Light‐element abundance variations in globular clusters

Star‐to‐star variations in abundances of the light elements carbon, nitrogen, oxygen, and sodium have been observed in stars of all evolutionary phases in all Galactic globular clusters that have been thoroughly studied. The data available for studying this phenomenon, and the hypotheses as to its origin, have both co‐evolved with observing technology; once high‐resolution spectra were available even for main‐sequence stars in globular clusters, scenarios involving multiple closely spaced stellar generations enriched by feedback from moderate‐ and high‐mass stars began to gain traction in the literature. This paper briefly reviews the observational history of globular cluster abundance inhomogeneities, discusses the presently favored models of their origin, and considers several aspects of this problem that require further study (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Investigation of the single neutron exposure model for the s-process: the primary nature of the neutron source

The primary nature of the ¹³C neutron source is very significant for the studies of the s -process nucleosynthesis. In this paper we present an attempt to fit the element abundances observed in 16 s -rich stars using parametric model of the single neutron exposure. The calculated results indicate that almost all s -elements were made in a single neutron exposure for nine sample stars. Although a large spread of neutron exposure is obtained, the maximum value of the neutron exposure will reach about 7.0 mbarn⁻¹, which is close to the theoretical predictions by the asymptotic giant branch (AGB) model. The calculated result is a significant evidence for the primary nature of the neutron source. Combining the result obtained in this work and the neutron exposure–initial mass relations, a large spread of neutron exposure can be explained by the different initial stellar mass and their time evolution. The possibility that the rotationally induced mixing process can lead to a spread of the neutron exposure in AGB stars is also existent.     

High resolution study of the abundance pattern of the heavy elements in very metal‐poor field stars

The abundances of heavy elements in EMP stars are not well explained by the simple view of an initial basic “rapid” process. In a careful and homogeneous analysis of the “First Stars” sample (eighty per cent of the stars have a metallicity [Fe/H] ≃ –3.1 ± 0.4), it has been shown that at this metallicity [Eu/Ba] is constant, and therefore the europium‐rich stars (generally called “r‐rich”) are also Ba‐rich. The very large variation of [Ba/Fe] (existence of “r‐poor” and “r‐rich” stars) induces that the early matter was not perfectly mixed. On the other hand, the distribution of the values of [Sr/Ba] vs. [Ba/Fe] appears with well defined upper and lower envelopes. No star was found with [Sr/Ba] < –0.5 and the scatter of [Sr/Ba] increases regularly when [Ba/Fe] decreases. To explain this behavior, we suggest that an early “additional” process forming mainly first peak elements would affect the initial composition of the matter. For a same quantity of accreted matter, this additional Sr production would barely affect the r‐rich matter (which already contains an important quantity of Sr) but would change significantly the composition of the r‐poor matter. The abundances found in the CEMP‐r+s stars reflect the transfer of heavy elements from a defunct AGB companion. But the abundances of the heavy elements in CEMP‐no stars present the same characteristics as the the abundances in the EMP stars. Direct stellar ages may be found from radioactive elements, the precision is limited by the precision in the measurements of abundances from faint lines in faint stars, and the uncertainty in the initial abundances of the radioactive elements. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Pycnonuclear burning of 34Ne in accreting neutron stars

We study the pycnonuclear burning of ³⁴Ne in the inner crust of an accreting neutron star. We show that the associated energy production rate can be calculated analytically for any arbitrary temporal variability of the mass accretion rate. We argue that the theoretical time-scale for ³⁴Ne burning is currently very uncertain and ranges from a fraction of a millisecond to a few years. The fastest allowable burning may change the composition of the accreted crust while the slowest burning leads to a time-independent nuclear energy generation rate for a variable accretion. The results are important for constructing self-consistent models of the accreted crust and deep crustal heating in neutron stars which enter soft X-ray transients.     

Nucleosynthesis in strong magnetic fields at statistical equilibrium

The atomic mass distribution of nuclides in an ultramagnetized astrophysical plasma is considered by employing a model of nuclear statistical equilibrium. The magnetization of atomic nuclei is shown to enhance the portion of light nuclear species in the iron region.     

