The role of epithermal neutrons in AGB stars: Boron synthesis

We consider the influence of the capture of epithermal neutrons on the nucleosynthesis in asymptotic giant branch stars (the Petrov-Shlyakhter effect). We show that epithermal neutrons can be captured by nitrogen through the hitherto unanalyzed channel ¹⁴N(n, α)¹¹B. Since the proton concentration in the partial mixing zone is low, this process results in an appreciable boron concentration. This boron can be brought to the stellar surface by peculiar processes. We analyze the boron concentration as a function of the assumed parameters for the partial mixing zone.     

AGB stars in the Magellanic Clouds

The stars that will be detectable in the Magellanic Clouds by the DENIS and 2MASS near infrared surveys are enumerated. All thermally-pulsing AGB stars will be observable in I, J, H and K, along with the top two magnitudes of both the early-AGB and the first giant branch. All carbon stars will be visible, and normal (N type) C stars will be easily distinguished by their large J-K colours. However, it will not be possible to separate faint, warm C stars from K and M stars using the photometry alone. Photometry of AGB stars in clusters will allow an accurate evaluation of the AGB tip luminosities as a function of initial mass. Random phase K magnitudes of LPVs and Cepheids should provide a better measure of the LMC tilt and distortions in the SMC. The K survey should turn up 100 to 150 objects undergoing superwind mass loss, these objects being OH/IR stars and the dust-enshrouded C star equivalents of OH/IR stars. It is shown that crowding should not be a problem even in the LMC bar.     

Short‐lived radioactivity in the early solar system: The Super‐AGB star hypothesis

Abstract– The composition of the most primitive solar system condensates, such as calcium‐aluminum‐rich inclusions (CAIs) and micron‐sized corundum grains, show that short‐lived radionuclides (SLR), e.g., ²⁶Al, were present in the early solar system. Their abundances require a local or stellar origin, which, however, is far from being understood. We present for the first time the abundances of several SLR up to ⁶⁰Fe predicted from stars with initial mass in the range approximately 7–11 M_⊙. These stars evolve through core H, He, and C burning. After core C burning they go through a “Super”‐asymptotic giant branch (Super‐AGB) phase, with the H and He shells activated alternately, episodic thermal pulses in the He shell, a very hot temperature at the base of the convective envelope (approximately 10⁸ K), and strong stellar winds driving the H‐rich envelope into the surrounding interstellar medium. The final remnants of the evolution of Super‐AGB stars are mostly O–Ne white dwarfs. Our Super‐AGB models produce ²⁶Al/²⁷Al yield ratios approximately 0.02–0.26. These models can account for the canonical value of the ²⁶Al/²⁷Al ratio using dilutions with the solar nebula of the order of 1 part of Super‐AGB mass per several 10² to several 10³ of solar nebula mass, resulting in associated changes in the O‐isotope composition in the range Δ¹⁷O from 3 to 20‰. This is in agreement with observations of the O isotopic ratios in primitive solar system condensates, which do not carry the signature of a stellar polluter. The radionuclides ⁴¹Ca and ⁶⁰Fe are produced by neutron captures in Super‐AGB stars and their meteoritic abundances are also matched by some of our models, depending on the nuclear and stellar physics uncertainties as well as the meteoritic experimental data. We also expect and are currently investigating Super‐AGB production of SLR heavier than iron, such as ¹⁰⁷Pd.     

Infrared photometry of AGB stars

The goal of this article is to show that, if one avoids star forming regions, mass-losing AGB stars can generally be selected from the data that will be provided by the near-infrared surveys which are presently considered. Also, if IRAS data are available, the separation between carbon-rich and oxygen-rich stars can be operated by their positions in infrared colour diagrams. In extragalactic systems, red supergiants can be discriminated from AGB stars by their luminosities.based on observations obtained with the ESO 1-m telescope     

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.     

