Developing a self-consistent AGB wind model – II. Non-classical, non-equilibrium polymer nucleation in a chemical mixture

ABSTRACT Unravelling the composition and characteristics of gas and dust lost by asymptotic giant branch (AGB) stars is important as these stars play a vital role in the chemical life cycle of galaxies. The general hypothesis of their mass-loss mechanism is a combination of stellar pulsations and radiative pressure on dust grains. However, current models simplify dust formation, which starts as a microscopic phase transition called nucleation. Various nucleation theories exist, yet all assume chemical equilibrium, growth restricted by monomers, and commonly use macroscopic properties for a microscopic process. Such simplifications for initial dust formation can have large repercussions on the type, amount, and formation time of dust. By abandoning equilibrium assumptions, discarding growth restrictions, and using quantum mechanical properties, we have constructed and investigated an improved nucleation theory in AGB wind conditions for four dust candidates, TiO2, MgO, SiO, and Al2O3. This paper reports the viability of these candidates as first dust precursors and reveals implications of simplified nucleation theories. Monomer restricted growth underpredicts large clusters at low temperatures and overpredicts formation times. Assuming the candidates are present, Al2O3 is the favoured precursor due to its rapid growth at the highest considered temperatures. However, when considering an initially atomic chemical mixture, only TiO2-clusters form. Still, we believe Al2O3 to be the prime candidate due to substantial physical evidence in presolar grains, observations of dust around AGB stars at high temperatures, and its ability to form at high temperatures and expect the missing link to be insufficient quantitative data of Al-reactions.

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Dynamics, temperature, chemistry, and dust: Ingredients for a self-consistent AGB wind

Proceedings of the International Astronomical Union ◽

10.1017/s1743921318005094 ◽

2018 ◽

Vol 14 (S343) ◽

pp. 129-133

Author(s):

J. Boulangier ◽

D. Gobrecht ◽

L. Decin

Keyword(s):

Life Time ◽

Vital Role ◽

Asymptotic Giant Branch ◽

Nucleation Theory ◽

Agb Stars ◽

Low Mass Stars ◽

Chemical Enrichment ◽

Self Consistent ◽

Evolutionary Phase ◽

Almost All

AbstractUnderstanding Asymptotic Giant Branch (AGB) stars is important as they play a vital role in the chemical life cycle of galaxies. AGB stars are in a phase of their life time where they have almost ran out of fuel and are losing vast amounts of material to their surroundings, via stellar winds. As this is an evolutionary phase of low mass stars, almost all stars go through this phase making them one of the main contributors to the chemical enrichment of galaxies. It is therefore important to understand what kind of material is being lost by these stars, and how much and how fast. This work summarises the steps we have taken towards developing a self-consistent AGB wind model. We improve on current models by firstly coupling chemical and hydrodynamical evolution, and secondly by upgrading the nucleation theory framework to investigate the creation of TiO2, SiO, MgO, and Al2O3 clusters.

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On the onset of dust formation in AGB stars

Proceedings of the International Astronomical Union ◽

10.1017/s1743921318006014 ◽

2018 ◽

Vol 14 (S343) ◽

pp. 119-128

Author(s):

David Gobrecht ◽

Stefan T. Bromley ◽

John M. C. Plane ◽

Leen Decin ◽

Sergio Cristallo

Keyword(s):