Heavy-element abundances in seven SC stars and several related stars

We employ spectra of resolution 20–35000 of seven SC stars, four S stars, two Ba stars and two K–M stars to derive abundances of a variety of elements from Sr to Eu relative to iron. Special attention is paid to Rb and Tc, and to the ratio of the heavy s-process species to the light s-process elements. Abundances are derived in LTE, both by using model atmospheres in which the carbon and oxygen abundances are nearly equal and by using curves of growth. Spectrum synthesis is used for critical lines such as the 5924-Å line of Tc and the 7800-Å line of Rb. For most of the heavy-element stars the enhancement of the s-process elements is about a factor of 10. The ratio of the heavy to light s-process species is not far from solar, except for RR Her for which the same ratio is +0.45 dex. For Tc the blending by other lines is severe. While we have probably detected the 5924-Å line, we can only present abundances in the less-than-or-equal-to category. For Rb, whose abundance is sensitive to the ⁸⁵Rb/⁸⁷Rb ratio and hence to the neutron density during s-process production, we find a considerable range of abundances, indicating a neutron density from 10⁶ to ≳10⁸ cm⁻³ for the SC stars. For the four S stars the range is from 10⁷ to ≳10⁸ cm⁻³. Recent calculations by Gallino et al. show that neutron densities near 10⁷ cm⁻³ favour the ¹³C source for neutrons, while densities greater than 10⁸ cm⁻³ may be associated with neutrons from the ²²Ne source.     

The deuterium abundance in the z= 0.7 absorber towards QSO PG1718+4807

We report on a further analysis of the ratio of deuterium to hydrogen (D/H) using Hubble Space Telescope ( HST ) spectra of the   z = 0.701  Lyman limit system towards the quasi-stellar object (QSO) PG1718+481. Initial analyses of this absorber found it gave a high D/H value, 1.8–  3.1 × 10⁻⁴  , inconsistent with several higher redshift measurements. It is thus important to critically examine this measurement. By analysing the velocity widths of the D  i , H  i and metal lines present in this system, Kirkman et al. report that the additional absorption in the blue wing of the Lyα line cannot be D  i , with a confidence level of 98 per cent. Here we present a more detailed analysis, taking into account possible wavelength shifts between the three sets of HST spectra used in the analysis. We find that the constraints on this system are not as strong as those claimed by Kirkman et al. The discrepancy between the parameters of the blue wing absorption and the parameters expected for D  i is marginally worse than 1σ.
Tytler et al. commented on the first analysis of Webb et al., reporting the presence of a contaminating lower redshift Lyman limit system, with  log[ N (H  i )] = 16.7  at   z = 0.602  , which biases the N (H  i ) estimate for the main system. Here we show that this absorber actually has  log[ N (H  i )] < 14.6  and does not impact on the estimate of N (H  i ) in the system of interest at   z = 0.701  .
The purpose of the present paper is to highlight important aspects of the analysis which were not explored in previous studies, and hence to help refine the methods used in future analyses of D/H in quasar spectra.     

Double detonations at the core–envelope boundary in Type Ia supernovae

An off-centre detonation propagating near the interface between a C–O core and a He envelope in a Type Ia supernova explosion is modelled as a steady two-dimensional similarity solution at a plane interface. We assume that in both regions the energy release occurs in an infinitely thin detonation, which produces material in nuclear statistical equilibrium (NSE) in He and in nuclear statistical quasi-equilibrium in C–O. An α-network is then used to determine the effect of the associated rarefaction wave in the C–O on the final abundance of intermediate elements. We find that, although there is a significant effect, the rarefaction is not strong enough to quench the reactions and prevent the C–O from burning to NSE.     

The role of gravitational supernovae in the Galactic evolution of the Li, Be and B isotopes

The observed Be and B relationships with metallicity clearly support the idea that both elements have a primary origin and that they are produced by the same class of objects. Spallation by particles accelerated during gravitational supernova events (SNII, SNIb/c) seems to be a likely origin. We show, in the context of a model of chemical evolution, that it is possible to solve the Li, Be and B abundance puzzle with the yields recently proposed by Ramaty et al., provided that SNII are unable to accelerate helium nuclei significantly and that different mechanisms are allowed to act simultaneously.     

