early-AGB Phase Research Articles

The intermediate neutron capture process

Context. The observed surface abundance distributions of carbon-enhanced metal-poor (CEMP) r/s stars suggest that these stars could have been polluted by an intermediate neutron capture process (the so-called i-process) occurring at intermediate neutron densities between the r- and s-processes. Triggered by the ingestion of protons inside a convective He-burning zone, the i-process could be hosted in several sites, a promising one being the early AGB phase of low-mass, low-metallicity stars. The i-process remains affected however by many uncertainties, including those of nuclear origin, since it involves hundreds of nuclei for which reaction rates have not yet been determined experimentally. Aims. We investigate both the systematic and statistical uncertainties associated with theoretical nuclear reaction rates of relevance during the i-process and explore their impact on the i-process elemental production, and subsequently on the surface enrichment, of a low-mass, low-metallicity star during the early AGB phase. Methods. We used the TALYS reaction code to estimate both the model and parameter uncertainties affecting the photon strength function and the nuclear level densities, and hence the radiative neutron capture rates. The impact of correlated systematic uncertainties was estimated by considering different nuclear models, as was detailed in Paper II. In contrast, the uncorrelated uncertainties associated with local variation in model parameters were estimated using a variant of the backward-forward Monte Carlo method to constrain the parameter changes to experimentally known cross sections before propagating them consistently to the neutron capture rates. The STAREVOL code (Siess 2006, A&A, 448, 717) was used to determine the impact of nuclear uncertainties on the i-process nucleosynthesis in a 1 M⊙ [Fe/H] = –2.5 model star during the proton ingestion event in the early AGB phase. A large nuclear network of 1160 species coherently coupled to the transport processes was solved to follow the i-process nucleosynthesis. Results. We find that the uncorrelated parameter uncertainties lead the surface abundance uncertainties of elements with Z ≥ 40 to range between 0.5 and 1.0 dex, with odd-Z elements displaying higher uncertainties. The correlated model uncertainties are of the same order of magnitude, and both model and parameter uncertainties have an important impact on potential observable tracers such as Eu and La. We find around 125 important (n, γ) reactions impacting the surface abundances, including 28 reactions that have a medium to high impact on the surface abundance of elements that are taken as observable tracers of i-process nucleosynthesis in CEMP stars. Conclusions. Both the correlated model and uncorrelated parameter uncertainties need to be estimated coherently before being propagated to astrophysical observables through multi-zone stellar evolution models. Many reactions are found to affect the i-process predictions and will require improved nuclear models guided by experimental constraints. Priority should be given to the reactions influencing the observable tracers.

Oxygen isotopic ratios in RGB & AGB stars

Stars in the mass range 1.2-7M⊙, with solar-like initial composition are evolved from the zero age main sequence till the early AGB phase. Their predicted oxygen isotopic ratios are compared to observationally inferred values in red giants to investigate how well standard evolutionary calculation reproduces available observations and to illustrate the role of convective mixing. We find that extra mixing beyond the convective boundary determined by the Schwarzschild criterion is needed to explain the observational oxygen isotopic ratios, particularly in low mass stars. The effect of recent determinations of proton capture reactions and their uncertainties on 16O/17O is also shown.

OH/IR stars and their superwinds as observed by theHerschelSpace Observatory

Aim : In order to study the history of mass loss in extreme OH/IR stars, we observed a number of these objects using CO as a tracer of the density and temperature structure of their circumstellar envelopes. Method : Combining CO observations from the Herschel Space Observatory with those from the ground, we trace mass loss rates as a function of radius in five extreme OH/IR stars. Using radiative transfer modelling, we modelled the dusty envelope as well as the CO emission. The high-rotational transitions of CO indicate that they originate in a dense superwind region close to the star while the lower transitions tend to come from a more tenuous outer wind which is a result of the mass loss since the early AGB phase. Result : The models of the circumstellar envelopes around these stars suggest that they have entered a superwind phase in the past 200 - 500 years. The low 18O/17O (~ 0.1 compared to the solar abundance ratio of ~ 5) and 12C/13C (3-30 cf. the solar value of 89) ratios derived from our study support the idea that these objects have undergone hot-bottom burning and hence that they are massive M >= 5 solar-mass AGB stars.

