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Due to their production sites, as well as to how they are processed and destroyed in stars, the light elements are excellent tools to investigate a number of crucial issues in modern astrophysics: from stellar structure and non-standard processes at work in stellar interiors to age dating of stars; from pre-main sequence evolution to the star formation histories of young clusters and associations and to multiple populations in globular clusters; from Big Bang nucleosynthesis to the formation and chemical enrichment history of the Milky Way Galaxy and its populations, just to cite some relevant examples. In this paper, we focus on lithium, beryllium, and boron (LiBeB) and on carbon, nitrogen, and oxygen (CNO). LiBeB are rare elements, with negligible abundances with respect to hydrogen; on the contrary, CNO are among the most abundant elements in the Universe, after H and He. Pioneering observations of light-element surface abundances in stars started almost 70 years ago and huge progress has been achieved since then. Indeed, for different reasons, precise measurements of LiBeB and CNO are difficult, even in our Sun; however, the advent of state-of-the-art ground- and space-based instrumentation has allowed the determination of high-quality abundances in stars of different type, belonging to different Galactic populations, from metal-poor halo stars to young stars in the solar vicinity and from massive stars to cool dwarfs and giants. Noticeably, the recent large spectroscopic surveys performed with multifiber spectrographs have yielded detailed and homogeneous information on the abundances of Li and CNO for statistically significant samples of stars; this has allowed us to obtain new results and insights and, at the same time, raise new questions and challenges. A complete understanding of the light-element patterns and evolution in the Universe has not been still achieved. Perspectives for further progress will open up soon thanks to the new generation instrumentation that is under development and will come online in the coming years.

The abundances of neutron (n)-capture elements in the carbon-enhanced metal-poor (CEMP)-r/s stars agree with predictions of intermediate n-density nucleosynthesis, at N n ∼ 1013–1015 cm−3, in rapidly accreting white dwarfs (RAWDs). We have performed Monte Carlo simulations of this intermediate-process (i -process) nucleosynthesis to determine the impact of (n,γ) reaction rate uncertainties of 164 unstable isotopes, from 131I to 189Hf, on the predicted abundances of 18 elements from Ba to W. The impact study is based on two representative one-zone models with constant values of N n = 3.16 × 1014 and 3.16 × 1013 cm−3 and on a multizone model based on a realistic stellar evolution simulation of He-shell convection entraining H in a RAWD model with [Fe/H] = −2.6. For each of the selected elements, we have identified up to two (n,γ) reactions having the strongest correlations between their rate variations constrained by Hauser–Feshbach computations and the predicted abundances, with the Pearson product–moment correlation coefficients | r P| > 0.15. We find that the discrepancies between the predicted and observed abundances of Ba and Pr in the CEMP-i star CS 31062−050 are significantly diminished if the rate of 137Cs(n,γ)138Cs is reduced and the rates of 141Ba(n,γ)142Ba or 141La(n,γ)142La increased. The uncertainties of temperature-dependent β-decay rates of the same unstable isotopes have a negligible effect on the predicted abundances. One-zone Monte Carlo simulations can be used instead of computationally time-consuming multizone Monte Carlo simulations in reaction rate uncertainty studies if they use comparable values of N n. We discuss the key challenges that RAWD simulations of i process for CEMP-i stars meet by contrasting them with recently published low- Z asymptotic giant branch (AGB) i process. [ABSTRACT FROM AUTHOR]

Since 7Li is easily destroyed in low temperatures, the surface lithium abundance decreases as stars evolve. This is supported by the lithium depletion observed in the atmosphere of most red giants. However, recent studies show that almost all of red clump stars have high lithium abundances A (Li) > −0.9, which are not predicted by the standard theory of the low-mass stellar evolution. In order to reconcile the discrepancy between the observations and the model, we consider additional energy loss channels that may come from physics beyond the Standard Model. A (Li) slightly increases near the tip of the red giant branch even in the standard model with thermohaline mixing because of the 7Be production by the Cameron–Fowler mechanism, but the resultant 7Li abundance is much lower than the observed values. We find that the production of 7Be becomes more active if there are additional energy loss channels, because themohaline mixing becomes more efficient and a heavier helium core is formed. [ABSTRACT FROM AUTHOR]

Ecologists have long acknowledged the importance of context dependency related to position along spatial gradients. It is also acknowledged that broad‐scale climate patterns can directly and indirectly alter population dynamics. What is not often addressed is whether climate patterns such as the Southern Oscillation interact with population‐level temporal patterns and affect the ability of time‐series data, such as long‐term state of the environment monitoring programmes, to detect change. Monitoring design criteria generally focus on number of data points, sampling frequency and duration, often derived from previous information on species seasonal and multi‐year temporal patterns. Our study questioned whether the timing of any changes relative to Southern Oscillation, interacting with species populations dynamics, would also be important. We imposed a series of simulated reductions on macrofaunal abundance data collected regularly over 29 years from two sites, using species selected for observed differences in temporal dynamics. We hypothesized that (1) high within‐year sampling frequency would increase detection ability for species with repeatable seasonality cycles and (2) timing of the reduction in abundance relative to the Southern Oscillation was only likely to affect detection ability for long‐lived species with multi‐year cyclic patterns in abundance. However, regardless of species population dynamics, we found both within‐year sampling frequency and the timing of the imposed reduction relative to the Southern Oscillation Index affected detection ability. The latter result, while apparently demonstrating a confounding influence on monitoring, offers the opportunity to improve our ability to detect and interpret analyses of monitoring data, and thus our ability to make recommendations to managers. [ABSTRACT FROM AUTHOR]

Sub-Chandrasekhar mass carbon-oxygen white dwarfs with a surface helium shell have been proposed as progenitors of Type Ia supernovae (SNe Ia). If true, the resulting thermonuclear explosions should be able to account for at least some of the range of SNe Ia observables. To study this, we conducted a parameter study based on three-dimensional simulations of double detonations in carbon-oxygen white dwarfs with a helium shell, assuming different core and shell masses. An admixture of carbon to the shell and solar metallicity are included in the models. The hydrodynamic simulations were carried out using the AREPO code. This allowed us to follow the helium shell detonation with high numerical resolution, and this improves the reliability of predicted nucleosynthetic shell detonation yields. The addition of carbon to the shell leads to a lower production of 56Ni, while including solar metallicity increases the production of intermediate mass elements. The production of higher mass elements is further shifted to stable isotopes at solar metallicity. Moreover, we find different core detonation ignition mechanisms depending on the core and shell mass configuration. This has an influence on the ejecta structure. We present the bolometric light curves predicted from our explosion simulations using the Monte Carlo radiative transfer code ARTIS and make comparisons with bolometric SNe Ia data. The bolometric light curves of our models show a range of brightnesses, which is able to account for subluminous to normal brightness SNe Ia. We show the model bolometric width-luminosity relation compared to data for a range of model viewing angles. We find that, on average, our brighter models lie within the observed data. The ejecta asymmetries produce a wide distribution of observables, which might account for outliers in the data. However, the models overestimate the extent of this compared to data. We also find that the bolometric decline rate over 40 days, Δm40(bol), appears systematically faster than data. [ABSTRACT FROM AUTHOR]

