Asymptotic Giant Branch Evolution Research Articles

The variable mass loss of the AGB star WX Piscium as traced by the CO J = 1-0 through 7-6 lines and the dust emission

Context. Low and intermediate mass stars lose a significant fraction of their mass through a dust-driven wind during the Asymptotic Giant Branch (AGB) phase. Recent studies show that winds from late-type stars are far from being smooth. Mass-loss variations occur on different time scales, from years to tens of thousands of years. The variations appear to be particularly prominent towards the end of the AGB evolution. The occurrence, amplitude and time scale of these variations are still not well understood. Aims. The goal of our study is to gain insight into the structure of the circumstellar envelope (CSE) of WX Psc and map the possible variability of the late-AGB mass-loss phenomenon. Methods. We have performed an in-depth analysis of the extreme infrared AGB star WX Psc by modeling (1) the CO J = 1−0 through 7−6 rotational line profiles and the full spectral energy distribution (SED) ranging from 0.7 to 1300 µm. We hence are able to trace a geometrically extended region of the CSE. Results. Both mass-loss diagnostics bear evidence of the occurrence of mass-loss modulations during the last ∼2000 yr. In particular, WX Psc went through a high mass-loss phase ( u M ∼ 5 × 10 −5 M� /yr) some 800 yr ago. This phase lasted about 600 yr and was followed by a long period of low mass loss ( u M ∼ 5 × 10 −8 M� /yr). The present day mass-loss rate is estimated to be ∼6 × 10 −6 M� /yr. Conclusions. The AGB star WX Psc has undergone strong mass-loss rate variability on a time scale of several hundred years during the last few thousand years. These variations are traced in the strength and profile of the CO rotational lines and in the SED. We have consistently simulated the behaviour of both tracers using radiative transfer codes that allow for non-constant mass-loss rates.

Tracing the Development of Dust around Evolved Stars: The Case of 47 Tuc

We observed mid-infrared (7.5-22 μm) spectra of asymptotic giant branch (AGB) stars in the globular cluster 47 Tuc with the Spitzer Space Telescope, and we find significant dust features of various types. A comparison of the characteristics of the dust spectra with the location of the stars in a (log P, K)-diagram shows that the dust mineralogy and the position on the AGB are related. A 13 μm feature is seen in spectra of low-luminosity AGB stars. More luminous AGB stars show a broad feature at 11.5 μm. The spectra of the most luminous stars are dominated by the amorphous silicate bending vibration centered at 9.7 μm. For 47 Tuc AGB stars, we conclude that early on the AGB dust (consisting primarily of Mg, Al, and Fe oxides) is formed. With further AGB evolution, amorphous silicates become the dominant species.

Spitzer spectroscopy of carbon stars in the Small Magellanic Cloud

We present Spitzer Space Telescope spectroscopic observations of 14 carbon-rich asymptotic giant branch (AGB) stars in the Small Magellanic Cloud (SMC). SiC dust is seen in most of the carbon-rich stars but it is weak compared to Large Magellanic Cloud (LMC) stars. The SiC feature is strong only for stars with significant dust excess, opposite to what is observed for Galactic stars. We argue that in the SMC, SiC forms at lower temperature than graphite dust, whereas in the Galaxy SiC and graphite condensate at more comparable temperatures. Dust input into the interstellar medium by AGB stars consists mostly of carbonaceous dust, with little SiC or silicate dust. Only the two coolest stars show a 30-μm band due to MgS dust. We suggest that this is due to the fact that, in the SMC, mass-losing AGB stars generally have low circumstellar (dust) optical depth and therefore effective heating of dust by the central star does not allow temperatures below the 650 K necessary for MgS to exist as a solid. Gas phase C2H2 bands are stronger in the SMC than in the LMC or Galaxy. This is attributed to an increasing C/O ratio at low metallicity. We present a colour–colour diagram based on Spitzer InfraRed Array Camera (IRAC) and Multiband Imaging Photometer for Spitzer (MIPS) colours to discriminate between O- and C-rich stars. We show that AGB stars in the SMC become carbon stars early in the thermal-pulsing AGB evolution, and remain optically visible for ∼6 × 105 yr. For the LMC, this lifetime is ∼3 × 105 yr. The superwind phase traced with Spitzer lasts for ∼104 yr. Spitzer spectra of a K supergiant and a compact H ii region are also given.

