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.     

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.     

Hipparcos parallaxes for Mira-like long-period variables

This paper concerns the calibration of the K period–luminosity relation for Mira variables using Hipparcos parallaxes. K magnitudes are available for 255 Mira-like variables which were observed by Hipparcos . Period–luminosity zero-points are evaluated for various subgroups of data. The best solution for oxygen-rich Miras, which uses 180 stars, omitting the short-period red group (which had different kinematics from the short-period blue stars) and the low-amplitude variables, provides a zero-point of     which implies a distance modulus for the Large Magellanic Cloud of     or perhaps slightly greater if a metallicity correction is required, in good agreement with the value derived from Cepheids. The zero-point of the period–luminosity relation for carbon stars is briefly discussed.
Linear diameters are derived for red variables with measured angular diameters and parallaxes, and are used to examine the long-standing question of the pulsation mode(s) of these stars. Evidence is presented to suggest that most of them are pulsating in the same mode and, if published model atmospheres are correct, this is probably the first overtone. Some discussion is given of sequences in the period–luminosity and period–colour diagrams and their bearing on the pulsation mode problem.     

Abundance analysis of the cool extreme helium star LSS 3378

Abundance analysis of the cool extreme helium (EHe) star LSS 3378 is presented. The abundance analysis is done using local thermodynamic equilibrium (LTE) line formation and LTE model atmospheres constructed for EHe stars.
The atmosphere of LSS 3378 shows evidence of H-burning, He-burning, and s -process nucleosynthesis. The derived abundances of iron peak and α-elements indicate the absence of selective fractionation or any other processes that can distort chemical composition of these elements. Hence, the Fe abundance [log ε(Fe) = 6.1] is adopted as an initial metallicity indicator. The measured abundances of LSS 3378 are compared with those of R Coronae Borealis (RCB) stars and with rest of the EHe stars as a group.     

Abundances in intermediate-mass AGB stars undergoing third dredge-up and hot-bottom burning

High-dispersion near-infrared spectra have been taken of seven highly evolved, variable, intermediate-mass (4–6 M_⊙) asymptotic giant branch (AGB) stars in the Large Magellanic Cloud and Small Magellanic Cloud in order to look for C, N and O variations that are expected to arise from third dredge-up and hot-bottom burning. The pulsation of the objects has been modelled, yielding stellar masses, and spectral synthesis calculations have been performed in order to derive abundances from the observed spectra. For two stars, abundances of C, N, O, Na, Al, Ti, Sc and Fe were derived and compared with the abundances predicted by detailed AGB models. Both stars show very large N enhancements and C deficiencies. These results provide the first observational confirmation of the long-predicted production of primary nitrogen by the combination of third dredge-up and hot-bottom burning in intermediate-mass AGB stars. It was not possible to derive abundances for the remaining five stars: three were too cool to model, while another two had strong shocks in their atmospheres which caused strong emission to fill the line cores and made abundance determination impossible. The latter occurrence allows us to predict the pulsation phase interval during which observations should be made if successful abundance analysis is to be possible.     

Carbon-rich Mira variables: radial velocities and distances

Optical radial velocities have been measured for 38 C-type Mira variables (C-Miras). These data together with others in the literature are used to study the differences between optical and CO millimetre (mm) observations for C-Miras and the necessary corrections to the optical velocities are derived in order to obtain the true radial velocities of the variables. The difference between absorption and emission-line velocities is also examined. A particularly large difference  (+30 km s ⁻¹)  is found in the case of the Hα line. A catalogue is given of 177 C-Miras with estimated distances and radial velocities. The distances are based on bolometric magnitudes derived in Paper I using South African Astronomical Observatory (SAAO) observations or (for 60 of the stars) using non-SAAO photometry. In the latter case, the necessary transformations to the SAAO system are derived. These data will be used in Paper III to study the kinematics of the C-Miras.     

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.     

The depletion of carbon by extra mixing in metal-poor giants

There is an apparent dichotomy between the metal-poor  ([Fe/H]≤−2)  yet carbon-normal giants and their carbon-rich counterparts. The former undergo significant depletion of carbon on the red giant branch after they have undergone first dredge-up, whereas the latter do not appear to experience significant depletion. We investigate this in the context that the extra mixing occurs via the thermohaline instability that arises due to the burning of  ³He  . We present the evolution of [C/Fe], [N/Fe] and  ¹²C/¹³C  for three models: a carbon-normal metal-poor star, and two stars that have accreted material from a  1.5 M_⊙  AGB companion, one having received  0.01 M_⊙  of material and the other having received  0.1 M_⊙  . We find the behaviour of the carbon-normal metal-poor stars is well reproduced by this mechanism. In addition, our models also show that the efficiency of carbon-depletion is significantly reduced in carbon-rich stars. This extra-mixing mechanism is able to reproduce the observed properties of both carbon-normal and carbon-rich stars.     

The origin of the lead-rich stars in the Galactic halo: investigation of model parameters for the s-process

Several stars at the low-metallicity extreme of the Galactic halo show large spreads of lead and associated 'heavy' s-process elements ([Pb/hs]). Theoretically, an s-process pattern should be obtained from an AGB star with a fixed metallicity and initial mass. For the third dredge-up and the s-process model, several important properties depend primarily on the core mass of AGB stars. Zijlstra reported that the initial-to-final mass relation steepens at low metallicity, due to low mass-loss efficiency. This might affect the model parameters of the AGB stars, e.g. the overlap factor and the neutron irradiation time, in particular at low metallicity. The calculated results do indeed show that the overlap factor and the neutron irradiation time are significantly small at low metallicities, especially for  3.0 M_⊙ AGB  stars. The scatter of [Pb/hs] found in low metallicities can therefore be explained naturally when varying the initial mass of the low-mass AGB stars.     

The role of ballast in the helium-shell flash

Calculations are presented of the evolutionary track of an asymptotic giant branch (AGB) star, after most of its envelope is consumed and it starts moving to the right in the Hertzsprung–Russell (HR) diagram. On looking for the conditions that may create a helium-shell flash at this stage, it is found that a necessary condition for the creation of a helium-shell flash at this stage is the presence of mass 'ballast' over the helium-burning shell. The effects of different amounts of such ballast are studied, and the detailed evolution of such a flash is presented.