Nucleosynthesis Results Research Articles

Element Formation in Radiation-hydrodynamics Simulations of Kilonovae

Understanding the details of r-process nucleosynthesis in binary neutron star merger (BNSM) ejecta is key to interpreting kilonova observations and identifying the role of BNSMs in the origin of heavy elements. We present a self-consistent, two-dimensional, ray-by-ray radiation-hydrodynamic evolution of BNSM ejecta with an online nuclear network (NN) up to a timescale of days. For the first time, an initial numerical relativity ejecta profile composed of the dynamical component and spiral-wave and disk winds is evolved including detailed r-process reactions and nuclear heating effects. A simple model for the jet energy deposition is also included. Our simulation highlights that the common approach of relating in postprocessing the final nucleosynthesis yields to the initial thermodynamic profile of the ejecta can lead to inaccurate predictions. Moreover, we find that neglecting the details of the radiation-hydrodynamic evolution of the ejecta in nuclear calculations can introduce deviations of up to 1 order of magnitude in the final abundances of several elements, including very light and second r-process peak elements. The presence of a jet affects element production only in the innermost part of the polar ejecta, and it does not alter the global nucleosynthesis results. Overall, our analysis shows that employing an online NN improves the reliability of nucleosynthesis and kilonova light-curve predictions.

Dependence of the Results of Nucleosynthesis on the Equation of State for Neutron-Star Matter

Dependence of the Results of Nucleosynthesis on the Equation of State for Neutron-Star Matter

Kilonovae of binary neutron star mergers leading to short-lived remnant neutron star formation

ABSTRACT We study kilonova emission from binary neutron star (BNS) mergers for the case that a remnant massive neutron star (MNS) forms and collapses to a black hole within 20 ms after the onset of the merger (which we refer to as ‘a short-lived case’) by consistently employing numerical relativity and nucleosynthesis results. We find that such kilonovae are fainter and last shorter than those for BNSs resulting in the formation of long-lived (${\gg} 1\, {\rm s}$) MNSs, in particular in the optical band. The resulting light curves are too faint and last for a too short duration to explain the kilonova observation for the BNS associated with GW170817, indicating that the merger remnant formed in GW170817 is unlikely to have collapsed to a black hole within a short period of time (∼20 ms) after the onset of the merger. Our present result implies that early observation is necessary to detect kilonovae associated with BNSs leading to short-lived MNS formation in particular for the optical blue band as well as that kilonovae could be hidden by the gamma-ray burst afterglow for nearly face-on observation. We provide a possible approximate scaling law for near-infrared light curves with the given reference time and magnitude when the decline power of the z-band magnitude, dMz/dlog10t, reaches 2.5. This scaling law suggests that the HK-band follow-up observation should be at least 1 mag deeper than that for the z-band reference magnitude and earlier than 4 times the reference time.

Chemical Evolution of R-process Elements in Stars (CERES)

Aims. The Chemical Evolution of R-process Elements in Stars (CERES) project aims to provide a homogeneous analysis of a sample of metal-poor stars ([Fe/H] < –1.5). We present the stellar parameters and the chemical abundances of elements up to Zr for a sample of 52 giant stars. Methods. We relied on a sample of high signal-to-noise UVES spectra. We determined stellar parameters from Gaia photometry and parallaxes. Chemical abundances were derived using spectrum synthesis and model atmospheres. Results. We determined chemical abundances of 26 species of 18 elements: Na, Mg, Al, Si, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Sr, Y, and Zr. For several stars, we were able to measure both neutral and ionised species, including Si, Sc, Mn, and Zr. We have roughly doubled the number of measurements of Cu for stars at [Fe/H] ≤ −2.5. The homogeneity of the sample made it possible to highlight the presence of two Zn-rich stars ([Zn/Fe] ∼ +0.7), one r-rich and the other r-poor. We report the existence of two branches in the [Zn/Fe] versus [Ni/Fe] plane and suggest that the high [Zn/Fe] branch is the result of hypernova nucleosynthesis. We discovered two stars with peculiar light neutron-capture abundance patterns: CES1237+1922 (also known as BS 16085-0050), which is ∼1 dex underabundant in Sr, Y, and Zr with respect to the other stars in the sample, and CES2250-4057 (also known as HE 2247-4113), which shows a ∼1 dex overabundance of Sr with respect to Y and Zr. Conclusions. The high quality of our dataset allowed us to measure hardly detectable ions. This can provide guidance in the development of line formation computations that take deviations from local thermodynamic equilibrium and hydrodynamical effects into account.