A Study on the Abundance Distribution of Heavy Elements in Metal-poor Stars by the Parameterization Method

Based on a large amount of observed data of element abundances in metal-poor stars, taking the abundance distribution of heavy elements in the solar system as a standard, and selecting Sr, Ba and Eu as the typical elements of the three nucleosynthetic processes in metal-poor stars, namely the weak sprocess, main s-process and r-process, we have studied the contributions of the three kinds of neutron-capture processes to the abundance distribution of heavy elements in metal-poor stars, with the parameterization method. It is found that the higher the metal abundance, the greater the contributions of the weak s-process and the chief s-process to the abundances of lighter neutron-capture elements. The heavier neutron-capture elements are mainly produced by the r-process and the chief s-process; and that at low metallicity, the abundances of heavy neutron-capture elements are mainly produced by the r-process. In the early Galaxy, the weak s-process has almost no contribution to the element abundance.     

Time-variable emission from transiently accreting neutron stars in quiescence due to deep crustal heating

Transiently accreting neutron stars in quiescence ( L _X ≲10³⁴ erg s⁻¹) have been observed to vary in intensity by factors of few, over time-scales of days to years. If the quiescent luminosity is powered by a hot neutron star core, the core cooling time-scale is much longer than the recurrence time, and cannot explain the observed, more rapid variability. However, the non-equilibrium reactions which occur in the crust during outbursts deposit energy in isodensity shells, from which the thermal diffusion time-scale to the photosphere is days to years. The predicted magnitude of variability is too low to explain the observed variability unless – as is widely believed – the neutrons beyond the neutron-drip density are superfluid. Even then, the variability due to this mechanism in models with standard core neutrino cooling processes is less than 50 per cent – still too low to explain the reported variability. However, models with rapid core neutrino cooling can produce a variability by a factor as great as 20, on time-scales of days to years following an outburst. Thus, the factors of ∼ few intensity variability observed from transiently accreting neutron stars can be accounted for by this mechanism only if rapid core cooling processes are active.     

The structure of steady detonation waves in Type Ia supernovae: pathological detonations in C–O cores

The structure of steady, one-dimensional detonation waves in C–O is investigated for initial densities in the range 2×10⁷ to 1×10⁹ g cm⁻³. At these and greater densities, the self-supporting detonation wave is of the pathological type. For such waves the detonation speed is an eigenvalue of the steady equations, and the reaction zone contains an internal frozen sonic point where the thermicity vanishes. The self-supporting flow downstream of this singular point is supersonic, and is very different from that in supported (overdriven) detonations. A method for determining the structure of pathological detonation waves is described. These waves are examined, and the self-sustaining wave is compared with and contrasted to the supported detonations considered previously by Khokhlov. We show that the thickness of the self-sustaining detonation is a few times the thickness of supported detonations, and that the self-sustaining detonation produces more of the iron-peak and less of the intermediate mass elements than do supported detonations. Implications for the cellular detonation instability are also discussed.     

On variations in the fine-structure constant and stellar pollution of quasar absorption systems

At redshifts   z _abs≲ 2  , quasar absorption-line constraints on space–time variations in the fine-structure constant, α, rely on the comparison of Mg  ii and Fe  ii transition wavelengths. One potentially important uncertainty is the relative abundance of Mg isotopes in the absorbers, which, if different from solar, can cause spurious shifts in the measured wavelengths and, therefore, α. Here we explore chemical evolution models with enhanced populations of intermediate-mass (IM) stars, which, in their asymptotic giant branch phase, are thought to be the dominant factories for heavy Mg isotopes at the low metallicities typical of quasar absorption systems. By design, these models partially explain recent Keck/HIRES evidence for a smaller α in   z _abs < 2  absorption clouds than on Earth. However, such models also overproduce N, violating observed abundance trends in high- z _abs damped Lyman-α (DLA) systems. Our results do not support the recent claim of Ashenfelter et al. that similar models of IM-enhanced initial mass functions (IMFs) may simultaneously explain the HIRES varying-α data and DLA N abundances. We explore the effect of the IM-enhanced model on Si, Al and P abundances, finding it to be much less pronounced than for N. We also show that the ¹³C/¹²C ratio, as measured in absorption systems, could constitute a future diagnostic of non-standard models of the high-redshift IMF.