We study the evolution of the [Eu/Fe] ratio in the Galactic halo by means of a stochastic chemical evolution model considering merging neutron stars as polluters of europium. We improved our previous stochastic chemical evolution model by adding a time delay distribution for the coalescence of the neutron stars, instead of constant delays. The stochastic chemical evolution model can reproduce the trend and the observed spread in the [Eu/Fe] data with neutron star mergers as unique producers if we assume: (i) a delay time distribution ∝ t −1.5, (ii) an |$M_{\boldsymbol{ Eu}}= 3 \times 10^{-6} \, \boldsymbol{M}_{\odot }$| per event, (iii) progenitors of neutron stars in the range |$9\!-\!50\, \boldsymbol{M}_{\odot}$|⁠ , and (iv) a constant fraction of massive stars in the initial mass function (0.02) that produce neutron star mergers. Our best model is obtained by relaxing point (iv) and assuming a fraction that varies with metallicity. We confirm that the mixed scenario with both merging neutron stars and supernovae as europium producers can provide a good agreement with the data relaxing the constraints on the distribution time delays for the coalescence of neutron stars. Adopting our best model, we also reproduce the dispersion of [Eu/Fe] at a given metallicity, which depends on the fraction of massive stars that produce neutron star mergers. Future high-resolution spectroscopic surveys, such as 4MOST and WEAVE, will produce the necessary statistics to constrain at best this parameter. [ABSTRACT FROM AUTHOR]

This study reports a probabilistic insight into the stellar‐mass and supernovae (SNe) explosion energy of five CEMP‐no stars' possible progenitors. As such, a direct comparison between the abundance ratios [X/Fe] of the light‐elements and the predicted nucleosynthetic yields of SN of high‐mass metal‐free stars has been performed. This comparison suggests that the possible progenitors have a stellar‐mass range of 11 − 22 M⊙ and explosion energies of 0.3 − 1.8 × 1051 erg. In addition, we investigate the kinematic signatures, derived from Gaia DR2, to conclude that these five CEMP‐no stars have halo‐like kinematic and do not enter the outer‐halo region. In addition, we link the abundance patterns with kinematic signatures to investigate the Gaia‐Sausage and Gaia‐Sequoia memberships. This chemo‐dynamical analysis suggests that these CEMP‐no stars are neither Gaia‐Sausage nor Gaia‐Sequoia remnant stars, but another accretion event might be responsible for the contribution of these stars to the Galactic Halo. [ABSTRACT FROM AUTHOR]

We investigate the impact of asymmetric neutrino emissions on explosive nucleosynthesis in core-collapse supernovae (CCSNe) of progenitors with a mass range of 9.5–25 M⊙. We perform axisymmetric, hydrodynamic simulations of the CCSN explosion with a simplified neutrino transport, in which anticorrelated dipolar emissions of ν e and |${\bar{\nu }}_{\rm e}$| are imposed. We then evaluate abundances and masses of the CCSN ejecta in a post-processing manner. We find that the asymmetric ν -emission leads to the abundant ejection of p - and n -rich matter in the high- ν e and - |${\bar{\nu }}_{\rm e}$| hemispheres, respectively. It substantially affects the abundances of the ejecta for elements heavier than Ni regardless of progenitors, although those elements lighter than Ca are less sensitive. Based on these results, we calculate the initial mass function-averaged abundances of the CCSN ejecta with taking into account the contribution from Type Ia SNe. For |$m_{\rm asy} = 10/3{{\ \rm per\ cent}}$| and |$10{{\ \rm per\ cent}}$|⁠ , where m asy denotes the asymmetric degree of the dipole components in the neutrino emissions, the averaged abundances for elements lighter than Y are comparable to those of the solar abundances, whereas those of elements heavier than Ge are overproduced in the case with |$m_{\rm asy} \ge 30{{\ \rm per\ cent}}$|⁠. Our result also suggests that the effect of the asymmetric neutrino emissions is imprinted in the difference of abundance ratio of [Ni/Fe] and [Zn/Fe] between the high- ν e and - |${\bar{\nu }}_{\rm e}$| hemispheres, indicating that the future spectroscopic X-ray observations of a CCSN remnant will bring evidence of the asymmetric neutrino emissions if exist. [ABSTRACT FROM AUTHOR]

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

We present the nucleosynthesis of magneto-rotational supernovae (MR-SNe) including neutrino-driven and magneto-rotational-driven ejecta based, for the first time, on 2D simulations with accurate neutrino transport. The models analysed here have different rotation and magnetic fields, allowing us to explore the impact of these two key ingredients. The accurate neutrino transport of the simulations is critical to analyse the slightly neutron-rich and proton-rich ejecta that are similar to the, also neutrino-driven, ejecta in standard supernovae. In the model with strong magnetic field, the r-process produces heavy elements up to the third r-process peak (A ∼ 195), in agreement with previous works. This model presents a jet-like explosion with proton-rich jets surrounded by neutron-rich material where the r-process occurs. We have estimated a lower limit for 56Ni of |$2.5\times 10^{-2} \, \mathrm{M}_\odot$|⁠ , which is still well below the expected hypernova value. Longer simulations including the accretion disc evolution are required to get a final prediction. In addition, we have found that the late evolution is critical in a model with weak magnetic field in which late-ejected neutron-rich matter produces elements up to the second r-process peak. Even if we cannot yet provide conclusions for hypernova nucleosynthesis, our results agree with observations of old stars and radioactive isotopes in supernova remnants. This makes MR-SNe a good additional scenario to neutron star mergers for the synthesis of heavy elements and brings us closer to understand their origin and the role of MR-SNe in the early Galaxy nucleosynthesis. [ABSTRACT FROM AUTHOR]