Emission Feature ◽

Stellar Atmospheres ◽

Nucleation Theory ◽

Classical Nucleation Theory ◽

Quantum Mechanical ◽

Dust Formation ◽

Agb Stars ◽

Promising Candidate ◽

Bending Modes ◽

Structure Calculations

AbstractA promising candidate to initiate dust formation in oxygen-rich AGB stars is alumina (Al2O3) showing an emission feature around ∼13μm attributed to Al−O stretching and bending modes (Posch+99,Sloan+03). The counterpart to alumina in carbon-rich AGB atmospheres is the highly refractory silicon carbide (SiC) showing a characteristic feature around 11.3μm (Treffers74). Alumina and SiC grains are thought to represent the first condensates to emerge in AGB stellar atmospheres. We follow a bottom-up approach, starting with the smallest stoichiometric clusters (i.e. Al4O6, Si2C2), successively building up larger-sized clusters. We present new results of quantum-mechanical structure calculations of (Al2O3)n, n = 1−10 and (SiC)n clusters with n = 1−16, including potential energies, rotational constants, and structure-specific vibrational spectra. We demonstrate the energetic viability of homogeneous nucleation scenarios where monomers (Al2O3 and SiC) or dimers (Al4O6 and Si2C2) are successively added. We find significant differences between our quantum theory based results and nanoparticle properties derived from (classical) nucleation theory.

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Imaging the dust and the gas around Mira using ALMA and SPHERE/ZIMPOL

Proceedings of the International Astronomical Union ◽

10.1017/s1743921318006245 ◽

2018 ◽

Vol 14 (S343) ◽

pp. 31-35

Author(s):

Theo Khouri ◽

Wouter H. T. Vlemmings ◽

Hans Olofsson ◽

Christian Ginski ◽

Elvire De Beck ◽

...

Keyword(s):

Mass Loss ◽

Formation Process ◽

Angular Resolution ◽

Asymptotic Giant Branch ◽

Stellar Disk ◽

High Angular Resolution ◽

Dust Formation ◽

Loss Mechanism ◽

Polarised Light ◽

Visible Wavelengths

AbstractThe mass-loss mechanism of asymptotic giant branch stars has long been thought to rely on two processes: stellar pulsations and dust formation. The details of the mass-loss mechanism have remained elusive, however, because of the overall complexity of the dust formation process in the very dynamical pulsation-enhanced atmosphere. Recently, our understanding of AGB stars and the associated mass loss has evolved significantly, thanks both to new instruments which allow sensitive and high-angular-resolution observations and the development of models for the convective AGB envelopes and the dust formation process. ALMA and SPHERE/ZIMPOL on the VLT have been very important instruments in driving this advance in the last few years by providing high-angular resolution images in the sub-mm and visible wavelengths, respectively. I will present observations obtained using these instruments at the same epoch (2.5 weeks apart) of the AGB star Mira that resolve even the stellar disk. The ALMA data reveals the distribution and dynamics of the gas around the star, while the polarised light imaged using SPHERE shows the distribution of the dust grains expected to drive the outflows. Moreover, the observations show a central source surrounded by asymmetric distributions of gas and dust, with complementary structures seen in the two components. We model the observed CO v = 1, J = 3−2 line to determine the density, temperature and velocity of gas close to the star. This model is then used to estimate the abundance of AlO. Our results show that only a very small fraction of aluminium (≲0.1%) is locked in AlO molecules. We also calculate models to fit the observed polarised light based on the gas densities we find. The low level of visible-light polarisation detected using ZIMPOL implies that, at the time of the observations, aluminium atoms are either not efficiently depleted into dust or the aluminium-oxide grains are relatively small (≲0.02μm).

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3D modelling of AGB stars with CO5BOLD

Proceedings of the International Astronomical Union ◽

10.1017/s1743921318006658 ◽

2018 ◽

Vol 14 (S343) ◽

pp. 9-18

Author(s):

Bernd Freytag ◽

Susanne Höfner ◽

Sofie Liljegren

Keyword(s):