The key role of massive stars in Oort cloud comet dynamics

The effects of a sample of 1300 individual stellar encounters spanning a wide range of parameter values (mass, velocity and encounter distance) are investigated. Power law fits for the number of injected comets demonstrate the long range effect of massive stars, whereas light stars affect comets mainly along their tracks. Similarly, we show that the efficiency of a star to fill the phase space region of the Oort cloud where the Galactic tides are able to inject comets into the observable region - the so-called “tidally active zone” (TAZ) - is also strongly dependent on the stellar mass. Power laws similar to those for direct injection are obtained for the efficiency of stars to fill the TAZ. This filling of the tidally active zone is crucial for the long term flux of comets from the Oort cloud. Based on long-term Monte Carlo simulations using a constant Galactic tide and a constant flux of stellar encounters, but neglecting the detailed effects of planetary perturbations, we show that this flux essentially results from a two step mechanism: (i) the stellar injection of comets into the TAZ; and (ii) the tidal injection of TAZ comets into the loss cone. We find that single massive stars are able to induce “comet drizzles” - corresponding to an increase of the cometary flux of about 40% - which may last for more than 100 Myr by filling the TAZ to a higher degree than normal. It appears that the stars involved in this process are the same that cause comet showers.     

X-ray emission from young stars and implications for the early solar system

Recent observations of soft X-ray emission from solar-type stars obtained with the Einstein X-Ray Observatory indicate that X-ray luminosity is inversely correlated with stellar age. If this result is applied to the Sun and if X-ray emission is a valid indicator of other manifestations of solar activity, then past solar wind and flare levels can be inferred. It can qualitatively explain the excess xenon and nitrogen found in the lunar regolith compared to the level expected from the comteporary solar wind. X-Ray emission from T Tauri and other low-mass pre-main-sequence stars is both highly luminous and variable, indicating the presence of flares ～4 × 10³ times stronger than the largest flares seen in the contemporary Sun. The proton flux from such solar flares during the 10⁶ to 10⁷-year pre-main-sequence phase would be sufficient to account for the ²⁶Al anomaly n meteorites.     

The evolution from visible to infrared variable AGB stars

Results of the Vassiliadis and Wood (1993) model for the evolution on the AGB are used to study how DENIS can complement an optical and IRAS based study of Long Period Variable AGB stars in a field towards the galactic bulge.     

Forming dust precursors in the inner envelopes of carbon-rich AGB stars

The formation of polycyclic aromatic hydrocarbon (PAH) molecules is studied in the inner envelope of a typical carbon-rich AGB star. The deep envelope is formed of layers of gas that experience the passage of strong periodic shocks forming close to the stellar photosphere. The parcels of gas then follow quasi-ballistic trajectories which are characterized by high gas densities. A chemical scheme based on combustion chemistry is applied to shocked layers of gas, and a PAH formation yield is calculated. PAHs up to coronene (C₂₄H₁₂) survive shocks with strengths of 10 km s^–1, and they accumulate in the gas parcel over several stellar pulsations. This result illustrates that any C-rich AGB star can nucleate dust precursors in its envelope.     

A phenomenological model for the extended zone above AGB stars

I suggest the existence of an extended zone above the surface of asymptotic giant branch (AGB), as well as similar stars experiencing high mass-loss rates. In addition to the escaping wind, in this zone there are parcels of gas that do not reach the escape velocity. These parcels of dense gas rise slowly and then fall back. The wind and bound gas exist simultaneously to distances of 100AU. I term this region the effervescent zone. In this phenomenological study I find that the density of the bound material in the effervescent zone falls as r^−5/2, not much faster than the wind density. The main motivation to propose the effervescent model is to allow wide binary companions to influence the morphology of the descendant planetary nebulae (PN) by accreting mass from the effervescent zone. Accretion from the effervescent zone is more efficient than accretion from the wind in forming an accretion disk around the companion. The companion might then blow two jets that will shape the descendant PN.     

On a magnetohydrodynamical process of mass loss in AGB stars

A spherically, time independent model is constructed to explain the mass loss observed in cool giants. It is suggested that a random magnetic field frozen within the gas is capable of ejecting matter from the photosphere of AGB (Mira or OH/IR) stars. The velocities at infinity of the ejected material are comparable to the observed velocities of expansion in circumstellar envelopes.     

Characteristics of convection and overshooting in RGB and AGB stars

Based on the turbulent convection model (TCM) of Li & Yang, we have studied the characteristics of turbulent convection in the envelopes of 2 and 5M⊙ stars at the red giant branch and asymptotic giant branch phases. The TCM has been successfully applied over the entire convective envelopes, including the convective unstable zone and the overshooting regions. We find that the convective motions become progressively stronger when the stellar models are located farther up along the Hayashi line. In the convect...     