Barium (Ba) stars are chemically peculiar stars that display in their atmospheres signatures of the slow neutron-capture (s -process) mechanism that operates within asymptotic giant branch (AGB) stars, an important contributor to the cosmic abundance. The observed chemical peculiarity in these objects is not due to self-enrichment, but to mass transfer between the components of a binary system. The atmospheres of Ba stars are therefore excellent astrophysical laboratories, providing strong constraints for the nucleosynthesis of the s -process in AGB stars. In particular, rubidium (Rb) is a key element for the s -process diagnostic because it is sensitive to the neutron density and hence its abundance points to the main neutron source of the s -process in AGB stars. We present Rb abundances for a large sample of 180 Ba stars from high-resolution spectra (R  = 48 000), and we compare the observed [Rb/Zr] ratios with theoretical predictions from s -process models in AGB stars. The target Ba stars in this study display [Rb/Zr] < 0, showing that Rb was not efficiently produced by the activation of the branching points at 85Kr and 86Rb. Model predictions from the Monash and FRUITY datasets of low-mass (≲4 M⊙) AGB stars are able to cover the Rb abundances observed in the program Ba stars. These observations indicate that the 13C(α,n)16O reaction is the main neutron source of the s -process in the low-mass AGB companions of the observed Ba stars. We have not found in the present study candidate companions for former IR/OH massive AGB stars. [ABSTRACT FROM AUTHOR]

Recent spectroscopic measurements of the equivalent widths of the resonant Be  ii doublet and Ca  ii K lines and their ratios in expanding nova ejecta indicate surprisingly high abundances of 7Be with a typical mass fraction X obs(7Be) = 10−4. This is an order of magnitude larger than theoretically predicted values of X theor(7Be) ∼ 10−5 for novae. We use an analytical solution of the 7Be production equations to demonstrate that X theor(7Be) is proportional to the 4He mass fraction Y in the nova accreted envelope and then we perform computations of 1D hydrostatic evolution of the |$1.15\, \mathrm{M}_\odot$| CO nova model that confirm our conclusion based on the analytical solution. Our assumption of enhanced 4He abundances helps to reduce, although not completely eliminate, the discrepancy between X obs(7Be) and X theor(7Be). It is supported by ultraviolet, optical, and infrared spectroscopy data that reveal unusually high values of Y in nova ejecta. We also show that a significantly increased abundance of 3He in nova accreted envelopes does not lead to higher values of X theor(7Be) because this assumption affects the evolution of nova models resulting in a decrease of both their peak temperatures and accreted masses and, as a consequence, in a reduced production of 7Be. [ABSTRACT FROM AUTHOR]

Our aim in this work is to identify and explain the necessary conditions required for an energetic explosion of a Chandrasekhar-mass white dwarf. We construct and analyse weakly compressible turbulence models with nuclear burning effects for carbon/oxygen plasma at a density expected for the deflagration-to-detonation transition (DDT) to occur. We observe the formation of carbon deflagrations and transient carbon detonations at early times. As turbulence becomes increasingly inhomogeneous, sustained carbon detonations are initiated by the Zel'dovich reactivity gradient mechanism. The fuel is suitably preconditioned by the action of compressive turbulent modes with wavelength comparable to the size of resolved turbulent eddies; no acoustic wave is involved in this process. Oxygen detonations are initiated, aided either by reactivity gradients or by collisions of carbon detonations. The observed evolutionary time-scales are found to be sufficiently short for the above process to occur in the expanding, centrally ignited massive white dwarf. The inhomogeneous conditions produced prior to the DDT might be of consequence for the chemical composition of the outer ejecta regions of Type Ia supernovae from the single degenerate channel, and offer the potential for validation of the proposed model. [ABSTRACT FROM AUTHOR]

The thermal structure of accreting neutron stars is affected by the presence of urca nuclei in the neutron star crust. Nuclear isobars harbouring urca nuclides can be produced in the ashes of Type I X-ray bursts, but the details of their production have not yet been explored. Using the code MESA , we investigate urca nuclide production in a one-dimensional model of Type I X-ray bursts using astrophysical conditions thought to resemble the source GS 1826-24. We find that high-mass (A ≥ 55) urca nuclei are primarily produced late in the X-ray burst, during hydrogen-burning freeze-out that corresponds to the tail of the burst light curve. The ∼0.4–0.6 GK temperature relevant for the nucleosynthesis of these urca nuclides is much lower than the ∼1 GK temperature most relevant for X-ray burst light curve impacts by nuclear reaction rates involving high-mass nuclides. The latter temperature is often assumed for nuclear physics studies. Therefore, our findings alter the excitation energy range of interest in compound nuclei for nuclear physics studies of urca nuclide production. We demonstrate that for some cases this will need to be considered in planning for nuclear physics experiments. Additionally, we show that the lower temperature range for urca nuclide production explains why variations of some nuclear reaction rates in model calculations impacts the burst light curve but not local features of the burst ashes. [ABSTRACT FROM AUTHOR]

In Pop III stellar models, convection-induced mixing between H- and He-rich burning layers can induce a burst of nuclear energy and thereby substantially alter the subsequent evolution and nucleosynthesis in the first massive stars. We investigate H–He shell and core interactions in 26 stellar evolution simulations with masses 15–140, M⊙, using five sets of mixing assumptions. In 22 cases H–He interactions induce local nuclear energy release in the range |$\sim 10^{9}\!-\!10^{13.5}\, \mathrm{L}_{\odot }$|⁠. The luminosities on the upper end of this range amount to a substantial fraction of the layer's internal energy over a convective advection time-scale, indicating a dynamic stellar response that would violate 1D stellar evolution modelling assumptions. We distinguish four types of H–He interactions depending on the evolutionary phase and convective stability of the He-rich material. H-burning conditions during H–He interactions give 12C/13C ratios between ≈ 1.5 to ∼1000 and [C/N] ratios from ≈ −2.3 to ≈ 3 with a correlation that agrees well with observations of CEMP (carbon-enhanced metal-poor) no stars. We also explore Ca production from hot CNO breakout and find the simulations presented here likely cannot explain the observed Ca abundance in the most Ca-poor CEMP-no star. We describe the evolution leading to H–He interactions, which occur during or shortly after core-contraction phases. Three simulations without an H–He interaction are computed to Fe-core infall and a |$140\, \mathrm{M}_{\odot }$| simulation becomes pair unstable. We also discuss present modelling limitations and the need for 3D hydrodynamic models to fully understand these stellar evolutionary phases. [ABSTRACT FROM AUTHOR]