Type Iax supernovae from deflagrations in Chandrasekhar mass white dwarfs

Context. Due to the ever increasing number of observations during the past decades, Type Ia supernovae are nowadays regarded as a heterogeneous class of optical transients consisting of several subtypes. One of the largest of these subclasses is the class of Type Iax supernovae. They have been suggested to originate from pure deflagrations in carbon-oxygen Chandrasekhar mass white dwarfs because the outcome of this explosion scenario is in general agreement with their subluminous nature. Aims. Although a few deflagration studies have already been carried out, the full diversity of the class has not been captured yet. This, in particular, holds for the faint end of the subclass. We therefore present a parameter study of single-spot ignited deflagrations in Chandrasekhar mass white dwarfs varying the location of the ignition spark, the central density, the metallicity, and the composition of the white dwarf. We also explore a rigidly rotating progenitor to investigate whether the effect of rotation can spawn additional trends. Methods. We carried out three-dimensional hydrodynamic simulations employing the LEAFS code. Subsequently, detailed nucleosynthesis results were obtained with the nuclear network code YANN. In order to compare our results to observations, we calculated synthetic spectra and light curves with the ARTIS code. Results. The new set of models extends the range in brightness covered by previous studies to the lower end. Our single-spot ignited explosions produce 56Ni masses from 5.8 × 10−3 to 9.2 × 10−2 M⊙. In spite of the wide exploration of the parameter space, the main characteristics of the models are primarily driven by the mass of 56Ni and form a one-dimensional sequence. Secondary parameters seem to have too little impact to explain the observed trend in the faint part of the Type Iax supernova class. We report kick velocities of the gravitationally bound explosion remnants from 6.9 to 369.8 km s−1. The magnitude as well as the direction of the natal kick is found to depend on the strength of the deflagration. Conclusions. This work corroborates the results of previous studies of deflagrations in Chandrasekhar mass white dwarfs. The wide exploration of the parameter space in initial conditions and viewing angle effects in the radiative transfer lead to a significant spread in the synthetic observables. The trends in observational properties toward the faint end of the class are, however, not reproduced. This motivates a quantification of the systematic uncertainties in the modeling procedure and the influence of the 56Ni-rich bound remnant to get to the bottom of these discrepancies. Moreover, while the pure deflagration scenario remains a favorable explanation for bright and intermediate luminosity Type Iax supernovae, our results suggest that other mechanisms also contribute to this class of events.

Dynamical ejecta of neutron star mergers with nucleonic weak processes – II: kilonova emission

ABSTRACT The majority of existing results for the kilonova (or macronova) emission from material ejected during a neutron-star (NS) merger is based on (quasi-) one-zone models or manually constructed toy-model ejecta configurations. In this study, we present a kilonova analysis of the material ejected during the first $\sim 10\,$ ms of a NS merger, called dynamical ejecta, using directly the outflow trajectories from general relativistic smoothed-particle hydrodynamics simulations, including a sophisticated neutrino treatment and the corresponding nucleosynthesis results, which have been presented in Part I of this study. We employ a multidimensional two-moment radiation transport scheme with approximate M1 closure to evolve the photon field and use a heuristic prescription for the opacities found by calibration with atomic-physics-based reference results. We find that the photosphere is generically ellipsoidal but augmented with small-scale structure and produces emission that is about 1.5–3 times stronger towards the pole than the equator. The kilonova typically peaks after $0.7\!-\!1.5\,$ d in the near-infrared frequency regime with luminosities between $3\!-\!7\times 10^{40}\,$ erg s−1 and at photospheric temperatures of $2.2\!-\!2.8\times 10^3\,$ K. A softer equation of state or higher binary-mass asymmetry leads to a longer and brighter signal. Significant variations of the light curve are also obtained for models with artificially modified electron fractions, emphasizing the importance of a reliable neutrino-transport modelling. None of the models investigated here, which only consider dynamical ejecta, produces a transient as bright as AT2017gfo. The near-infrared peak of our models is incompatible with the early blue component of AT2017gfo.

