}$Sn. The resulting dipole strengths have been implemented in Hauser-Feshbach calculations of astrophysical neutron capture rates relevant for r-process nucleosynthesis studies. The RQTBA calculations show the presence of enhanced dipole strength at energies around the neutron threshold for neutron rich nuclei. The computed neutron capture rates are sensitive to the fine structure of the low lying dipole strength, which emphasizes the importance of a reliable knowledge of this excitation mode., Comment: 15 pages, 4 figures, Accepted in Nucl. Phys. A

We propose to advance investigations of electromagnetic radiation originating in atomic nuclei beyond its current infancy to a true astronomy. This nuclear emission is independent from conditions of gas, thus complements more traditional stronomical methods used to probe the nearby universe. Radioactive gamma-rays arise from isotopes which are made in specific locations inside massive stars, their decay in interstellar space traces an otherwise not directly observable hot and tenuous phase of the ISM, which is crucial for feedback from massive stars. Its intrinsic clocks can measure characteristic times of processes within the ISM. Frontier questions that can be addressed with studies in this field are the complex interiors of massive stars and supernovae which are key agents in galactic dynamics and chemical evolution, the history of star-forming and supernova activity affecting our solar-system environment, and explorations of occulted and inaccessible regions of young stellar nurseries in our Galaxy., Comment: 7 pages, white paper for US Nat.Acad.Sci. Decadal Survey "Astro2010"

While the high-entropy wind (HEW) of Type II supernovae remains one of the more promising sites for the rapid neutron-capture (r-) process, hydrodynamic simulations have yet to reproduce the astrophysical conditions under which the latter occurs. We have performed large-scale network calculations within an extended parameter range of the HEW, seeking to identify or to constrain the necessary conditions for a full reproduction of all r-process residuals N_{r,\odot}=N_{\odot}-N_{s,\odot} by comparing the results with recent astronomical observations. A superposition of weighted entropy trajectories results in an excellent reproduction of the overall N_{r,\odot}-pattern beyond Sn. For the lighter elements, from the Fe-group via Sr-Y-Zr to Ag, our HEW calculations indicate a transition from the need for clearly different sources (conditions/sites) to a possible co-production with r-process elements, provided that a range of entropies are contributing. This explains recent halo-star observations of a clear non-correlation of Zn and Ge and a weak correlation of Sr - Zr with heavier r-process elements. Moreover, new observational data on Ru and Pd seem to confirm also a partial correlation with Sr as well as the main r-process elements (e.g. Eu)., Comment: 15 pages, 1 table, 4 figures; To be published in the Astrophysical Journal Letters

The r-process constitutes one of the major challenges in nuclear astrophysics. Its astrophysical site has not yet been identified but there is observational evidence suggesting that at least two possible sites should contribute to the solar system abundance of r-process elements and that the r-process responsible for the production of elements heavier than Z=56 operates quite robustly producing always the same relative abundances. From the nuclear-physics point of view the r-process requires the knowledge of a large number of reaction rates involving exotic nuclei. These include neutron capture rates, beta-decays and fission rates, the latter for the heavier nuclei produced in the r-process. We have developed for the first time a complete database of reaction rates that in addition to neutron-capture rates and beta-decay half-lives includes all possible reactions that can induce fission (neutron-capture, beta-decay and spontaneous fission) and the corresponding fission yields. In addition, we have implemented these reaction rates in a fully implicit reaction network. We have performed r-process calculations for the neutrino-driven wind scenario to explore whether or not fission can contribute to provide a robust r-process pattern.