Millihertz quasi-periodic oscillations (mHz QPOs) observed in neutron-star low-mass X-ray binaries (NS LMXBs) are generally explained as marginally stable thermonuclear burning on the neutron star surface. We report the discovery of mHz QPOs in an XMM–Newton observation of the transient 1RXS J180408.9−342058, during a regular bursting phase of its 2015 outburst. We found significant periodic signals in the March observation, with frequencies in the range 5–8 |$\, \mathrm{mHz}$|⁠ , superimposed on a strong ∼1/ f power-law noise continuum. Neither the QPO signals nor the power-law noise were present during the April observation, which exhibited a 2.5 × higher luminosity and had correspondingly more frequent bursts. When present, the QPO signal power decreases during bursts and disappears afterwards, similar to the behaviour in other sources. 1RXS J180408.9−342058 is the eighth source known to date that exhibits such QPOs driven by thermonuclear burning. We examine the range of properties of the QPO signals in different sources. Whereas the observed oscillation profile is similar to that predicted by numerical models, the amplitudes are significantly higher, challenging their explanation as originating from marginally stable burning. [ABSTRACT FROM AUTHOR]

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The core collapse of massive, rapidly-rotating stars are thought to be the progenitors of long-duration gamma-ray bursts (GRB) and their associated hyperenergetic supernovae (SNe). At early times after the collapse, relatively low angular momentum material from the infalling stellar envelope will circularize into an accretion disc located just outside the black hole horizon, resulting in high accretion rates necessary to power a GRB jet. Temperatures in the disc mid-plane at these small radii are sufficiently high to dissociate nuclei, while outflows from the disc can be neutron-rich and may synthesize r -process nuclei. However, at later times, and for high progenitor angular momentum, the outer layers of the stellar envelope can circularize at larger radii ≳ 107 cm, where nuclear reactions can take place in the disc mid-plane (e.g. 4He + 16O → 20Ne +  γ). Here we explore the effects of nuclear burning on collapsar accretion discs and their outflows by means of hydrodynamical α -viscosity torus simulations coupled to a 19-isotope nuclear reaction network, which are designed to mimic the late infall epochs in collapsar evolution when the viscous time of the torus has become comparable to the envelope fall-back time. Our results address several key questions, such as the conditions for quiescent burning and accretion versus detonation and the generation of 56Ni in disc outflows, which we show could contribute significantly to powering GRB SNe. Being located in the slowest, innermost layers of the ejecta, the latter could provide the radioactive heating source necessary to make the spectral signatures of r -process elements visible in late-time GRB-SNe spectra. [ABSTRACT FROM AUTHOR]

The large fraction of carbon-enhanced metal-poor (CEMP) stars at lower metallicities makes them an interesting class of objects to be probed further in greater detail. They show different abundance patterns of neutron-capture elements and based on that CEMP stars are further divided into four categories. Abundances of C, N and O, along with other elements, are required to understand the different nucleosynthetic origins of the subclasses and their progenitors. We studied nine bright carbon-enhanced stars from the Milky Way halo in a metallicity range from −0.8 to −2.5. They show enhancement in C, N, O and Ba and exhibit radial velocity variation. This indicates the presence of a binary companion which might have contributed to the enhanced carbon and s-process abundance through mass transfer during its asymptotic-giant-branch (AGB) phase of evolution. Their abundance pattern of C, N and O favors low-mass nature for their binary companion. [ABSTRACT FROM AUTHOR]

We measured the thermonuclear burning efficiency as a function of accretion rate for the Type I X-ray bursts of five low-mass X-ray binary systems. We chose sources with measured neutron star spins and a substantial population of bursts from a large observational sample. The general trend for the burst rate is qualitatively the same for all sources; the burst rate first increases with the accretion rate up to a maximum, above which the burst rate declines, despite the increasing accretion rate. At higher accretion rates, when the burst rate decreases, the α-value (the ratio of accretion energy and burst energy) increases by up to a factor of 10 above that in the rising burst rate regime. These observations are contrary to the predictions of 1D numerical models, but can be explained as the consequence of a zone of stable burning on the neutron star surface, which expands with increasing accretion rate. The stable burning also 'pollutes' the unstable burning layer with ashes, contributing to the change in burst properties measured in the falling burst rate regime. We find that the mass accretion rate at which the burst rate begins to decrease is anticorrelated with the spin of the neutron star. We conclude that the neutron star spin is a key factor, moderating the nuclear burning stability, via the local accretion rate and fuel composition over the star. [ABSTRACT FROM AUTHOR]

Observations of abundances and isotopic ratio determinations in stars yield powerful constraints on stellar models. In particular, the oxygen isotopic ratios are of particular interest because they are affected not only by nucleosynthesis but also by mixing processes, which are not very well-understood yet. This review is focused on the measurements via the Trojan Horse Method (THM) that have been carried out to investigate the low-energy cross sections of proton and neutron-induced reactions on 17O as well as the proton-induced reaction on 18O, overcoming extrapolation procedures and enhancement effects due to electron screening. The (p,α) reactions induced on these oxygen isotopes are of paramount importance for the nucleosynthesis in a number of stellar sites, including red giants (RGs), asymptotic giant branch (AGB) stars, massive stars, and classical novae. In detail, the indirect measurement of the low-energy region of 17O(p,α)14N was performed. The strength of the narrow resonance at 65 keV was evaluated, and it was used to renormalize the corresponding resonance strength in the 17O+p radiative capture channel. The reaction rate was then evaluated for both the 17O(p,α)14N and the 17O(p,γ)18F reactions, and significant differences of 30 and 20% with respect the literature data were found, respectively, in the temperature range relevant for RG, AGB, and massive stars nucleosynthesis. Regarding the 18O(p,α)15N reaction, the strength of the 20 keV resonance was extracted, which is the main contribution to the reaction rate for astrophysical applications. This approach has allowed us to improve the data accuracy of a factor 8.5, as it is based on the measured strength instead of educated guesses or spectroscopic measurements. Finally, the 17O(n,α)14C reaction was studied because of its role during the s-process nucleosynthesis as a possible neutron poison reaction. This study represents the extension of THM to resonant neutron-induced reactions. In this measurement, the subthreshold level centered at −7 keV in the center-of-mass system, corresponding to the 8.039 MeV 18O excited level, was observed. Moreover, the THM measurements showed a clear agreement with the available direct measurements and the additional contribution of the 8.121 MeV 18O level, strongly suppressed in direct measurements because of its l = 3 angular momentum. The contributions of those levels to the total reaction rate were than evaluated for future astrophysical applications.