The Discovery of Fluorine in Cool Extreme Helium Stars

Neutral fluorine (F I) lines are identified in the optical spectra of cool EHe stars. These are the first identification of F I lines in a star's spectrum, and provide the first measurement of fluorine abundances in EHe stars. The results show that fluorine is overabundant in EHe stars. The overabundance of fluorine provides evidence for the synthesis of fluorine in these stars, that is discussed in the light of asymptotic giant branch (AGB) evolution, and the expectation from accretion of an He white dwarf by a C-O white dwarf.

The Central Regions of M31 in the 3-5 μm Wavelength Region

Images obtained with NIRI on the Gemini North telescope are used to investigate the photometric properties of the central regions of M31 in the 3-5 μm wavelength range. The light distribution in the central arcsecond differs from what is seen in the near-infrared in the sense that the difference in peak brightness between P1 and P2 is larger in M' than in K'; no obvious signature of P3 is detected in M'. These results can be explained if there is a source of emission that contributes ~20% of the peak M' light of P1, has an effective temperature of no more than a few hundred K, and is located between P1 and P2. Based on the red K - M' color of this source, it is suggested that the emission originates in a circumstellar dust shell surrounding a single bright asymptotic giant branch (AGB) star. Tests of this hypothesis are described. A bright source that is ~8'' from the center of the galaxy is also detected in M'. This object has red colors and an absolute brightness in M' that is similar to the most highly evolved AGB stars in the solar neighborhood; hence, it is likely to be a very evolved AGB star embedded in a circumstellar envelope. The K-band brightness of this star is close to the peak expected for AGB evolution, and an age of only a few hundred million years is estimated, which is comparable to that of the P3 star cluster. Finally, using high angular resolution near-infrared adaptive optics images as a guide, a sample of unblended AGB stars outside of the central few arcseconds is defined in L'. The (L', K - L') color-magnitude diagram of these sources shows a dominant AGB population with a peak L' brightness and a range of K - L' colors that are similar to those of the most luminous M giants in the Galactic bulge.

The Elemental Abundances in Bare Planetary Nebula Central Stars and the Shell Burning in AGB Stars

ABSTRACTWe review the observed properties of extremely hot, hydrogen‐deficient post–asymptotic giant branch (AGB) stars of spectral type [WC] and PG1159. Their H deficiency is probably caused by a (very) late helium‐shell flash or an AGB final thermal pulse, laying bare interior stellar regions that are usually kept hidden below the hydrogen envelope. Thus, the photospheric elemental abundances of these stars allow us to draw conclusions about details of nuclear burning and mixing processes in the precursor AGB stars. We summarize the state of the art of stellar evolution models that simulate AGB evolution and the occurrence of a late He‐shell flash. We compare predicted elemental abundances to those determined by quantitative spectral analyses performed with advanced non‐LTE model atmospheres. Good qualitative and quantitative agreement is found. Future work can contribute to an even more complete picture of the nuclear processes in AGB stars.

Toward a Working Model for the Abundance Variations in Stars within Globular Clusters

A popular self-enrichment scenario for the formation of globular clusters assumes that the abundance anomalies seen in the stars in many clusters are due to a second stage of star formation occurring from the matter lost by the winds of massive asymptotic giant branch (AGB) stars. Until today, the modelings of the AGB evolution by several different groups has failed, for different reasons, to account for the patterns of chemical anomalies. Here we show that our own modeling can provide a consistent picture if we constrain the three main parameters that regulate AGB evolution, by (1) adopting a high efficiency convection model, (2) adopting rates of mass loss with a high dependence on the stellar luminosity, and (3) assuming a very small overshooting below the formal convective regions during the thermal pulse (TP) phase. The first assumption is needed to obtain an efficient oxygen depletion in the AGB envelopes, and the second is needed to lose the whole stellar envelope within few thermal pulses, so that the sum of CNO elements does not increase too much, consistent with observations. The third assumption is needed to fully understand the sodium production. We also show that the Mg-Al anticorrelation can be explained by adopting the higher limit of the NACRE rates for proton captures by 25Mg and 26Mg; the models are consistent with the recently discovered F-Al correlation. Problems remain in fully explaining the observed Mg isotope ratios.

Evolution of Asymptotic Giant Branch Stars

▪ Abstract The current status of modeling the evolution and nucleosynthesis of asymptotic giant branch (AGB) stars is reviewed. The principles of AGB evolution have been investigated in recent years leading to improved and refined models, for example with regard to hot-bottom burning or the third dredge-up. The postprocessing s-process model yields quantitative results that reproduce many observations. However, these and most other processes in AGB stars are intimately related to the physics of stellar mixing. Mixing in AGB stars is currently not well-enough understood for accurate yield predictions. Several constraints and methods are available to improve the models. Some regimes of AGB evolution have not yet been studied in sufficient detail. These include the super-AGB stars and AGB stars at extremely low or ultra low metallicity.