Sensitivity in the models of massive stars

Two related problems are discussed in this article: The width of the Main-Sequence of massive stars and sensitivity peatures introduced into the evolutionary tracks of massive stars by mass loss and core-overshooting. It is suggested that core-overshooting may not necessarily be implied by the observations of the width of the Main-Sequence band. It is also noted that models evolved with both mass loss and/or core-overshooting reveal the presence of a large and unexplained expansion of the stellar models under certain conditions. This sensitivity feature would seem to be a fundamental feature inherent to the structure of massive stars.     

Detection of detached dust envelopes around oxygen-rich AGB stars

Extended emission components are clearly found in the IRAS scan data of optically visible oxygen-rich AGB stars which show no 10µm silicate band feature in the IRAS LRS spectra but a strong infrared excess in the IRAS photometric data. It is most likely that these stars really have their circumstellar dust envelopes, which are detached from the central stars, indicating a halting of mass loss for a significant period.     

The production of 26Al, 60Fe and 44Ti in massive stars of solar metallicity

We discuss the production of the three gamma ray emitters ²⁶Al, ⁶⁰Fe and ⁴⁴Ti in a 15 M_⊙ and a 25 M_⊙ star of solar metallicity.     

The criterion of appearance of unstable thermonuclear reaction in the helium burning shell of AGB stars

A criterion of the appearance of unstable thermonuclear reaction in the helium burning shell of thermal pulsating AGB (TP-AGB) star is established. The new criterion contains abundant physical information. It involves not only the geometric parameters of the helium burning shell, but also its mechanical, thermal and chemical parameters.The following mechanism of the occurrence and disappearance of unstable thermonuclear reaction in the helium burning shell of TP-AGB star is proposed: The appearance of a region of unstable convection in the helium burning shell of the TP-AGB star triggers unstable thermonuclear reaction which will promote a rapid expansion and a rapid geometric deformation of the shell, thereby removing the unstable thermonuclear reaction.Using the improved program of stellar evolution of Kippenhahn, the evolution of a 5Mo star is followed from the main sequence to the TP-AGB stage. The results show that the new criterion can well reflect the status of the thermonuclear reaction in the helium burning shell of the star. Besides, it is revealed that in the sixth period of thermal pulsation of the star the elements that are dredged up to the surface of the star, are synthesized mainly by thermonuclear reaction under the conditions, temperature lgT₂/K < 8.155 and density 4.0 < lg P2 /9 . CM-3 < 4.6.     

Searching for stars closely encountering with the solar system

Based on a new version of the Hipparcos catalog and currently available radial velocity data, we have searched for stars that either have encountered or will encounter the solar neighborhood within less than 3 pc in the time interval from −2 Myr to +2 Myr. Nine new candidates within 30 pc of the Sun have been found. To construct the stellar orbits relative to the solar orbit, we have used the epicyclic approximation. We show that, given the errors in the observational data, the probability that the well-known star HIP 89 825 (GL 710) encountering with the Sun most closely falls into the Oort cloud is 0.86 in the time interval 1.45 ± 0.06 Myr. This star also has a nonzero probability, 1 × 10⁻⁴, of falling into the region d < 1000 AU, where its influence on Kuiper Belt objects becomes possible.     

Deuterium in the early solar system

An attempt is made to construct a self-consistent picture of the deuterium abundance in the early Solar System based on the assumption of chemical equilibrium in the solar nebula. A recent determination of the $\text{D}\text{H}$ ratio for the atmosphere of Jupiter is consistent with a previous estimate of the $\text{D}\text{H}$ ratio for the proto-Sun. The high (> 1.5 × 10^?4) $\text{D}\text{H}$ ratios determined from analyses of carbonaceous meteorites imply an equilibrium temperature < 270°K, in marked disagreement with the equilibrium temperature determined for the same material by oxygen isotope cosmothermometry.     

Yields of massive stars and their role in galactic chemical evolution studies

We review the yields of intermediate mass elements (from C to Zn) from massive stars and their associated uncertainties, in the light of recent theoretical results. We consider the role of those yields for our understanding of the chemical evolution of the solar neighbourhood and of the halo of our Galaxy. Current yields reproduce in a satisfactory way the solar system composition, but several problems remain concerning abundance ratios in halo stars.