Radiation Transport ◽

Three Dimensional ◽

Asymptotic Giant Branch ◽

Small Scale ◽

Dust Formation ◽

Radiation Hydrodynamics ◽

Agb Stars ◽

Outer Atmosphere ◽

Solar Type ◽

Dust Distribution

AbstractLocal three-dimensional radiation-hydrodynamics simulations of patches of the surfaces of solar-type stars, that are governed by small-scale granular convection, have helped analyzing and interpreting observations for decades. These models contributed considerably to the understanding of the atmospheres and indirectly also of the interiors and the active layers above the surface of these stars. Of great help was of course the availability of a close-by prototype of these stars – the sun.In the case of an asymptotic-giant-branch (AGB) star, the convective cells have sizes comparable to the radius of the giant. Therefore, the extensions of the solar-type-star simulations to AGB stars have to be global and cover the entire object, including a large part of the convection zone, the molecule-formation layers in the inner atmosphere, and the dust-formation region in the outer atmosphere. Three-dimensional radiation-hydrodynamics simulations with CO5BOLD show how the interplay of large and small convection cells, waves, pulsations, and shocks, but also molecular and dust opacities of AGB stars create conditions very different from those in the solar atmosphere.Recent CO5BOLD models account for frequency-dependent radiation transport and the formation of two independent dust species for an oxygen-rich composition. The drop of the comparably smooth temperature distribution below a threshold determines to onset of dust formation, further in, at higher temperatures, for aluminium oxides (Al2O3) than for silicates (Mg2SiO4). An uneven dust distribution is mostly caused by inhomogeneities in the density of the shocked gas.

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From molecules to dust grains: The role of alumina cluster seeds

Proceedings of the International Astronomical Union ◽

10.1017/s1743921319009384 ◽

2019 ◽

Vol 15 (S350) ◽

pp. 245-248

Author(s):

David Gobrecht ◽

John M.C. Plane ◽

Stefan T. Bromley ◽

Leen Decin ◽

Sergio Cristallo

Keyword(s):

Bulk Material ◽

Spectral Feature ◽

Chemical Kinetic ◽

Asymptotic Giant Branch ◽

Nucleation Theory ◽

Classical Nucleation Theory ◽

Circumstellar Envelope ◽

Agb Stars ◽

Dust Grains ◽

Alumina Cluster

AbstractAsymptotic Giant Branch (AGB) stars contribute a major part to the global dust budget in galaxies. Owing to their refractory nature alumina (stoichiometric formula AlO) is a promising candidate to be the first condensate emerging in the atmospheres of oxygen-rich AGB stars. Strong evidence for that is supplied by the presence of alumina in pristine meteorites and a broad spectral feature observed around ∼ 13 μm. The emergence of a specific condensate depends on the thermal stability of the solid, the gas density and its composition. The evaluation of the condensates is based on macroscopic bulk properties. The growth and size distribution of dust grains is commonly described by Classical Nucleation Theory (CNT). We question the applicability of CNT in an expanding circumstellar envelope as CNT presumes thermodynamic equilibrium and requires, in practise, seed nuclei on which material can condense. However, nano-sized molecular clusters differ significantly from bulk analogues. Quantum effects of the clusters lead to non-crystalline structures, whose characteristics (energy, geometry) differ substantially, compared to the bulk material. Hence, a kinetic quantum-chemical treatment involving various transition states describes dust nucleation most accurately. However, such a treatment is prohibitive for systems with more than 10 atoms. We discuss the viability of chemical-kinetic routes towards the formation of the monomer (Al2O3) and the dimer (Al4O6) of alumina.

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Dust production scenarios in galaxies at z ∼6–8.3

Astronomy and Astrophysics ◽

10.1051/0004-6361/201935149 ◽

2019 ◽

Vol 624 ◽

pp. L13 ◽

Cited By ~ 9

Author(s):

Aleksandra Leśniewska ◽

Michał Jerzy Michałowski

Keyword(s):

Interstellar Medium ◽

Grain Growth ◽

Early Universe ◽

High Redshift ◽

Asymptotic Giant Branch ◽

Maximum Efficiency ◽

Dust Formation ◽

Agb Stars ◽

Dust Production

Context. The mechanism of dust formation in galaxies at high redshift is still unknown. Asymptotic giant branch (AGB) stars and explosions of supernovae (SNe) are possible dust producers, and non-stellar processes may substantially contribute to dust production, for example grain growth in the interstellar medium. Aims. Our aim is to determine the contribution to dust production of AGB stars and SNe in nine galaxies at z ∼ 6−8.3, for which observations of dust have been recently attempted. Methods. In order to determine the origin of the observed dust we have determined dust yields per AGB star and SN required to explain the total amounts of dust in these galaxies. Results. We find that AGB stars were not able to produce the amounts of dust observed in the galaxies in our sample. In order to explain these dust masses, SNe would have to have maximum efficiency and not destroy the dust which they formed. Conclusions. Therefore, the observed amounts of dust in the galaxies in the early universe were formed either by efficient supernovae or by a non-stellar mechanism, for instance the grain growth in the interstellar medium.

