3D modelling of AGB stars with CO5BOLD

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.

scholarly journals Heading Gaia to measure atmospheric dynamics in AGB stars

2018 ◽  
Vol 617 ◽  
pp. L1 ◽  
Author(s):  
A. Chiavassa ◽  
B. Freytag ◽  
M. Schultheis
Keyword(s):  
Large Scale ◽  
Three Dimensional ◽  
Complex Surface ◽  
Asymptotic Giant Branch ◽  
Stellar Surface ◽  
Small Scale ◽  
Substantial Part ◽  
Agb Stars ◽  
Stellar Radius ◽  
The Impact

Context. Asymptotic giant branch (AGB) stars are characterised by complex stellar surface dynamics that affect the measurements and amplify the uncertainties on stellar parameters. The uncertainties in observed absolute magnitudes have been found to originate mainly from uncertainties in the parallaxes. The resulting motion of the stellar photocentre could have adverse effects on the parallax determination with Gaia. Aims. We explore the impact of the convection-related surface structure in AGBs on the photocentric variability. We quantify these effects to characterise the observed parallax errors and estimate fundamental stellar parameters and dynamical properties. Methods. We use three-dimensional (3D) radiative hydrodynamics simulations of convection with CO5BOLD and the post-processing radiative transfer code OPTIM3D to compute intensity maps in the Gaia G band [325–1030 nm]. From those maps, we calculate the intensity-weighted mean of all emitting points tiling the visible stellar surface (i.e. the photocentre) and evaluate its motion as a function of time. We extract the parallax error from Gaia data-release 2 (DR2) for a sample of semi-regular variables in the solar neighbourhood and compare it to the synthetic predictions of photocentre displacements. Results. AGB stars show a complex surface morphology characterised by the presence of few large-scale long-lived convective cells accompanied by short-lived and small-scale structures. As a consequence, the position of the photocentre displays temporal excursions between 0.077 and 0.198 AU (≈5 to ≈11% of the corresponding stellar radius), depending on the simulation considered. We show that the convection-related variability accounts for a substantial part of the Gaia DR2 parallax error of our sample of semi-regular variables. Finally, we present evidence for a correlation between the mean photocentre displacement and the stellar fundamental parameters: surface gravity and pulsation. We suggest that parallax variations could be exploited quantitatively using appropriate radiation-hydrodynamics (RHD) simulations corresponding to the observed star.

scholarly journals Modelling the Mass Loss of Cool AGB Stars

2003 ◽  
Vol 210 ◽  
pp. 387-401 ◽  
Author(s):  
Peter Woitke
Keyword(s):  
Theoretical Research ◽  
Cloud Formation ◽  
Current Status ◽  
Small Scale ◽  
Driving Mechanism ◽  
Agb Stars ◽  
Physical Description ◽  
Stellar Pulsation ◽  
Outer Atmosphere ◽  
Present Understanding

The current status of modelling the wind driving mechanism of AGB stars is reviewed. Our present understanding of these winds is substantially guided by spherically symmetric, time-dependent models which reveal a complex interplay between stellar pulsation, hydrodynamics, small-scale waves, chemistry, dust formation and radiative transfer. The level of approximation used for the physical description of the various processes, and the degree of consistence in the models are crucial and different approaches lead to rather different results. Beside reviewing the frontiers of the present theoretical research on AGB star winds, this paper presents some new results concerning the cooling of the gas behind propagating shock waves in the outer atmosphere and the self-organisation of dust-forming gases (cloud formation) caused by a radiative/thermal instability of dust forming gases.

Separation of gas and dust in the winds of AGB stars

2018 ◽  
Vol 14 (S343) ◽  
pp. 462-463
Author(s):  
Lars Mattsson ◽  
Christer Sandin ◽  
Paolo Ventura
Keyword(s):  
High Resolution ◽  
Asymptotic Giant Branch ◽  
Radiation Hydrodynamics ◽  
Symmetric Model ◽  
Spherically Symmetric ◽  
Agb Stars ◽  
First Results ◽  
Forced Turbulence ◽  
Spherically Symmetric Model ◽  
Entire Picture

AbstractWe present first results from a project aiming at a better understanding of how gas and dust interact in dust-driven winds from Asymptotic Giant Branch (AGB) stars. We are at the final stage of developing a new parallelised radiation-hydrodynamics (RHD) code for AGB-wind modelling including a new generalised implementation of drift. We also discuss first results from high-resolution box simulations of forced turbulence intended to give quantitative “3D corrections” to dust-driven winds from AGB stars. It is argued that modelling of dust-driven winds of AGB stars is a problem that may need to be treated in a less holistic way, where some parts of the problem are treated separately in detailed simulations and are parameterised back into a less detailed (1D spherically symmetric) model describing the entire picture.