Grids of stellar models with rotation

[abridged] Many topical astrophysical research areas, such as the properties of planet host stars, the nature of the progenitors of different types of supernovae and gamma ray bursts, and the evolution of galaxies, require complete and homogeneous sets of stellar models at different metallicities in order to be studied during the whole of cosmic history. We present here a first set of models for solar metallicity, where the effects of rotation are accounted for in a homogeneous way. We computed a grid of 48 different stellar evolutionary tracks, both rotating and non-rotating, at Z=0.014, spanning a wide mass range from 0.8 to 120 Msun. For each of the stellar masses considered, electronic tables provide data for 400 stages along the evolutionary track and at each stage, a set of 43 physical data are given. These grids thus provide an extensive and detailed data basis for comparisons with the observations. The rotating models start on the ZAMS with a rotation rate Vini/Vcrit=0.4. The evolution is computed until the end of the central carbon-burning phase, the early AGB phase, or the core helium-flash for, respectively, the massive, intermediate, and both low and very low mass stars. The initial abundances are those deduced by Asplund and collaborators, which best fit the observed abundances of massive stars in the solar neighbourhood. We update both the opacities and nuclear reaction rates, and introduce new prescriptions for the mass-loss rates as stars approach the Eddington and/or the critical velocity. We account for both atomic diffusion and magnetic braking in our low-mass star models. [...]

Dust properties in the circumstellar shells of evolved stars: Observational constraints from ISO and Spitzer infrared spectroscopy

AbstractWe present the results of a systematic analysis of the solid state features identified in the circumstellar environments of a large sample of evolved stars with ISO/SWS and Spitzer/IRS spectroscopy. The sample includes several hundred stars with a wide variety of progenitor masses evolving from the early AGB phase to the PN stage. Our observations are used to propose an evolutionary scheme in which the results obtained can be interpreted as a consequence of the nucleosynthesis processes that take place in this short phase of the stellar evolution, in particular the third dredge-up and hot bottom burning, which in turn are also strongly modulated by the stellar metallicity.

The Chemical Composition of Carbon Stars. II. The J‐Type Stars

Abundances of lithium, heavy elements, and carbon isotope ratios have been measured in 12 J-type Galactic carbon stars. The abundance analysis shows that in these stars the abundances of s-process elements with respect to the metallicity are nearly normal. Tc is not present in most of them, although upper limits have been derived for WZ Cas and WX Cyg, perhaps two SC-type rather than J-type carbon stars. The Rb abundances, obtained from the resonance λ7800 Rb I line, are surprisingly low, probably owing to strong non-LTE effects in the formation of this line in cool carbon-rich stars. Lithium and 13C are found to be enhanced in all the stars. These results together with the nitrogen abundances and oxygen isotope ratios measured by Lambert et al. and Harris et al. are used to discuss the origin of J stars. The luminosity and variability class of the stars studied would indicate that they are low-mass (M ≲ 2-3 M☉), less evolved objects than the normal carbon stars, although the presence of some luminous (Mbol < -5.5) J stars in our Galaxy (WZ Cas may be an example) and in other galaxies, suggests the existence of at least two types of J stars, with different formation scenarios depending upon the initial mass of the parent star. Standard evolutionary AGB models are difficult to reconcile with all the observed chemical characteristics. In fact, they suggest the existence of an extra mixing mechanism that transports material from the convective envelope down to hotter regions where some nuclear burning occurs. This mechanism would act preferably on the early-AGB phase in low-mass stars. Mixing at the He-core flash and the binary system hypothesis are also discussed as alternatives to the above scenario.

Understanding the Li Production in AGB stars: the J-type Stars

A full abundance analysis have been carried out in 12 galactic J-stars. The analysis shows: a) 12C/13C ratios are low, b) the abundance of s-process and rare-earth elements with respect to the stellar metallicity is nearly-solar, [&lt; heavy &gt; /Fe] = 0.10 ± 0.11 c) Tc is not present in most of the stars although upper limits are placed in WZ Cas and WX Cyg, probably two SC-type stars, and d) all the stars are Li-rich, log ∈(Li)&gt; 1. The location of J-stars in an empirical H-R diagram when compared with that of the normal N-type carbon stars, suggests that J-stars may be in the early-AGB phase. The different mechanism proposed to be responsible of this Li production are briefly discussed.