The Barents Sea is a nursery area for many commercially and ecologically important fish stocks, and this whole region is presently subject to rapid climatic change from a cold period in the 1980s to a record warm period in the latest decade, with a peak in 2016. The present study focuses exclusively on year 2016, which was characterized by record warm air and seawater and an exceptionally large horizontal coverage of Atlantic waters. Earlier studies have suggested that environmental conditions during the first year of life are the most critical for year class strength and development of fish stocks. We focus on 8 fish species (age 0) and document spatial distributions of their abundances and lengths as well as ambient environmental conditions. Data for most of the Barents Sea obtained from the ecosystem survey (BESS) were used to explore if the record-warm conditions in 2016 limited 0-group fish distributions, abundances and size. Abundances and lengths for the 8 species were related to physical conditions (seawater temperature and salinity) and biological features (biomass of mesozooplankton and biomass of the jellyfish). In 2016, 0-group capelin, haddock, herring and long rough dab were more abundant and all species except long rough dab were larger than the long term mean (1980–2015). Larger individuals and higher abundances were observed mainly in the areas covered by relatively warm water masses apparently holding a sufficient amount of plankton. Most of the 0-group fishes were distributed within their thermal habitats, but with some geographical shift most likely reflecting a shift in the distribution of water masses. A significantly lower abundance of polar cod was observed in 2016, with very few individuals registered within the traditional core area in the south eastern Barents Sea. The increased temperature and low plankton biomass may have limited polar cod distribution and abundance there. A spatial analysis showed that biomass of C. capillata was positively related to abundances of 0-group herring, capelin and cod, indicating that they were inhabiting similar water masses. The high abundances of capelin, haddock, herring and long rough dab, and generally large individuals of most species, may suggest suitable living and feeding conditions in 2016.

We discuss the evolution of stars through the asymptotic giant branch, focusing on the physical mechanisms potentially able to alter the surface chemical composition and on how changes in the chemistry of the external regions affect the physical properties of the star and the duration of this evolutionary phase. We focus on the differences between the evolution of low-mass stars, driven by the growth of the core mass and by the surface carbon enrichment, and that of their higher mass counterparts, which experience hot bottom burning. In the latter sources, the variation of the surface chemical composition reflects the equilibria of the proton capture nucleosynthesis experienced at the base of the convective envelope. The pollution expected from this class of stars is discussed, outlining the role of mass and metallicity on the chemical composition of the ejecta. To this aim, we considered evolutionary models of 0.7–8 M⊙ stars in a wide range of metallicities, extending from the ultra-metal-poor domain to super-solar chemistries.

We present a new code, CASTRO, that solves the multicomponent compressible hydrodynamic equations for astrophysical flows including self-gravity, nuclear reactions, and radiation. CASTRO uses an Eulerian grid and incorporates adaptive mesh refinement (AMR). Our approach to AMR uses a nested hierarchy of logically rectangular grids with simultaneous refinement in both space and time. The radiation component of CASTRO will be described in detail in the next paper, Part II, of this series. © 2010. The American Astronomical Society. All rights reserved.

The reanalysis of the previously published abundance pattern of mild barium star HD202109 (ζ Cyg) and the chemical compositions of 129 thin disk barium stars facilitated the search for possible correlations of different stellar parameters with second ionization potentials of chemical elements. Results show that three valuable correlations exist in the atmospheres of barium stars. The first is the relationship between relative abundances and second ionization potentials. The second is the age dependence of mean correlation coefficients of relative abundances vs. second ionization potentials, and the third one is the changes in correlation coefficients of relative abundances vs. second ionization potentials as a function of stellar spatial velocities and overabundances of s-process elements. These findings demonstrate the possibility of hydrogen and helium accretion from the interstellar medium on the atmospheres of barium stars.

The analysis of the high-resolution spectra of 31 Magellanic Clouds Cepheid variables enabled the identification of thorium lines. The abundances of thorium were found with spectrum synthesis method. The calculated thorium abundances exhibit correlations with the abundances of other chemical elements and atmospheric parameters of the program stars. These correlations are similar for both Clouds. The correlations of iron abundances of thorium, europium, neodymium, and yttrium relative to the pulsational periods are different in the Large Magellanic Cloud (LMC) and the Small Magellanic Cloud (SMC), namely the correlations are negative for LMC and positive or close to zero for SMC. One of the possible explanations can be the higher activity of nucleosynthesis in SMC with respect to LMC in the recent several hundred million years.

The chemical composition of helium-rich hot subluminous O stars plays an important role to understand and model their formation history. We present a spectroscopic analysis of four He-sdO stars,CD-31° 4800, [CW83] 0904- 02, LSS 1274 and LS IV +10° 9. The analysis is based on archival optical and UV high-resolution spectra. We used Tlusty200/Synspec48 to compute line blanketed non-LTE model atmospheres and their corresponding synthetic spectra and derive the atmospheric parameters as well as the abundances of the most prominent elements. All stars have helium-dominated atmospheres with hardly any hydrogen and temperatures between 42000 K and 47000 K while their surface gravity spans between log g = 5.4 and 5.7. CD-31° 4800 shows an enrichment of nitrogen and the characteristic pattern of hydrogen burning via the CNO-cycle, while the rest of the elements have about the solar abundance. This points to the slow merger of two helium white dwarfs as the most likely origin for this system. The other three stars are enriched in carbon, nitrogen and neon while their intermediate mass element’s abundance scatters around the solar value. They were possibly formed in the deep mixing late hot flasher scenario.

We report the determination of abundances and isotopic ratios for C, O, and Si in the photosphere of the red giant (RG) component of the recurrent nova (RN) T Coronae Borealis from new 2.284–2.402 μ m and 3.985–4.155 μ m spectroscopy. Abundances and isotopic ratios in the photosphere may be affected by (i) processes in the RG interior which are brought to the surface during dredge-up and (ii) contamination of the RG, either during the common envelope phase of the binary evolution or by material synthesized in RN eruptions, or a combination of the two. We find that the abundances of C, O, and Si are reasonably consistent with the expected composition of an RG after first dredge-up, as is the 16O/17O ratio. The 28Si/29Si ratio is found to be 8.6 ± 3.0, and that for 28Si/30Si is 21.5 ± 3.0. The 12C/13C ratio (10 ± 2) is somewhat lower than expected for first dredge-up. The 16O/18O ratio (41 ± 3) is highly inconsistent with that expected either from RG evolution (∼550) or from contamination of the RG by the products of a nova thermonuclear runaway. In particular, the C and O isotopic ratios taken in combination are a puzzle. We urge confirmation of our results using spectroscopy at high resolution. We also encourage a thorough theoretical study of the effects on the secondary star in an RN system of contamination by ejecta having anomalous abundances and isotopic ratios. [ABSTRACT FROM AUTHOR]

