An explosive end to intermediate-mass zero-metallicity stars and early Universe nucleosynthesis

We use the Cambridge stellar evolution code stars to model the evolution of 5 and  7 M_⊙  zero-metallicity stars. With enhanced resolution at the hydrogen- and helium-burning shell in the asymptotic giant branch (AGB) phases, we are able to model the entire thermally pulsing AGB (TP-AGB) phase. The helium luminosities of the thermal pulses are significantly lower than in higher metallicity stars so there is no third dredge-up. The envelope is enriched in nitrogen by hot-bottom burning of carbon that was previously mixed in during second dredge-up. There is no s -process enrichment owing to the lack of third dredge-up. The thermal pulses grow weaker as the core mass increases and they eventually cease. From then on the star enters a quiescent burning phase which lasts until carbon ignites at the centre of the star when the CO core mass is  1.36 M_⊙  . With such a high degeneracy and a core mass so close to the Chandrasekhar mass, we expect these stars to explode as type 1.5 supernovae, very similar to type Ia supernovae but inside a hydrogen-rich envelope.     

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.     

Continuum emission around AGB stars at 1.2 mm

It is generally acknowledged that the mass-loss of asymptotic giant branch (AGB) stars undergoes variations on different time-scales. We constructed models for the dust envelopes for a sample of AGB stars to assess whether mass-loss variations influence the spectral energy distribution. To constrain the variability, extra observations at millimetre wavelengths (1.2 mm) were acquired. From the analysis of the dust models, two indications for the presence of mass-loss variations can be found, being (1) a dust temperature at the inner boundary of the dust envelope that is far below the dust condensation temperature and (2) an altered density distribution with respect to  ρ( r ) ∝ r ⁻²  resulting from a constant mass-loss rate. For five out of the 18 studied sources a two-component model of the envelope is required, consisting of an inner region with a constant mass-loss rate and an outer region with a less steep density distribution. For one source an outer region with a steeper density distribution was found. Moreover, in a search for time variability in our data set at 1.2 mm, we found that WX Psc shows a large relative time variation of 34 per cent which might partially be caused by variable molecular line emission.     

AGB variables and the Mira period–luminosity relation

Published data for large-amplitude asymptotic giant branch variables in the Large Magellanic Cloud (LMC) are re-analysed to establish the constants for an infrared ( K ) period–luminosity relation of the form   M _K =ρ[log  P − 2.38]+δ  . A slope of  ρ=−3.51 ± 0.20  and a zero-point of  δ=−7.15 ± 0.06  are found for oxygen-rich Miras (if a distance modulus of 18.39 ± 0.05 is used for the LMC). Assuming this slope is applicable to Galactic Miras we discuss the zero-point for these stars using the revised Hipparcos parallaxes together with published very long baseline interferometry (VLBI) parallaxes for OH masers and Miras in globular clusters. These result in a mean zero-point of  δ=−7.25 ± 0.07  for O-rich Galactic Miras. The zero-point for Miras in the Galactic bulge is not significantly different from this value.
Carbon-rich stars are also discussed and provide results that are consistent with the above numbers, but with higher uncertainties. Within the uncertainties there is no evidence for a significant difference between the period–luminosity relation zero-points for systems with different metallicity.     

Self-enrichment by asymptotic giant branch stars in globular clusters: comparison between intermediate and high metallicities

We present theoretical evolutionary sequences of intermediate-mass stars  ( M = 3 − 6.5 M_⊙)  with metallicity   Z = 0.004  . Our goal is to test whether the self-enrichment scenario by massive asymptotic giant branch stars may work for the high-metallicity globular clusters, after previous works by the same group showed that the theoretical yields by this class of objects can reproduce the observed trends among the abundances of some elements, namely the O–Al and O–Na anticorrelations, at intermediate metallicities, i.e.  [Fe/H]=−1.3  . We find that the increase in the metallicity favours only a modest decrease of the luminosity and the temperature at the bottom of the envelope for the same core mass, and also the efficiency of the third dredge-up is scarcely altered. On the contrary, differences are found in the yields, due to the different impact that processes with the same efficiency have on the overall abundance of envelopes with different metallicities. We expect the same qualitative patterns as in the intermediate-metallicity case, but the slopes of some of the relationships among the abundances of some elements are different. We compare the sodium–oxygen anticorrelation for clusters of intermediate metallicity ( Z ≈ 10⁻³) and clusters of metallicity large as in these new models. Although the observational data are still too scarce, the models are consistent with the observed trends, provided that only stars of   M ≳ 5 M_⊙  contribute to self-enrichment.     

