Dust and molecular shells in asymptotic giant branch stars

Zhao-Geisler, R.; Quirrenbach, A.; Köhler, Rainer H.; López, B.

doi:10.1051/0004-6361/201118150

Cited by 40 publications

(44 citation statements)

References 161 publications

(243 reference statements)

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“…10), and has already been suggested to explain similar features seen in mid-infrared interferometric measurements of other comparable sources (e.g. Wittkowski et al 2007;Zhao-Geisler et al 2012).…”

Section: Morphology Of Rt Vir In the Mid-infraredsupporting

confidence: 62%

“…Finally, interferometric measurements in the mid-infrared have already pointed to the presence of Al 2 O 3 dust around two stellar radii in Mira and semi-regular M-type AGB stars (e.g. Wittkowski et al 2007;Zhao-Geisler et al 2012). We therefore decided to add corundum in the a-posteriori COMA radiative transfer computation, using the small-particle limit approximation.…”

Section: Exploring Extra Dust Opacitiesmentioning

confidence: 99%

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The wind of the M-type AGB star RT Virginis probed by VLTI/MIDI

Sacuto

Ramstedt

Höfner

et al. 2013

A&A

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Aims. We study the circumstellar environment of the M-type AGB star RT Vir using mid-infrared high spatial resolution observations from the ESO-VLTI focal instrument MIDI. The aim of this study is to provide observational constraints on theoretical prediction that the winds of M-type AGB objects can be driven by photon scattering on iron-free silicate grains located in the close environment (about 2 to 3 stellar radii) of the star. Methods. We interpreted spectro-interferometric data, first using wavelength-dependent geometric models. We then used a selfconsistent dynamic model atmosphere containing a time-dependent description of grain growth for pure forsterite dust particles to reproduce the photometric, spectrometric, and interferometric measurements of RT Vir. Since the hydrodynamic computation needs stellar parameters as input, a considerable effort was first made to determine these parameters. Results. MIDI differential phases reveal the presence of an asymmetry in the stellar vicinity. Results from the geometrical modeling give us clues to the presence of aluminum and silicate dust in the close circumstellar environment (<5 stellar radii). Comparison between spectro-interferometric data and a self-consistent dust-driven wind model reveals that silicate dust has to be present in the region between 2 to 3 stellar radii to reproduce the 59 and 63 m baseline visibility measurements around 9.8 μm. This gives additional observational evidence in favor of winds driven by photon scattering on iron-free silicate grains located in the close vicinity of an M-type star. However, other sources of opacity are clearly missing to reproduce the 10−13 μm visibility measurements for all baselines.Conclusions. This study is a first attempt to understand the wind mechanism of M-type AGB stars by comparing photometric, spectrometric, and interferometric measurements with state-of-the-art, self-consistent dust-driven wind models. The agreement of the dynamic model atmosphere with interferometric measurements in the 8−10 μm spectral region gives additional observational evidence that the winds of M-type stars can be driven by photon scattering on iron-free silicate grains. Finally, a larger statistical study and progress in advanced self-consistent 3D modeling are still required to solve the remaining problems.

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Section: Morphology Of Rt Vir In the Mid-infraredsupporting

confidence: 62%

Section: Exploring Extra Dust Opacitiesmentioning

confidence: 99%

The wind of the M-type AGB star RT Virginis probed by VLTI/MIDI

Sacuto

Ramstedt

Höfner

et al. 2013

A&A

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“…Using image reconstruction techniques, Zhao-Geisler et al (2015) were able to spatially resolve the dust formation region close to the star and deduced that the data signal the presence of a dust layer of presumably amorphous aluminium oxide (Al 2 O 3 ) at ∼2 R . The observations suggest that the formation of amorphous Al 2 O 3 occurs mainly around or after visual minimum (Zhao-Geisler et al 2012).…”