Optimization of Starburst99 for Intermediate‐Age and Old Stellar Populations

We have incorporated the latest release of the Padova models into the evolutionary synthesis code Starburst99. The Padova tracks were extended to include the full asymptotic giant branch (AGB) evolution until the final thermal pulse over the mass range 0.9 to 5 solar mass. With this addition, Starburst99 accounts for all stellar phases that contribute to the integrated light of a stellar population with arbitrary age from the extreme ultraviolet to the near-infrared. AGB stars are important for ages between 0.1 and 2 Gyr, with their contribution increasing at longer wavelengths. We investigate similarities and differences between the model predictions by the Geneva and the Padova tracks. The differences are particularly pronounced at ages > 1 Gyr, when incompleteness sets in for the Geneva models. We also perform detailed comparisons with the predictions of other major synthesis codes and found excellent agreement. Our synthesized optical colors are compared to observations of old, intermediate-age, and young populations. Excellent agreement is found for the old globular cluster system of NGC 5128 and for old and intermediate-age clusters in NGC 4038/39. In contrast, the models fail for red supergiant dominated populations with sub-solar abundances. This failure can be traced back to incorrect red supergiant parameters in the stellar evolutionary tracks. Our models and the synthesis code are publicly available as version 5.0 of Starburst99 at this http URL

The evolution of planetary nebulae

We investigate and discuss the expansion properties of planetary nebulae by means of 1D radiation-hydrodynamics models computed for different initial envelope configurations and central star evolutionary tracks. In particular, we study how the expansion depends on the initial density gradient of the circumstellar envelope and show that it is possible to derive information on the very last mass-loss episodes during the star's final evolution along and off the asymptotic giant branch. To facilitate the comparison of the models with real objects, we have also computed observable quantities like surface brightness and emission-line profiles. With the help of newly acquired high-resolution emission-line profiles for a sample of planetary nebulae we show that models with initial envelopes based on the assumption of a stationary wind outflow fail to explain the observed expansion speeds of virtually all of the observed planetary nebulae. Instead it must be assumed that during the very last phase of evolution along the final asymptotic giant branch evolution the mass-loss rate increases in strength, resulting in a much steeper slope of the circumstellar radial density distribution. Under these conditions, the expansion properties of the nebular gas differ considerably from the self-similar solutions found for isothermal conditions. Furthermore, the mass loss must remain at a rather high level until the stellar remnant begins to evolve quickly towards the central star regime. Current theoretical computations of dust-driven mass-loss which are restricted to rather low temperatures cannot be applied during the star's departure from the asymptotic giant branch.

Astrophysical Significance of Asymptotic Giant Branch Stellar Wind Energies Recorded in Meteoritic SiC Grains

Using model calculations for ion implantation into spherical grains in free space, we have identified two noble gas components implanted with high and low energies into presolar SiC grains isolated from the Murchison meteorite. The low-energy component seems to be associated with the stellar winds of expanding asymptotic giant branch (AGB) stars during the main stage of their evolution. In contrast, the high-energy component was probably generated during the late, thermally pulsing peak of AGB evolution and was implanted into SiC grains as a high-speed, post-AGB wind during the planetary nebula formation stage. If true, this is the first time that such a planetary nebular component has been identified in the laboratory and provides astrophysics with a new arena of research possibilities. Possible involvement of chemically active elements in the implantation events established for noble gases is also discussed. Although we have concentrated on evidence provided by the implantation of noble gases, we also extend the discussions to chemically active elements such as Ba and Sr.

A new synthetic model for asymptotic giant branch stars

We present a synthetic model for thermally pulsing asymptotic giant branch (TPAGB) evolution constructed by fitting expressions to full evolutionary models in the metallicity range 0.0001 ≤Z≤ 0.02. Our model includes parametrizations of third dredge-up and hot-bottom burning with mass and metallicity. The Large Magellanic Cloud and Small Magellanic Cloud carbon star luminosity functions are used to calibrate third dredge-up. We calculate yields appropriate for galactic chemical evolution models for 1H, 4He, 12C, 13C, 14N, 15N, 16O and 17O. The initial–final mass relation is examined for our stars and found to fit to within 0.1 M⊙ of the observations. We also reproduce well the white dwarf mass function for masses above about 0.58 M⊙. The new model is to be implemented in a rapid binary star evolution code.