Big bang nucleosynthesis constraints on sterile neutrino and lepton asymmetry of the Universe

We consider the cosmological effects of sterile neutrinos with the masses of 150–450 MeV . The decay of sterile neutrinos changes the thermal history of the Universe and affects the energy density of radiation at recombination and the big bang nucleosynthesis (BBN) results. We derive severe constraints on the parameters of sterile neutrinos from the primordial abundances of helium-4 and deuterium. We also find that in a particular model the constraints can be considerably relaxed by assuming a large lepton asymmetry in the active neutrinos. In this case, the consistent parameters result in Neff ≃ 3.2–3.4 and can alleviate the Hubble tension.

Nucleosynthesis Constraints on the Explosion Mechanism for Type Ia Supernovae

Abstract Observations of type Ia supernovae (SNe Ia) include information about the characteristic nucleosynthesis associated with these thermonuclear explosions. We consider observational constraints from iron-group elemental and isotopic ratios, to compare with various models obtained with the most realistic recent treatment of electron captures (ECs). The nucleosynthesis is sensitive to the highest white-dwarf central densities. Hence, nucleosynthesis yields can distinguish high-density Chandrasekhar-mass models from lower-density burning models such as white-dwarf mergers. We discuss new results of post-processing nucleosynthesis for two spherical models (deflagration and/or delayed-detonation models) based upon new EC rates. We also consider cylindrical and 3D explosion models (including deflagration, delayed-detonation, or a violent merger model). Although there are uncertainties in the observational constraints, we identify some trends in the observations and the models. We make a new comparison of the models with elemental and isotopic ratios from five observed supernovae and three supernova remnants. We find that the models and data tend to fall into two groups. In one group, low-density cores such as in a 3D merger model are slightly more consistent with the nucleosynthesis data, while the other group is slightly better identified with higher-density cores such as in single-degenerate 1D–3D deflagration models. Hence, we postulate that both types of environments appear to contribute nearly equally to observed SN Ia. We also note that observational constraints on the yields of 54Cr and 54Fe, if available, might be used as a means to clarify the degree of geometrical symmetry of SN Ia explosions.

The impact of (n, γ) reaction rate uncertainties of unstable isotopes near N = 50 on the i-process nucleosynthesis in He-shell flash white dwarfs

The first-peak s-process elements Rb, Sr, Y and Zr in the post-AGB star Sakurai's object (V4334 Sagittarii) have been proposed to be the result of i-process nucleosynthesis in a post-AGB very-late thermal pulse event. We estimate the nuclear physics uncertainties in the i-process model predictions to determine whether the remaining discrepancies with observations are significant and point to potential issues with the underlying astrophysical model. We find that the dominant source in the nuclear physics uncertainties are predictions of neutron capture rates on unstable neutron rich nuclei, which can have uncertainties of more than a factor 20 in the band of the i-process. We use a Monte Carlo variation of 52 neutron capture rates and a 1D multi-zone post-processing model for the i-process in Sakurai's object to determine the cumulative effect of these uncertainties on the final elemental abundance predictions. We find that the nuclear physics uncertainties are large and comparable to observational errors. Within these uncertainties the model predictions are consistent with observations. A correlation analysis of the results of our MC simulations reveals that the strongest impact on the predicted abundances of Rb, Sr, Y and Zr is made by the uncertainties in the (n, γ) reaction rates of 85Br, 86Br, 87Kr, 88Kr, 89Kr, 89Rb, 89Sr, and 92Sr. This conclusion is supported by a series of multi-zone simulations in which we increased and decreased to their maximum and minimum limits one or two reaction rates per run. We also show that simple and fast one-zone simulations should not be used instead of more realistic multi-zone stellar simulations for nuclear sensitivity and uncertainty studies of convective–reactive processes. Our findings apply more generally to any i-process site with similar neutron exposure, such as rapidly accreting white dwarfs with near-solar metallicities.