Section: Infrared Continuum Emissionmentioning

confidence: 86%

Study of the aluminium content in AGB winds using ALMA

Decin

Richards

Waters

et al. 2017

A&A

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Context. The condensation of inorganic dust grains in the winds of evolved stars is poorly understood. As of today, it is not yet known which molecular clusters form the first dust grains in oxygen-rich (C/O < 1) asymptotic giant branch (AGB) winds. Aluminium oxides and iron-free silicates are often put forward as promising candidates for the first dust seeds. Aims. We aim to constrain the dust formation histories in the winds of oxygen-rich AGB stars. Methods. We obtained Atacama Large Millimeter/sub-millimeter array (ALMA) observations with a spatial resolution of 120 × 150 mas tracing the dust formation region of the low mass-loss rate AGB star, R Dor, and the high mass-loss rate AGB star, IK Tau. We detected emission line profiles of AlO, AlOH, and AlCl in the ALMA data and used these line profiles to derive a lower limit of atomic aluminium incorporated in molecules. This constrains the aluminium budget that can condense into grains. Results. Radiative transfer models constrain the fractional abundances of AlO, AlOH, and AlCl in IK Tau and R Dor. We show that the gas-phase aluminium chemistry is completely different in both stars with a remarkable difference in the AlO and AlOH abundance stratification. The amount of aluminium locked up in these three molecules is small, ≤1.1 × 10 −7 w.r.t. H 2 , for both stars, i.e. only ≤2% of the total aluminium budget. An important result is that AlO and AlOH, which are the direct precursors of alumina (Al 2 O 3 ) grains, are detected well beyond the onset of the dust condensation, which proves that the aluminium oxide condensation cycle is not fully efficient. The ALMA observations allow us to quantitatively assess the current generation of theoretical dynamical-chemical models for AGB winds. We discuss how the current proposed scenario of aluminium dust condensation for low mass-loss rate AGB stars within a few stellar radii from the star, in particular for R Dor and W Hya, poses a challenge if one wishes to explain both the dust spectral features in the spectral energy distribution (SED) in interferometric data and in the polarized light signal. In particular, the estimated grain temperature of Al 2 O 3 is too high for the grains to retain their amorphous structure. We advocate that large gas-phase (Al 2 O 3 ) n clusters (n > 34) can be the potential agents of the broad 11 µm feature in the SED and in the interferometric data and we propose potential formation mechanisms for these large clusters.

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“…While Norris et al (2012) found a halo of large (∼0.3 µm), transparent grains close to the photosphere (at r ≤ 2 R ) in a few O-rich AGBs, Zhao-Geisler et al (2012) reported evidence for the coexistence of a warm layer of H 2 O and a Al 2 O 3 dust shell close to the stellar surface in several O-rich evolved stars. The presence of two dust shells of alumina and silicate composition, where the alumina shell is located below the silicate shell, has been inferred from the observation of a few O-rich stars (ZhaoGeisler et al 2012;Karovicova et al 2013).…”

Section: Summary and Discussionmentioning

confidence: 99%

Dust formation in the oxygen-rich AGB star IK Tauri

Gobrecht

Cherchneff

Sarangi

et al. 2015

A&A

181

289

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Aims. We model the synthesis of molecules and dust in the inner wind of the oxygen-rich Mira-type star IK Tau by considering the effects of periodic shocks induced by the stellar pulsation on the gas and by following the non-equilibrium chemistry in the shocked gas layers between 1 R and 10 R . We consider a very complete set of molecules and dust clusters, and combine the nucleation phase of dust formation with the condensation of these clusters into dust grains. We also test the impact of increasing the local gas density. Our derived molecular abundances and dust properties are compared to the most recent observational data. Methods. A semi-analytical formalism based on parameterised fluid equations is used to describe the gas density, velocity, and temperature in the inner wind. The chemistry is described by using a chemical kinetic network of reactions and the condensation mechanism is described by a Brownian formalism. A set of stiff, ordinary, coupled differential equations is solved, and molecular abundances, dust cluster abundances, grain size distributions and dust masses are derived. Results. The shocks drive an active non-equilibrium chemistry in the dust formation zone of IK Tau where the collision destruction of CO in the post-shock gas triggers the formation of C-bearing species such as HCN and CS. Most of the modelled molecular abundances agree well with the latest values derived from Herschel data, except for SO 2 and NH 3 , whose formation may not occur in the inner wind. Clusters of alumina, Al 2 O 3 , are produced within 2 R and lead to a population of alumina grains close to the stellar surface. Clusters of silicates (Mg 2 SiO 4 ) form at larger radii (r > 3 R ), where their nucleation is triggered by the formation of HSiO and H 2 SiO. They efficiently condense and reach their final grain size distribution between ∼6 R and 8 R with a major population of medium size grains peaking at ∼200 Å. This two dust-shell configuration agrees with recent interferometric observations. The derived dust-to-gas mass ratio for IK Tau is in the range 1-6 × 10 −3 and agrees with values derived from observations of O-rich Mira-type stars. Conclusions. Our results confirm the importance of periodic shocks in chemically shaping the inner wind of AGB stars and providing gas conditions conducive to the efficient synthesis of molecules and dust by non-equilibrium processes. They indicate that the wind acceleration will possibly develop in the radius range 4-8 R in IK Tau.

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Dust and molecular shells in asymptotic giant branch stars

Cited by 40 publications

References 161 publications

The wind of the M-type AGB star RT Virginis probed by VLTI/MIDI

The wind of the M-type AGB star RT Virginis probed by VLTI/MIDI

Study of the aluminium content in AGB winds using ALMA

Dust formation in the oxygen-rich AGB star IK Tauri

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