We investigated how the thermal beta+ decay rates and their corresponding neutrino energy loss rates affect the properties of X-ray bursts. Previous studies carried out showed how the beta+ decay rates alone affect the properties of X-ray bursts. Here, we performed the sensitivity studies on neutrino energy loss rates and their impact on X-ray bursts. Furthermore, all the experimental beta+ decay rates related to X-ray bursts were measured. However, the temperature of X-ray bursts can be as high as 1.5 GK. Thus, at such a high temperature, the nuclei can be in excited states due to thermal effects. Experiments cannot directly measure the thermal beta decay rates and thermal neutrino loss rates. We performed sensitivity tests on the thermal beta+ decay rates and their thermal neutrino losses in one-zone X-ray burst models in different ignition conditions. We discovered that beta decays due to thermal effects do affect the light curves in some ignition conditions. The thermal beta decay rates of 68Se also have major effects on the final abundances in most circumstances. We further found that a neutrino energy loss rate of 64Ga has noticeable effects on the light curves of X-ray bursts in general. [ABSTRACT FROM AUTHOR]

Kilonovae are optical flashes produced in the aftermath of neutron star–neutron star mergers or neutron star–black hole mergers. In this work, we use the Millennium Simulation, combined with semi-analytic galaxy formation model gabe (Galaxy Assembly with Binary Evolution) to explore the cosmic event rate of kilonovae, and the properties of their host galaxies in a cosmological context. We find that model with supernova kick velocity of |$V_{\rm kick}=0\, \, {\rm km}\, \, {\rm s}^{-1}$| fits the observation best, satisfying the preference for low kick velocity binary system in theoretical models. With |$V_{\rm kick}=0\, \, {\rm km}\, \, {\rm s}^{-1}$|⁠ , the cosmic event rate of NNMs and NBMs at z = 0 are 283 and |$91\, \, {\rm Gpc}^{-3}\, \, {\rm yr}^{-1}$|⁠ , respectively, marginally consistent with the constraint from LVC GWTC-1. For Milky Way-mass galaxies, we predict the NNM rate is |$25.7^{+59.6}_{-7.1}\, {\rm Myr}^{-1}$|⁠ , which is also in good agreement with the observed properties of binary neutron stars in the Milky Way. Taking all the kilonovae into account in the history of Milky Way-mass galaxies, we find that the averaged r-process elements yield (A > 79) in an NNM/NBM event should be |$0.01\, {\rm M}_\odot$| to be consistent with observation. We conclude that NGC 4993, the host galaxy of GW170817, is a typical host galaxy for NNMs. However, generally, NNMs and NBMs tend to reside in young, blue, star-forming, late-type galaxies, with stellar mass, and gaseous metallicity distribution peaking at |$M_{\rm *}=10^{10.65}\, {\rm M}_\odot$| and 12 + log (O/H) = 8.72 − 8.85, respectively. By exploring the connection between kilonovae and their host galaxies in the cosmological background, it is promising to constrain model better when we have more events in the forthcoming future. [ABSTRACT FROM AUTHOR]

Context. Monte Carlo methods can be used to evaluate the uncertainty of a reaction rate that arises from many uncertain nuclear inputs. However, until now no attempt has been made to find the effect of correlated energy uncertainties in input resonance parameters. Aims. Our goal is to investigate the impact of correlated resonance energy uncertainties on reaction rates. Methods. Using a combination of numerical and Monte Carlo variation of resonance energies, the effect of correlations are investigated. Five reactions are considered: two fictional, illustrative cases and three reactions whose rates are of current interest. Results. The effect of correlations in resonance energies depends on the specific reaction cross section and temperatures considered. When several resonances contribute equally to a reaction rate, and when they are located on either side of the Gamow peak, correlations between their energies dilute their effect on reaction rate uncertainties. If they are both located above or below the maximum of the Gamow peak, however, correlations between their resonance energies can increase the reaction rate uncertainties. This effect can be hard to predict for complex reactions with wide and narrow resonances contributing to the reaction rate. [ABSTRACT FROM AUTHOR]

In this work, we report a list of 91 strong Th  ii spectral lines in the visible wavelength region that could be used as cosmochronometers in future investigations to determine stellar ages from thorium radioactive decay. This list was established on the basis of a pseudo-relativistic Hartree–Fock model including core-polarization corrections (HFR+CPOL) allowing us to compute the corresponding radiative decay parameters, the latter being found to be in good overall agreement with the most accurate experimental data when available. Besides this semi-empirical approach, the fully relativistic ab initio multiconfiguration Dirac–Hartree–Fock (MCDHF) method was attempted, showing large discrepancies with both the present HFR+CPOL and the most recent experimental oscillator strengths, although the calculated wavelengths from theoretical energy levels agreed with observed spectral line wavelengths within 2.4  per cent. [ABSTRACT FROM AUTHOR]

Molybdenum abundances in the stars from 13 different open clusters have been determined. High-resolution stellar spectra have been obtained using the VLT telescope equipped with the UVES spectrograph on Cerro Paranal, Chile. The Mo abundances have been derived in the LTE approximation from the Mo I lines at 5506 Å and 5533 Å. A comparative analysis of the behaviour of molybdenum in the sampled stars of open clusters and Galactic disc show similar trends of decreasing Mo abundances with increasing metallicities; such a behaviour pattern suggests a common origin of the examined populations. On the other hand, the scatter of Mo abundances in the open cluster stars is slightly greater, 0.14 dex versus 0.11 dex. The results are discussed, considering the abundance trends with the age of clusters and distances from the center of the Galaxy. [ABSTRACT FROM AUTHOR]

The recent GAIA DR2 measurements of distances to galactic novae have allowed to re-analyse some properties of nova populations in the Milky Way and in external galaxies on new and more solid empirical bases. In some cases, we have been able to confirm results previously obtained, such as the concept of nova populations into two classes of objects, that is disk and bulge novae and their link with the Tololo spectroscopic classification in Fe II and He/N novae. The recent and robust estimates of nova rates in the Magellanic Clouds galaxies provided by the OGLE team have confirmed the dependence of the normalized nova rate (i.e., the nova rate per unit of luminosity of the host galaxy) with the colors and/or class of luminosity of the parent galaxies. The nova rates in the Milky Way and in external galaxies have been collected from literature and critically discussed. They are the necessary ingredient to asses the contribution of novae to the nucleosynthesis of the respective host galaxies, particularly to explain the origin of the overabundance of lithium observed in young stellar populations. A direct comparison between distances obtained via GAIA DR2 and maximum magnitude vs. rate of decline (MMRD) relationship points out that the MMRD can provide distances with an uncertainty better than 30%. Multiwavelength observations of novae along the whole electromagnetic spectrum, from radio to gamma rays, have revealed that novae undergo a complex evolution characterized by several emission phases and a non-spherical geometry for the nova ejecta. [ABSTRACT FROM AUTHOR]