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Exploring the innermost dust formation region of the oxygen-rich AGB star IK Tauri with VLT/SPHERE-ZIMPOL and VLTI/AMBER

Astronomy and Astrophysics ◽

10.1051/0004-6361/201834999 ◽

2019 ◽

Vol 628 ◽

pp. A132 ◽

Cited By ~ 4

Author(s):

C. Adam ◽

K. Ohnaka

Keyword(s):

Spectral Resolution ◽

Mass Loss Rate ◽

Outer Boundary ◽

Surrounding Medium ◽

Wavelength Region ◽

Asymptotic Giant Branch ◽

High Spectral Resolution ◽

Dust Formation ◽

Agb Stars ◽

Polarimetric Imaging

Context. Low- and intermediate-mass stars on the asymptotic giant branch (AGB) are known to be prevalent dust providers to galaxies, replenishing the surrounding medium with molecules and dust grains. However, the mechanisms responsible for the formation and acceleration of dust in the cool extended atmospheres of AGB stars are still open to debate. Aims. We present visible polarimetric imaging observations of the oxygen-rich AGB star IK Tau obtained with the high-resolution polarimetric imager VLT/SPHERE-ZIMPOL at post-maximum light (phase 0.27) as well as high-spectral resolution long-baseline interferometric observations with the AMBER instrument at the Very Large Telescope Interferometer (VLTI). We aim to spatially resolve the dust and molecule formation regions, and to investigate their physical and chemical properties within a few stellar radii of IK Tau. Methods. IK Tau was observed with VLT/SPHERE-ZIMPOL at three wavelengths in the pseudo-continuum (645, 748, and 820 nm), in the Hα line at 656.3 nm, and in the TiO band at 717 nm. The VLTI/AMBER observations were carried out in the wavelength region of the CO first overtone lines near 2.3 μm with a spectral resolution of 12 000. Results. The excellent polarimetric imaging capabilities of SPHERE-ZIMPOL have allowed us to spatially resolve clumpy dust clouds at 20–50 mas from the central star, which corresponds to 2–5 R⋆ when combined with a central star’s angular diameter of 20.7 ± 1.53 mas measured with VLTI/AMBER. The diffuse, asymmetric dust emission extends out to ~73 R⋆. We find that the TiO emission extends to 150 mas (15 R⋆). The AMBER data in the individual CO lines also suggest a molecular outer atmosphere extending to ~1.5 R⋆. The results of our 2D Monte Carlo radiative transfer modelling of dust clumps suggest that the polarized intensity and degree of linear polarization can be reasonably explained by small-sized (0.1 μm) grains of Al2O3, MgSiO3, or Mg2SiO4 in an optically thin shell (τ550 nm = 0.5 ± 0.1) with an inner and outer boundary radius of 3.5 R⋆ and ≳25 R⋆, respectively. The observed clumpy structures can be reproduced by a density enhancement of a factor of 3.0 ± 0.5. However, the model still predicts the total intensity profiles to be too narrow compared to the observed data, which may be due to the TiO emission and/or grains other than homogeneous, filled spheres. Conclusions. IK Tau’s mass-loss rate is 20–50 times higher than the previously studied AGB stars W Hya, R Dor, and o Cet. Nevertheless, our observations of IK Tau revealed that clumpy dust formation occurs close to the star as seen in those low mass-rate AGB stars.