Hydrodynamic simulations of second generation star formation in a young globular cluster

2019 ◽  
Vol 14 (S351) ◽  
pp. 269-272
Author(s):  
Francesco Calura
Keyword(s):  
Star Formation ◽  
Globular Cluster ◽  
Second Generation ◽  
Three Dimensional ◽  
Asymptotic Giant Branch ◽  
Central Density ◽  
Agb Stars ◽  
Hydrodynamic Simulations ◽  
Numerical Studies

AbstractI will present results obtained by means of three-dimensional hydrodynamic simulations of the formation of second generation (SG) stars in a young globular cluster (GC). Our setup includes the mass return from Asymptotic Giant branch (AGB) stars, the accretion of pristine gas as well as star formation of SG stars, three ingredients which have never been simultaneously taken into account in previous 3D numerical studies of GC formation. The cluster is set in motion with respect to a distribution of gas and allowed to accrete mass from it. Formation of SG stars occurs out of the gas shed by AGB stars and from the gas accreted during the motion of the cluster. We consider two models characterised by different densities of the external gas. In both cases, we find that a very compact SG subsystem with central density > 105M⊙/pc3 forms in the innermost regions of the cluster.

scholarly journals Dust production scenarios in galaxies at z ∼6–8.3

2019 ◽  
Vol 624 ◽  
pp. L13 ◽  
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.

scholarly journals Maser emission from the CO envelope of the asymptotic giant branch star W Hydrae

2021 ◽  
Vol 654 ◽  
pp. A18
Author(s):  
W. H. T. Vlemmings ◽  
T. Khouri ◽  
D. Tafoya
Keyword(s):  
Mass Loss ◽  
Radiation Field ◽  
Asymptotic Giant Branch ◽  
Small Scale ◽  
High Angular Resolution ◽  
Circumstellar Envelope ◽  
Agb Stars ◽  
Maser Emission ◽  
Co Emission ◽  
Loss Rates

Context. Observation of CO emission around asymptotic giant branch (AGB) stars is the primary method to determine gas mass-loss rates. While radiative transfer models have shown that molecular levels of CO can become mildly inverted, causing maser emission, CO maser emission has yet to be confirmed observationally. Aims. High-resolution observations of the CO emission around AGB stars now have the brightness temperature sensitivity to detect possible weak CO maser emission. Methods. We used high angular resolution observations taken with the Atacama Large Millimeter/submillimeter Array (ALMA) to observe the small-scale structure of CO J = 3−2 emission around the oxygen-rich AGB star W Hya. Results. We find CO maser emission amplifying the stellar continuum with an optical depth τ ≈−0.55. The maser predominantly amplifies the limb of the star because CO J = 3−2 absorption from the extended stellar atmosphere is strongest towards the centre of the star. Conclusions. The CO maser velocity corresponds to a previously observed variable component of high-frequency H2O masers and with the OH maser that was identified as the amplified stellar image. This implies that the maser originates beyond the acceleration region and constrains the velocity profile since we find the population inversion primarily in the inner circumstellar envelope. We find that inversion can be explained by the radiation field at 4.6 μm and that the existence of CO maser emission is consistent with the estimated mass-loss rates for W Hya. However, the pumping mechanism requires a complex interplay between absorption and emission lines in the extended atmosphere. Excess from dust in the circumstellar envelope of W Hya is not sufficient to contribute significantly to the required radiation field at 4.6 μm. The interplay between molecular lines that cause the pumping can be constrained by future multi-level CO observations.

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

2019 ◽  
Vol 489 (4) ◽  
pp. 4890-4911 ◽  
Author(s):  
Jels Boulangier ◽  
D Gobrecht ◽  
L Decin ◽  
A de Koter ◽  
J Yates
Keyword(s):  
High Temperatures ◽  
Vital Role ◽  
Asymptotic Giant Branch ◽  
Nucleation Theory ◽  
Dust Formation ◽  
Agb Stars ◽  
Loss Mechanism ◽  
General Hypothesis ◽  
Chemical Mixture ◽  
Large Clusters

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.

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

2019 ◽  
Vol 628 ◽  
pp. A132 ◽  
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.

scholarly journals A mid-infrared imaging survey of post-AGB stars

2011 ◽  
Vol 7 (S283) ◽  
pp. 59-62
Author(s):  
Eric Lagadec ◽  
Tijl Verhoelst ◽  
Djamel Mékarnia ◽  
Olga Suárez ◽  
Albert A. Zijlstra ◽  
...  
Keyword(s):  
Binary Systems ◽  
Infrared Imaging ◽  
Morphological Changes ◽  
Asymptotic Giant Branch ◽  
Central Core ◽  
Binary Interaction ◽  
Agb Stars ◽  
Mid Infrared ◽  
Evolved Stars ◽  
Dust Distribution

AbstractPost-AGB stars are key objects for the study of the dramatic morphological changes of low- to intermediate-mass stars on their evolution from the Asymptotic Giant Branch (AGB) towards the planetary nebula stage. There is growing evidences that binary interaction processes may very well have a determining role in the shaping process of many objects, but so far direct evidence is still weak. We aim at a systematic study of the dust distribution around a large sample of post-AGB stars as a probe of the symmetry breaking in the nebulae around these systems. We used imaging in the mid-infrared to study the inner part of these evolved stars to probe direct emission from dusty structures in the core of post-AGB stars in order to better understand their shaping mechanisms. We imaged a sample of 93 evolved stars and nebulae in the mid-infrared using VISIR/VLT, T-Recs/Gemini South and Michelle/Gemini North. We found that all the the proto-planetary nebulae we resolved show a clear departure from spherical symmetry. 59 out of the 93 observed targets appear to be non resolved. The resolved targets can be divided in two categories. The nebulae with a dense central core, that are either bipolar and multipolar. The nebulae with no central core have an elliptical morphology. The dense central torus observed likely host binary systems which triggered fast outflows that shaped the nebulae.

scholarly journals Studying Asymptotic Giant Branch Stars in the JWST Era

Universe ◽  
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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