Heavy-element yields and abundances of asymptotic giant branch models with a Small Magellanic Cloud metallicity

We present new theoretical stellar yields and surface abundances for asymptotic giant branch (AGB) models with a metallicity appropriate for stars in the Small Magellanic Cloud (SMC, $Z= 0.0028$, [Fe/H] $\approx -0.7$). New evolutionary sequences and post-processing nucleosynthesis results are presented for initial masses between 1$M_{\odot}$ and 7$M_{\odot}$, where the 7$M_{\odot}$ is a super-AGB star with an O-Ne core. Models above 1.15$M_{\odot}$ become carbon rich during the AGB, and hot bottom burning begins in models $M \ge 3.75 M_{\odot}$. We present stellar surface abundances as a function of thermal pulse number for elements between C to Bi and for a selection of isotopic ratios for elements up to Fe and Ni (e.g., $^{12}$C/$^{13}$C), which can be compared to observations. The integrated stellar yields are presented for each model in the grid for hydrogen, helium and all stable elements from C to Bi. We present evolutionary sequences of intermediate-mass models between 4--7$M_{\odot}$ and nucleosynthesis results for three masses ($M=3.75, 5, 7M_{\odot}$) including $s$-process elements for two widely used AGB mass-loss prescriptions. We discuss our new models in the context of evolved AGB stars and post-AGB stars in the Small Magellanic Clouds, barium stars in our Galaxy, the composition of Galactic globular clusters including Mg isotopes with a similar metallicity to our models, and to pre-solar grains which may have an origin in metal-poor AGB stars.

Strong magnetic field generated by the extreme oxygen-rich red supergiant VY Canis Majoris

Abstract Evolved stars experience high mass-loss rates forming thick circumstellar envelopes (CSEs). The circumstellar material is made of the result of stellar nucleosynthesis and, as such, plays a crucial role in the chemical evolution of galaxies and the universe. Since asymmetric geometries of CSEs are common, and with very complex structures for some cases, radiative pressure from the stars can explain only a small portion of the mass-loss processes; thus the essential driving mechanism is still unknown, particularly for high-mass stars. Here we report on magnetic field measurements associated with the well-known extreme red supergiant (RSG) VY Canis Majoris (VY CMa). We measured the linear polarization and the Zeeman splitting of the SiO v = 0, J = 1–0 transition using a sensitive radio interferometer. The measured magnetic field strengths are surprisingly high; their upper limits range between 150 and 650 G within 530 au (∼80 R*) of the star. The lower limit of the field strength is expected to be at least ∼10 G based on the high degree of linear polarization. Since the field strengths are very high, the magnetic field must be a key element in understanding the stellar evolution of VY CMa, as well as the dynamical and chemical evolution of the complex CSE of the star. M-type RSGs, with large stellar surface, were thought to be very slow rotators. This would seem to make a dynamo in operation difficult, and would also dilute any fossil magnetic field. At least for VY CMa, we expect that powerful dynamo processes must still be active to generate the intense magnetic field.