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AGB population as probes of galaxy structure and evolution

Proceedings of the International Astronomical Union ◽

10.1017/s1743921318006671 ◽

2018 ◽

Vol 14 (S343) ◽

pp. 283-290

Author(s):

Atefeh Javadi ◽

Jacco Th. van Loon

Keyword(s):

Star Formation ◽

Mass Loss ◽

Galaxy Evolution ◽

Local Group ◽

Near Field ◽

Asymptotic Giant Branch ◽

Star Formation History ◽

Agb Stars ◽

Dust Production ◽

Loss Mechanism

AbstractThe evolution of galaxies is driven by the birth and death of stars. AGB stars are at the end points of their evolution and therefore their luminosities directly reflect their birth mass; this enables us to reconstruct the star formation history. These cool stars also produce dust grains that play an important role in the temperature regulation of the interstellar medium (ISM), chemistry, and the formation of planets. These stars can be resolved in all of the nearby galaxies. Therefore, the Local Group of galaxies offers us a superb near-field cosmology site. Here we can reconstruct the formation histories, and probe the structure and dynamics, of spiral galaxies, of the many dwarf satellite galaxies surrounding the Milky Way and Andromeda, and of isolated dwarf galaxies. It also offers a variety of environments in which to study the detailed processes of galaxy evolution through studying the mass-loss mechanism and dust production by cool evolved stars. In this paper, I will first review our recent efforts to identify mass-losing Asymptotic Giant Branch (AGB) stars and red supergiants (RSGs) in Local Group galaxies and to correlate spatial distributions of the AGB stars of different mass with galactic structures. Then, I will outline our methodology to reconstruct the star formation histories using variable pulsating AGB stars and RSGs and present the results for rates of mass–loss and dust production by pulsating AGB stars and their analysis in terms of stellar evolution and galaxy evolution.

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Studying Asymptotic Giant Branch Stars in the JWST Era

Universe ◽

10.3390/universe6120223 ◽

2020 ◽

Vol 6 (12) ◽

pp. 223

Author(s):

Paolo Ventura ◽

Ester Marini ◽

Silvia Tosi ◽

Flavia Dell’Agli

Keyword(s):

Large Magellanic Cloud ◽

Magellanic Cloud ◽

Asymptotic Giant Branch ◽

Dust Formation ◽

Asymptotic Giant Branch Stars ◽

Agb Stars ◽

Space Telescope ◽

James Webb Space Telescope ◽

Giant Branch Stars

We explore the potential offered by the incoming launch of the James Webb Space Telescope, to study the stars evolving through the asymptotic giant branch (AGB) phase. To this aim we compare data of AGB stars in the Large Magellanic Cloud, taken with the IRS spectrograph, with the results from modelling of AGB evolution and dust formation in the wind. We find that the best diagrams to study M- and C-stars are, respectively, ([F770W]−[F2500W], [F770W]) and ([F770W]−[F1800W], [F1800W]). ([F770W]−[F2500W], [F770W]) turns out to be the best way of studying the AGB population in its entirely.

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The evolution of DARWIN: current status of wind models for AGB stars

Proceedings of the International Astronomical Union ◽

10.1017/s1743921318005926 ◽

2018 ◽

Vol 14 (S343) ◽

pp. 99-107

Author(s):

Sara Bladh

Keyword(s):

Mass Loss ◽

Momentum Transfer ◽

Asymptotic Giant Branch ◽

Dust Particles ◽

Current Status ◽

Dust Formation ◽

Agb Stars ◽

Future Plans

AbstractThe slow, dense winds observed in evolved asymptotic giant branch (AGB) stars are usually attributed to a combination of dust formation in the dynamical inner atmosphere of these stars and momentum transfer from stellar photons interacting with the newly formed dust particles. Wind models calculated with the DARWIN code, using this mass-loss scenario, have successfully produced outflows with dynamical and photometric properties compatible with observations, for both C-type and M-type AGB stars. Presented here is an overview of the DARWIN models currently available and what output these models produce, as well as future plans.

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