Following the recent detection of 7Be  ii  in the outburst spectra of classical novae, we report the search for this isotope in the outbursts of four recent bright novae by means of high-resolution Ultraviolet and Visual Echelle Spectrograph (UVES) observations. The 7Be  ii  λλ313.0583, 313.1228 nm doublet resonance lines are detected in the high-velocity components of Nova Mus 2018 and ASASSN-18fv during outbursts. However, 7Be  ii is not detected in ASASSN-17hx and possibly not in Nova Cir 2018, which shows that 7Be is not always ejected in the thermonuclear runaway. Taking into account the 7Be decay, we find X (7Be)/ X (H) ≈ 1.5 × 10−5 and 2.2 × 10−5 in Nova Mus 2018 and ASASSN-18fv, respectively. A value of 7Be/H ≈ 2 × 10−5 is found in five out of the seven extant measurements, and it can be considered as a typical 7Be yield for novae. However, this value is almost one order of magnitude larger than predicted by current theoretical models. We argue that the variety of high 7Be/H abundances could be the result of a higher than solar content of 3He in the donor star. The cases with 7Be not detected might be related to the small mass of the white dwarf (WD) or to relatively little mixing with the core material of the WD. The 7Be/H, or 7Li/H, abundance is ≈ 4 dex above meteoritic abundance, thus confirming the novae as the main sources of 7Li in the Milky Way. [ABSTRACT FROM AUTHOR]

We present a systematic search for short-duration gamma-ray bursts (GRBs) in the local Universe based on 14 yr of observations with the Neil Gehrels Swift Observatory. We cross-correlate the GRB positions with the GLADE catalogue of nearby galaxies, and find no event at a distance ≲100 Mpc and four plausible candidates in the range 100 Mpc ≲ D  ≲ 200 Mpc. Although affected by low statistics, this number is higher than the one expected for chance alignments to random galaxies, and possibly suggests a physical association between these bursts and nearby galaxies. By assuming a local origin, we use these events to constrain the range of properties for X-ray counterparts of neutron star mergers. Optical upper limits place tight constraints on the onset of a blue kilonova, and imply either low masses (⁠|$\lesssim 10^{-3}\, \mathrm{M}_{\odot }$|⁠) of lanthanide-poor ejecta or unfavorable orientations (θobs ≳ 30 deg). Finally, we derive that the all-sky rate of detectable short GRBs within 200 Mpc is |$1.3^{+1.7}_{-0.8}$| yr−1 (68 per cent confidence interval), and discuss the implications for the GRB outflow structure. If these candidates are instead of cosmological origin, we set a upper limit of ≲2.0 yr−1 (90 per cent confidence interval) to the rate of nearby events detectable with operating gamma-ray observatories, such as Swift and Fermi. [ABSTRACT FROM AUTHOR]

Interactions between species are influenced by different ecological mechanisms, such as morphological matching, phenological overlap and species abundances. How these mechanisms explain interaction frequencies across environmental gradients remains poorly understood. Consequently, we also know little about the mechanisms that drive the geographical patterns in network structure, such as complementary specialization and modularity. Here, we use data on morphologies, phenologies and abundances to explain interaction frequencies between hummingbirds and plants at a large geographical scale. For 24 quantitative networks sampled throughout the Americas, we found that the tendency of species to interact with morphologically matching partners contributed to specialized and modular network structures. Morphological matching best explained interaction frequencies in networks found closer to the equator and in areas with low-temperature seasonality. When comparing the three ecological mechanisms within networks, we found that both morphological matching and phenological overlap generally outperformed abundances in the explanation of interaction frequencies. Together, these findings provide insights into the ecological mechanisms that underlie geographical patterns in resource specialization. Notably, our results highlight morphological constraints on interactions as a potential explanation for increasing resource specialization towards lower latitudes. [ABSTRACT FROM AUTHOR]

We investigate the significance of isomers on a kilonova associated with neutron star mergers (NSMs) for the first time. We calculate the evolution of abundances and energy generation rates (⁠|$\dot{\epsilon }_{\rm nuc}$|⁠) of ejecta from NSMs, taking into account β− decay through isomers. We find that for ejecta with electron fraction (Y e) of 0.2–0.3, |$\dot{\epsilon }_{\rm nuc}$| is appreciably changed from those without isomers, due to the large change in timing of β− decay through the isomeric states (ISs) of |${}^{123,125,127} \rm Sn$| and |${}^{128} \rm Sb$|⁠. In particular, the effects of the isomers on |$\dot{\epsilon }_{\rm nuc}$| are prominent for ejecta of Y e ∼ 0.25, which could emit a fraction of an early, blue component of a kilonova observed in GW170817. When the excitation from a ground state to its IS is important, the isomers of |${}^{129} \rm Sb$| and |${}^{129, 131} \rm Te$| also cause additional and appreciable change in |$\dot{\epsilon }_{\rm nuc}$|⁠. Furthermore, we demonstrate that larger amounts of lanthanide-free ejecta result in a better fit of an observed light curve of the kilonova in GW170817 if the isomers are taken into account. [ABSTRACT FROM AUTHOR]

The availability of multi-wavelength observations and parallaxes from the space missions and very comprehensive models of AGB evolution that include the accretion of matter from the circumbinary disc have strongly impacted our understanding of these enigmatic objects. The important developments made in the recent times are summarized here. The revised estimates of luminosities (derived from better-defined Spectral Energy Distributions (SEDs) and new distances from Gaia DR2) further support the opinion that RV Tauri stars contain a mixture of post-AGB stars and post-RGB stars. Their locations in HR diagram also indicate that the instability strip (IS) of RV Tauri stars have a broader extension in the cooler edge than that of classical Cepheids. A new P−L relation has been calibrated for the galactic Cepheids which have a steeper slope than that derived for the Population II Cepheids and RV Tauri stars in Magellanic clouds. The most significant chemical peculiarity exhibited by RV Tauri stars and other post-AGB stars is the selective depletion of refractory elements that correlates with their condensation temperatures. A large range in the size of depletion as well as in the shapes of the depletion curves has been observed. Earlier models to explain this effect were mostly qualitative. Recent investigators model these depletions using evolutionary codes (e.g. MESA) to evolve stars in the post-AGB phase, while including accretion of metal-poor gas from circumbinary disc. These authors model the accretion rate onto a the binary post-AGB star from a viscously evolving disc for a range of initial accretion rates and disc masses. It is reported that large initial accretion rates and disc masses are required to explain the large depletion and saturated depletion curve that could extend the evolution time of post-AGB star. It is also proposed that the unsaturated depletion curve (with a plateau) are likely to be caused by post-RGB stars. [ABSTRACT FROM AUTHOR]