Light-curve and spectral properties of ultrastripped core-collapse supernovae leading to binary neutron stars

We investigate light-curve and spectral properties of ultra-stripped core-collapse supernovae. Ultra-stripped supernovae are the explosions of heavily stripped massive stars which lost their envelopes via binary interactions with a compact companion star. They eject only ~ 0.1 Msun and may be the main way to form double neutron-star systems which eventually merge emitting strong gravitational waves. We follow the evolution of an ultra-stripped supernova progenitor until iron core collapse and perform explosive nucleosynthesis calculations. We then synthesize light curves and spectra of ultra-stripped supernovae using the nucleosynthesis results and present their expected properties. Ultra-stripped supernovae synthesize ~ 0.01 Msun of radioactive 56Ni, and their typical peak luminosity is around 1e42 erg/s or -16 mag. Their typical rise time is 5 - 10 days. Comparing synthesized and observed spectra, we find that SN 2005ek, some of the so-called calcium-rich gap transients, and SN 2010X may be related to ultra-stripped supernovae. If these supernovae are actually ultra-stripped supernovae, their event rate is expected to be about 1 per cent of core-collapse supernovae. Comparing the double neutron-star merger rate obtained by future gravitational-wave observations and the ultra-stripped supernova rate obtained by optical transient surveys identified with our synthesized light-curve and spectral models, we will be able to judge whether ultra-stripped supernovae are actually a major contributor to the binary neutron star population and provide constraints on binary stellar evolution.

Light-curve and spectral properties of ultra-stripped core-collapse supernovae

AbstractWe discuss light-curve and spectral properties of ultra-stripped core-collapse supernovae. Ultra-stripped supernovae are supernovae with ejecta masses of only ~0.1M⊙ whose progenitors lose their envelopes due to binary interactions with their compact companion stars. We follow the evolution of an ultra-stripped supernova progenitor until core collapse and perform explosive nucleosynthesis calculations. We then synthesize light curves and spectra of ultra-stripped supernovae based on the nucleosynthesis results. We show that ultra-stripped supernovae synthesize ~0.01M⊙ of the radioactive 56Ni, and their typical peak luminosity is around 1042 erg s−1 or −16 mag. Their typical rise time is 5 − 10 days. By comparing synthesized and observed spectra, we find that SN 2005ek and some of so-called calcium-rich gap transients like PTF10iuv may be related to ultra-stripped supernovae.

STELLAR YIELDS FROM METAL-RICH ASYMPTOTIC GIANT BRANCH MODELS

ABSTRACT We present new theoretical stellar yields and surface abundances for three grids of metal-rich asymptotic giant branch (AGB) models. Post-processing nucleosynthesis results are presented for stellar models with initial masses between 1 M ☉ and 7.5 M ☉ for Z = 0.007, and 1 M ☉ and 8 M ☉ for Z = 0.014 (solar) and Z = 0.03. We include stellar surface abundances as a function of thermal pulse on the AGB for elements from C to Bi and for a selection of isotopic ratios for elements up to Fe and Ni (e.g., / ), which can be obtained from observations of molecules in stars and from the laboratory analysis of meteoritic stardust grains. Ratios of elemental abundances of He/H, C/O, and N/O are also included, which are useful for direct comparison to observations of AGB stars and their progeny, including planetary nebulae. The integrated elemental stellar yields are presented for each model in the grid for hydrogen, helium, and all stable elements from C to Bi. Yields of Li are also included for intermediate-mass models with hot bottom burning. We present the first slow neutron-capture (s-process) yields for super solar metallicity AGB stars with Z = 0.03, and the first complete s-process yields for models more massive than 6 M ☉ at all three metallicities.

Primordial nucleosynthesis in theRh=ctcosmology: pouring cold water on the simmering Universe

Primordial nucleosynthesis is rightly hailed as one of the great successes of the standard cosmological model. Here we consider the initial forging of elements in the recently proposed Rh = ct universe, a cosmology that demands linear evolution of the scale factor. Such a universe cools extremely slowly compared to standard cosmologies, considerably depleting the available neutrons during nucleosynthesis; this has significant implications for the resultant primordial abundances of elements, predicting a minuscule quantity of helium which is profoundly at odds with observations. The production of helium can be enhanced in such a "simmering universe" by boosting the baryon to photon ratio, although more than an order of magnitude increase is required to bring the helium mass fraction into accordance with observations. However, in this scenario, the prolonged period of nucleosynthesis results of the efficient cooking of lighter into heavier elements, impacting the resultant abundances of all elements so that, other than hydrogen and helium, there are virtually no light elements present in the universe. Without the addition of substantial new physics in the early universe, it is difficult to see how the Rh = ct universe can be considered a viable cosmological model.