The evolution of planetary nebulae

We follow hydrodynamically the evolution of spherical model planetary nebulae subject to different initial conditions and with various central stars, investigating how combinations of central-star mass and asymptotic giant branch mass-loss rate determine the shape and kinematics of a planetary nebula. With this approach we aim at constituting a framework useful for the interpretation of the evolutionary status and previous mass-loss history of observed individual nebulae, making use of their kinematical properties and surface brightness characteristics. In particular, the models are compared with the observed morphologies and kinematics of double shell nebulae. The dynamical structure of all the models is characterized by a more or less complicated shock wave pattern set up by ionization and wind interaction whose combined action results in general in a typical double-shell structure. We have found that models with simple initial structures based on a constant AGB mass-loss rate fail to comply with observed shell morphologies and surface-brightness distributions. A reasonable agreement with the observations is only found for a model where the mass-loss rate is strongly increasing towards the end of the asymptotic giant-branch evolution. Depending on the central star's evolutionary speed and the density of the cool wind expelled along the asymptotic giant-branch, planetary nebulae may never get optically thin. This is primarily the case for the more massive central stars, and this fact offers a rather natural explanation for the long standing problem of the very existence of molecular hydrogen in the immediate vicinity of hot central stars. We also show that distances to planetary nebulae based on expansion parallaxes are systematically too small by a significant amount.

Post–Asymptotic Giant Branch Evolution in the Large Magellanic Cloud: A Study of the Central Stars of Planetary Nebulae

We present medium- and broadband Hubble Space Telescope (HST) photometry of a sample of 35 central stars (CSs) of planetary nebulae (PNs) in the Large Magellanic Cloud (LMC). The observations were made with the Wide Field Planetary Camera 2 (WFPC2) and Space Telescope Imaging Spectrograph instruments on board the HST. By observing LMC objects, our sample is free of the distance uncertainty that is the dominant source of error in the determination of CS luminosities in Galactic PNs. By observing with the HST we resolve the nebula, and therefore we often detect the CSs unambiguously. We obtain core masses of 16 of the objects by comparing their positions on the H-R diagram with theoretical evolutionary tracks, once we determine the stellar effective temperature through Zanstra analysis. This sample of CS masses is the largest and most reliable set obtained in an extragalactic environment. We find an average mass of 0.65 M?, although a few of the objects have very high mass. This average value is consistent with the average mass of the white dwarf population in the Galaxy. As the immediate precursors of white dwarfs, the study of the mass distribution of PN CSs should help to constrain the initial-to-final mass relation within environments of differing metallicity. Finally, by exploring the physical connections between the star and the nebula, we establish the importance of the study of PNs in the LMC to constrain the energy input from the wind during the post-asymptotic giant branch phase.

Near-Infrared Monitoring of the Carbon Star IRC +10 216: A High Spatial-Resolution Time Sequence of Dust-Shell Evolution

The carbon star IRC +10 216 is a long-period Asymptotic Giant Branch (AGB) star suffering from strong stellar winds (several 10-5 M⊙/yr; Loup et al. 1993) which have led to an almost complete obscuration of the star by dust. Due to the high mass-loss rate, long period of P = 649 d (Le Bertre 1992), and carbon-rich chemistry of the dust-shell, IRC +10 216 is obviously in a very advanced stage of its AGB evolution. High-resolution near-infrared imaging of IRC +10 216 has revealed that on sub-arcsecond scales (100 mas) its dust shell is clumpy, bipolar, and changing on a time scale of only ~1 yr (Weigelt et al. 1997, 1998, Haniff & Buscher 1998, Osterbart et al. 2000, Tuthill et al. 2000). Since most dust shells around AGB stars are known to be spherically symmetric, whereas most proto-planetary nebulae (PPN) show an axisymmetric geometry (Olofsson 1996), it appears likely that IRC +10 216 has already entered the transition phase to the PPN stage. This suggests that the break of the dust-shell symmetry between the AGB and post-AGB phase already takes place at the end of the AGB evolution.

Structure and Evolution of Planetary Nebula Haloes

The density structure of the extended haloes of Planetary Nebulae (PN) is generally believed to reflect the previous history of heavy mass loss during the final stages of stellar evolution on the asymptotic giant-branch (AGB). In this review, we discuss different interpretations of the observed PN halo structures in the light of recent numerical simulations combining detailed AGB and post-AGB stellar evolution calculations with time-dependent hydrodynamical wind models.