Type Ia supernovae are the result of explosive thermonuclear burning in CO white dwarfs. The progenitors of the Ia supernovae are white dwarfs in an interacting binary system. The donor companion is either a degenerate star (white dwarf) or a non-degenerate star (e.g. red giant). Recurrent novae are interacting binaries with a massive white dwarf accreting from either a main sequence, slightly evolved, or a red giant star. The white dwarf in these systems is a massive, hot white dwarf, accreting at a high rate. Recurrent novae are thought to be the most promising single degenerate progenitors of Type Ia supernovae. Presented here are the properties of a few recurrent novae based on recent outbursts. The elemental abundances and their distribution in the ejected shell are discussed. [ABSTRACT FROM AUTHOR]

The cosmic origin of fluorine, the ninth element of the periodic table, is still under debate. The reason for this fact is the large difficulties in observing stellar diagnostic lines, which can be used for the determination of the fluorine abundance in stars. Here we discuss some recent work on the chemical evolution of fluorine in the Milky Way and discuss the main contributors to the cosmic budget of fluorine. [ABSTRACT FROM AUTHOR]

Several anomalous elemental abundance ratios have been observed in the metal-poor star HD94028. We assume that its high [As/Ge] ratio is a product of a weak intermediate (i) neutron-capture process. Given that observational errors are usually smaller than predicted nuclear physics uncertainties, we have first set-up a benchmark one-zone i-process nucleosynthesis simulation results of which provide the best fit to the observed abundances. We have then performed Monte Carlo simulations in which 113 relevant (n,γ) reaction rates of unstable species were randomly varied within Hauser–Feshbach model uncertainty ranges for each reaction to estimate the impact on the predicted stellar abundances. One of the interesting results of these simulations is a double-peaked distribution of the As abundance, which is caused by the variation of the 75Ga (n,γ) cross-section. This variation strongly anticorrelates with the predicted As abundance, confirming the necessity for improved theoretical or experimental bounds on this cross-section. The 66Ni (n,γ) reaction is found to behave as a major bottleneck for the i-process nucleosynthesis. Our analysis finds the Pearson product–moment correlation coefficient r P > 0.2 for all of the i-process elements with 32 ≤ Z ≤ 42, with significant changes in their predicted abundances showing up when the rate of this reaction is reduced to its theoretically constrained lower bound. Our results are applicable to any other stellar nucleosynthesis site with the similar i-process conditions, such as Sakurai's object (V4334 Sagittarii) or rapidly accreting white dwarfs. [ABSTRACT FROM AUTHOR]

Molecular oxygen (O2) is essential to human beings on the earth. Although elemental oxygen is rather abundant, O2 is rare in the interstellar medium. It was only detected in two galactic and one extra-galactic region. The inconsistency between observations and theoretical studies is a big challenge for astrochemical models. Here we report a two-phase modeling research of molecular oxygen, using the Nautilus gas-grain code. We apply the isothermal cold dense models in the interstellar medium with two typical sets of initial elemental abundances, as well as the warm-up models with various physical conditions. Under cold dense conditions, we find that the timescales for gas-phase CO, O2 and H2O to reach peak values are dependent on the hydrogen density and are shortened when hydrogen density increases. In warm-up models, O2 abundances are in good agreement with observations at temperatures rising after 105 yr. In both isothermal and warm-up models, the steady-state O2 fractional abundance is independent of the hydrogen density, as long as the temperature is high enough (>30 K), at which O2 is prevented from significant depleting onto grain surface. In addition, low density is preferable for the formation of O2, whether molecular oxygen is under cold conditions or in warm regions. [ABSTRACT FROM AUTHOR]

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We have revisited the ultraviolet and optical spectra of the blue horizontal branch star Feige 86. The new analysis finds the star cooler and more compact than previously determined. The IUE spectrum of Feige 86 holds numerous unidentified spectral lines of heavy metals, indicating efficient atomic diffusion in the atmosphere. Because diffusion plays a key role in the atmospheres of hot subdwarfs as well, it is indispensable to a better understanding of subdwarf pulsations and evolution. Feige 86 looks like an ideal target to confront diffusion theory with observations and test spectroscopic techniques. Therefore, to advance our general understanding of diffusion in stellar atmospheres we urge for new ultraviolet spectroscopy of Feige 86 at the highest possible resolution with HST/STIS.

In a series of diver surveys, biologists documented relatively high densities of kelp forest fishes, including kelp bass, within and around the nearly four-decades old San Diego–La Jolla Ecological Reserve. Based on the fishes observed during the surveys, scientists recommend expanding the small, 210-hectare, “no-take” reserve to include a greater proportion of nearby rocky reef and submarine canyon habitats. A 100-year archive of fishes collected from the area suggests a larger reserve could benefit more than 200 species.

Chemical abundances and abundance ratios measured in galaxies provide precious information about the mechanisms, modes, and time-scales of the assembly of cosmic structures. Yet, the nucleogenesis and chemical evolution of elements heavier than helium are dictated mostly by the physics of the stars and the shape of the stellar mass spectrum. In particular, estimates of CNO isotopic abundances in the hot, dusty media of high-redshift starburst galaxies offer a unique glimpse into the shape of the stellar initial mass function (IMF) in extreme environments that cannot be accessed with direct observations (star counts). Underlying uncertainties in stellar evolution and nucleosynthesis theory, however, may hurt our chances of getting a firm grasp of the IMF in these galaxies. In this work, we adopt new yields for massive stars, covering different initial rotational velocities. First, we implement the new yield set in a well-tested chemical evolution model for the Milky Way. The calibrated model is then adapted to the specific case of a prototype submillimetre galaxy (SMG). We show that, if the formation of fast-rotating stars is favoured in the turbulent medium of violently star-forming galaxies irrespective of metallicity, the IMF needs to be skewed towards high-mass stars in order to explain the CNO isotopic ratios observed in SMGs. If, instead, stellar rotation becomes negligible beyond a given metallicity threshold, as is the case for our own Galaxy, there is no need to invoke a top-heavy IMF in starbursts. [ABSTRACT FROM AUTHOR]