Sio observations of mass-losing AGB stars

We have observed the first overtone rotation-vibration absorption bands of SiO near = 4µm for a sample of 23 oxygen-rich Mira and Semiregular variables using the ESO NTT and IRSPEC. We discuss the strength of the SiO absorption in terms of the near infrared, IRAS and pulsational properties of the stars. Especially among the Miras there are big differences in the strength of the SiO bands between individual objects, which are probably due to pulsational variability.This work is supported by the Austrian Science Fund Project P9638-AST.     

Distributions of Neutron Exposures in AGB Stars and the Galaxy

Based on the s-process nucleosynthesis model with the 13C(α,n)16O reaction oc- curring under radiative conditions in the interpulse phases, we investigate the characteristics of the distribution of neutron exposure in low-mass Asymptotic Giant Branch (AGB) stars. We introduce a new concept, the distribution of neutron exposures of the Galaxy (NEG), to study the chemical evolution characteristics of the Galaxy for s-process elements. Using a chemical evolution model of the Galaxy, we develop a model for the NEG and obtain the evolution results of the NEG in different epochs. The present results appear to reasonably re- produce the distribution of neutron exposures of the solar system (hereafter NES). The main component and the strong component in the NES are built up in different epochs. The strong component of the s-process is mainly synthesised in the low-mass and metal-poor AGB stars, and the main component is produced by the s-process in the low-mass AGB stars with higher metallicities.     

Solar-like Oscillations in Semiregular Variables

Power spectra of the light curves of semiregular variables, based on visualmagnitude estimates spanning many decades, show clear evidence forstochastic excitation. This supports the suggestion byChristensen-Dalsgaard et al. (2001) that oscillations in these stars aresolar-like, i.e., stochastically excited by convection, with mode lifetimesranging from years to decades.     

Pulsation–decline relationships in R Coronae Borealis stars

Decline onset times were measured in long-term visual light curves for five R Coronae Borealis (RCB) variable stars. These included RY Sgr and V854 Cen, the two RCB stars previously reported to have a relationship between dust formation events and pulsational variations. Analysis of the decline epochs showed that all decline onsets for a given star obey a linear ephemeris tied to the object's dominant radial pulsation period. Thus, in addition to confirming the pulsation–decline correlation for RY Sgr and V854 Cen, this same behaviour was demonstrated in UW Cen, R CrB and S Aps for the first time. This general result firmly establishes the connection between radial pulsation and mass loss in the RCB stars. The dominant pulsation period of ≈40 d for all of these objects therefore represents the characteristic time-scale on which these stars produce dust.     

The evolution of low-metallicity asymptotic giant branch stars and the formation of carbon-enhanced metal-poor stars

We investigate the behaviour of asymptotic giant branch (AGB) stars between metallicities   Z = 10⁻⁴  and 10⁻⁸. We determine which stars undergo an episode of flash-driven mixing, where protons are ingested into the intershell convection zone, as they enter the thermally pulsing AGB phase and which undergo third dredge-up. We find that flash-driven mixing does not occur above a metallicity of   Z = 10⁻⁵  for any mass of star and that stars above  2 M_⊙  do not experience this phenomenon at any metallicity. We find carbon ingestion (CI), the mixing of carbon into the tail of hydrogen-burning region, occurs in the mass range  2 M_⊙  to around  4 M_⊙  . We suggest that CI may be a weak version of the flash-driven mechanism. We also investigate the effects of convective overshooting on the behaviour of these objects. Our models struggle to explain the frequency of Carbon-Enhanced Metal-Poor (CEMP) stars that have both significant carbon and nitrogen enhancement. Carbon can be enhanced through flash-driven mixing, CI or just third dredge-up. Nitrogen can be enhanced through hot bottom burning and the occurrence of hot dredge-up also converts carbon into nitrogen. The C/N ratio may be a good indicator of the mass of the primary AGB stars.