H- and He-burning Central Stars and the Evolution to White Dwarfs

The structure and evolution of central stars of planetary nebulae (CSPNe) is reviewed. CSPNe represent the rapid transitional stage between the Asymptotic Giant Branch (AGB) and the white-dwarf domain. It is shown that the whole evolution off the AGB through the central-star regime depends on the evolutionary history. The detailed evolution into a white dwarf is controlled by the internal stellar structure which, in turn, is determined by the duration of the preceding AGB evolution and therefore by the AGB mass-loss history. The evolution of hydrogen-deficient central stars has been a matter of debate since many years. Convective overshoot appears to be a key ingredient to model these objects. Various thermal-pulse scenarios with inclusion of overshoot are discussed, leading to surface abundances in general agreement with those observed for Wolf-Rayet central stars.

Parameterizing the Third Dredge up in AGB Stars

We present new evolutionary sequences for low and intermediate mass stars (1M⊙ to 6M⊙) for three different metallicities, z = 0.02, 0.008 and 0.004. We evolve the models from the pre-main sequence to the thermally-pulsing asymptotic giant branch (AGB) phase. We have two sequences of models for each mass, one which includes mass-loss and one without mass-loss. For an overview of AGB evolution and nucleosynthesis, see Herwig (2002) and Lattanzio (2002).

The Dynamical Evolution of the Circumstellar Gas around Low‐ and Intermediate‐Mass Stars. II. The Planetary Nebula Formation

We have studied the effect of the mass of the central star (CS) on the gas evolution during the planetary nebula (PN) phase. We have performed numerical simulations of PN formation using CS tracks for six stellar core masses corresponding to initial masses from 1 to 5 Msun. The gas structure resulting from the previous asymptotic giant branch (AGB) evolution is used as the starting configuration. The formation of multiple shells is discussed in the light of our models, and the density, velocity and halpha emission brightness profiles are shown for each stellar mass considered. We have computed the evolution of the different shells in terms of radius, expansion velocity, and halpha peak emissivity. We find that the evolution of the main shell is controlled by the ionization front rather than by the thermal pressure provided by the hot bubble during the early PN stages. This effect explains why the kinematical ages overestimate the age in young CSs. At later stages in the evolution and for low mass progenitors the kinematical ages severely underestimate the CS age. Large (up to 2.3 pc), low surface brightness shells (less than 2000 times the brightness of the main shell) are formed in all of our models (with the exception of the 5 Msun model). These PN halos contain most of the ionized mass in PNe, which we find is greatly underestimated by the observations because of the low surface brightness of the halos.

Lithium during the AGB evolution in young open clusters of the Large Magellanic Cloud

We present the results of mid-resolution spectroscopy in the LiI 6708 A spectral region of Asymptotic Giant Branch (AGB) stars belonging to young open clusters of the Large Magellanic Cloud. Most stars belong to the clusters NGC 1866 and NGC 2031, which have an age of � 150 Myr. Lithium lines of different strength are detected in the spectra of stars evolving along the AGB, in agreement with theoretical predictions. According to stellar evolutionary models, at the start of the AGB the stars should all show a low residual lithium abundance as a consequence of dilution during the previous evolutionary phases. The most luminous and cooler thermally pulsating AGB stars, if they are massive enough, once in the AGB go first through a phase of Li destruction, which is followed by a phase of strong lithium production and further destruction. The production of lithium on the AGB is in particular explained by the onset of the Hot Bottom Burning (HBB) process. Our most conclusive results are obtained for the populous cluster NGC 1866 where: the early-AGB stars show a weak Li line, which can be attributed to the dilution of the initial abundance; one of the two luminous stars seem to have completely depleted lithium, as no line is detected; the second one shows a deep lithium line, whose strength can be explained by lithium production. The bolometric magnitude of these stars are consistent with the values predicted by the theory, for the mass evolving on the AGB of NGC 1866, at which lithium is first destroyed and then produced (Mbol �− 6). We also analyze the infrared luminosities (ISOCAM data) of these stars, to discuss if their evolutionary phase precedes or follows the lithium production stage. More intriguing and less clear results are obtained for the most luminous stars in NGC 2031: the brightest star seems as well to have destroyed lithium, while the second one shows a strong lithium line. However its bolometric luminosity - derived from the near-IR photometry, is much lower (Mbol �− 5.2 ± 0.15) than that expected from HBB models. Although low luminosity lithium rich AGB stars are also known, whose appearance is attributed to non-canonical mixing processes, it is not clear why two almost coeval clusters show such a different behaviour. It is also possible that this star does not belong to NGC 2031. Finally we suggest the observational tests that could shed further light on this matter.