Neutron excess number and nucleosynthesis of heavy elements in a type Ia supernova explosion

Type Ia supernovae produce very powerful burst of light, which can be observed to high redshift. This fact is very attractive for cosmological applications. For supernova light curve modeling, it is very important to know the amount of Fe and Ni, formed during the explosion. In this paper, we explore both the chemical composition of the ejected supernova shells and the possibility of weak r-process under increased neutron excess number based on a set of trajectories of tracer particles, calculated in a hydrodynamic model of SNIa explosion. It is shown that no r-process elements are synthesized in the considered supernova model, even for an increased neutron excess number (Ye ∼ 0.4) because of the slow evolution of temperature and density along chosen trajectories. The results of explosive nucleosynthesis are discussed.

$ f(R,T_{\mu\nu}T^{\mu\nu})$ gravity and Cardassian-like expansion as one of its consequences

We propose a new model of gravity where the Ricci scalar $ (R)$ in the Einstein-Hilbert action is replaced by an arbitrary function of $ R$ and of the norm of energy-momentum tensor, i.e., $ f(R,T_{\mu\nu}T^{\mu\nu})$ . Field equations are derived in the metric formalism. We find that the equation of motion of massive test particles is non-geodesic and these test particles are acted upon by a force which is orthogonal to the four-velocity of the particles. We also find the Newtonian limit of the model to calculate the extra acceleration which can affect the perihelion of Mercury. There is a deviation from the general relativistic (GR) result unless the energy density of fluid is constant. Arranging the $ \alpha$ parameter gives an opportunity to cure the inconsistency between the observational values for the abundance of light elements and the standard Big Bang nucleosynthesis results. Even the dust-dominated universe undergoes an accelerated expansion without using a cosmological constant in Model II. With this specific choice of $ f(R,T_{\mu\nu}T^{\mu\nu})$ , we get Cardassian-like expansion.

The long-term evolution of neutron star merger remnants - I. The impact of r-process nucleosynthesis

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.

A new candidate for probing Population III nucleosynthesis with carbon-enhanced damped Lyα systems

We report the identification of a very metal poor damped Lyα system (DLA) at zabs = 3.067 295 that is modestly carbon enhanced, with an iron abundance of ∼1/700 solar ([Fe/H] =−2.84) and [C,O/Fe] ≃ +0.6. Such an abundance pattern is likely to be the result of nucleosynthesis by massive stars. On the basis of 17 metal absorption lines, we derive a 2σ upper limit on the DLA's kinetic temperature of TDLA ≤ 4700 K, which is broadly consistent with the range of spin temperature estimates for DLAs at this redshift and metallicity. While the best-fitting abundance pattern shows the expected hallmarks of Population III nucleosynthesis, models of high-mass Population II stars can match the abundance pattern almost as well. We discuss current limitations in distinguishing between these two scenarios and the marked improvement in identifying the remnants of Population III stars expected from the forthcoming generation of 30-m class telescopes.

Sulphur abundances in halo giants from the [S ı] line at 1082 nm and the S ı triplet around 1045 nm

Context. It is still debated whether or not the Galactic chemical evolution of sulphur in the halo follows the flat trend with [Fe/H] that is ascribed to the result of explosive nucleosynthesis in type II SNe. It has been suggested that the disagreement between different investigations of sulphur abundances in halo stars might be owing to problems with the diagnostics used, that a new production source of sulphur might be needed in the early Universe, like hypernovae, or that the deposition of supernova ejecta into the interstellar medium is time-delayed.