Grids of stellar models

We present a new grid of stellar models from 0.8 to at , with mass loss and moderate overshooting, from the zero age main sequence to either the helium flash (low mass stars), the early AGB phase (intermediate-mass stars) or the end of carbon burning (massive stars). The calculations are done with opacities provided by Iglesias & Rogers (1993), completed by those of Alexander & Ferguson (1994) at low temperatures. This grid is a homogeneous extension to very high metallicity of the previous grids published by the Geneva group. It is useful for the study of galactic bulges, elliptical galaxies and eventually quasars. Calculations of stars more massive than are not presented as these objects lose almost their entire mass during their main sequence phase, and are likely to end their life as white dwarfs.

The evolution of the Magellanic Clouds

The observed maximum luminosities of AGB stars in the Magellanic Cloud clusters were compared with luminosities obtained theoretically in order to estimate the ages of the clusters. Several rates of mass loss by AGB stars are considered. The ages for 10 clusters in the SMC and 25 in the LMC are presented in the tables with the compilations of ages from the literature quoted. The discrepancies between ages derived from the AGB peak luminosities and from the Main-Sequence turn off and maximum luminosity are explained by allowing for intensive mass-loss during the AGB evolutionary phase. The method is useful only if the brightest star is on the thermally pulse AGB phase but not in the early-AGB phase.

On the evolution of binary components which first fill their Roche lobes after the exhaustion of central helium

view Abstract Citations (34) References (42) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS On the Evolution of Binary Components Which First Fill Their Roche Lobes after the Exhaustion of Central Helium Iben, I., Jr. Abstract The evolution of model close binary components of initial mass 3-7 Msun and of Population I composition is followed from the beginning of the core helium-burning phase. For one set of models, mass loss is initiated at an arbitrary point during the early AGB phase (early case C events) and evolution is followed to the degenerate dwarf stage. For another set of models, mass loss is initiated when hydrogen is reignited and thermal pulses begin (late case C events). It is found that the inclusion of overshoot and semiconvection during the core helium-burning phase has little effect on the relationship between progenitor mass and final degenerate dwarf mass. A new channel for producing both oxygen-neon (ONe) degenerate dwarfs and neutron stars is identified. Involved are stars of initial mass 7. 8.5 Msun which undergo early case C events. For such stars, mass loss from the surface proceeds sufficiently more rapidly than the mass of the hydrogen-exhausted core can be decreased by convective dredge-up that carbon ignition at the center occurs before degeneracy sets in; the remnant of the mass transfer event can evolve into either an ONe degenerate dwarf or a neutron star at a combined frequency that is comparable to the observed rate of supernova formation in the Galaxy. In binary systems in which the primary becomes a neutron star or an ONe degenerate dwarf, the secondary will normally evolve into a CO degenerate dwarf, and common envelope action will lead in some instances to small enough orbital separations that orbital shrinkage due to gravitational wave radiation will again lead to Roche-lobe filling by the CO dwarf. If before the merging and explosion process begins, the heavier compact component is an ONe degenerate dwarf, a supernova-like explosion fueled by the conversion of carbon and oxygen into iron peak nuclei may result, and the light curve and spectrum may be of the Type I "peculiar" variety. The compact remnant remaining after the explosion may be either an ONe degenerate dwarf or a neutron star. Publication: The Astrophysical Journal Pub Date: May 1986 DOI: 10.1086/164154 Bibcode: 1986ApJ...304..201I Keywords: Binary Stars; Neutron Stars; Stellar Evolution; Stellar Models; White Dwarf Stars; Convective Heat Transfer; Helium; Hertzsprung-Russell Diagram; Roche Limit; Stellar Mass; Astrophysics; STARS: BINARIES; STARS: EVOLUTION; STARS: INTERIORS; STARS: NEUTRON; STARS: SUPERNOVAE; STARS: WHITE DWARFS full text sources ADS |