Dynamic atmospheres and winds of cool luminous giants

Höfner, Susanne; Bladh, Sara; Aringer, B.; Ahuja, Rajeev

doi:10.1051/0004-6361/201628424

Cited by 101 publications

(180 citation statements)

References 64 publications

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“…This allows the material to potentially grow in size, and Γ can become larger than 1 only once their size is larger than 0.1-0.3 µm. This idea of a GBDS was recently shown to fit well with the theoretical dynamical wind models of Höfner et al (2016), who allowed for the formation of composite grains with an Al 2 O 3 core and a silicate mantle at slightly larger distances than the Al 2 O 3 GBDS shell, thereby giving the wind a kick start. The location of the GBDS coincides with the region in which large transparent grains are detected in polarized light images (Norris et al 2012;Ohnaka et al 2016), albeit this does not imply by necessity that the species responsible for the Al 2 O 3 emission in the SED are the same species that cause the stellar light to be scattered.…”

Section: Infrared Continuum Emissionsupporting

confidence: 56%

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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Section: Infrared Continuum Emissionsupporting

confidence: 56%

Study of the aluminium content in AGB winds using ALMA

Decin

Richards

Waters

et al. 2017

A&A

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“…In contrast to the models of Gobrecht et al (2016), Höfner et al (2016) present dynamical models in which the grain growth and the radiation pressure on dust grains are taken into account in a self-consistent manner, but the amount of the seed nuclei for dust formation is treated as a free parameter. They also included the formation of silicate mantle on Al 2 O 3 grains, which was previously proposed by, e.g., Kozasa & Sogawa (1997a, 1997b.…”

Section: Discussionmentioning

confidence: 99%

“…5, it is not possible to draw a definitive conclusion about the nature of the dust found in the extended region. The models of Höfner et al (2016) also suggest that the radiation pressure on Al 2 O 3 grains forming very close to the star is too low to drive mass loss, and Al 2 O 3 grains with silicate mantle forming farther out may be more efficient in driving stellar winds by the scattering of stellar photons. SPHERE-ZIMPOL observations with higher cadence will help us to better understand the dynamics of the dust clouds and clarify where the effective wind acceleration takes place.…”

Section: Discussionmentioning

confidence: 99%

“…The detection of large, transparent grains close to the star lends support to the scenario proposed by Höfner (2008) that the radiation pressure due to the scattering of stellar photonsinstead of absorption-by large, micron-sized grains can drive the mass loss in oxygen-rich AGB stars. Dust formation in Mira-type AGB stars is strongly affected by the periodic passage of shocks generated by large-amplitude stellar pulsation, as demonstrated by self-consistent hydrodynamical models for AGB stellar winds (e.g., Fleischer et al 1991Fleischer et al , 1992 for carbon-rich stars, Jeong et al 2003 andHöfner et al 2016 for oxygen-rich stars). The nucleation and growth process of dust grains from the gas is a complicated, nonequilibrium process, which is not yet completely understood (e.g., Gobrecht et al 2016;Gail et al 2016).…”

Section: Introductionmentioning

confidence: 99%

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Clumpy dust clouds and extended atmosphere of the AGB star W Hydrae revealed with VLT/SPHERE-ZIMPOL and VLTI/AMBER

Ohnaka

Weigelt

Hofmann

2016

A&A

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Aims. Our recent visible polarimetric images of the well-studied AGB star W Hya taken at pre-maximum light (phase 0.92) with VLT/SPHERE-ZIMPOL have revealed clumpy dust clouds close to the star at ∼2 R . We present second-epoch SPHERE-ZIMPOL observations of W Hya at minimum light (phase 0.54) as well as high-spectral resolution long-baseline interferometric observations with the AMBER instrument at the Very Large Telescope Interferometer (VLTI). Methods. We observed W Hya with VLT/SPHERE-ZIMPOL at three wavelengths in the 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 high-spatial resolution polarimetric images obtained with SPHERE-ZIMPOL have allowed us to detect clear time variations in the clumpy dust clouds as close as 34-50 mas (1.4-2.0 R ) to the star. We detected the formation of a new dust cloud as well as the disappearance of one of the dust clouds detected at the first epoch. The Hα and TiO emission extends to ∼150 mas (∼6 R ), and the Hα images obtained at two epochs reveal time variations. The degree of linear polarization measured at minimum light, which ranges from 13 to 18%, is higher than that observed at pre-maximum light. The power-law-type limb-darkened disk fit to the AMBER data in the continuum results in a limb-darkened disk diameter of 49.1 ± 1.5 mas and a limb-darkening parameter of 1.16±0.49, indicating that the atmosphere is more extended with weaker limb-darkening compared to pre-maximum light. Our Monte Carlo radiative transfer modeling shows that the second-epoch SPHERE-ZIMPOL data can be explained by a shell of 0.1 µm grains of Al 2 O 3 , Mg 2 SiO 4 , and MgSiO 3 with a 550 nm optical depth of 0.6±0.2 and an inner and outer radii of 1.3 R and 10±2 R , respectively. Our modeling suggests the predominance of small (0.1 µm) grains at minimum light, in marked contrast to the predominance of large (0.5 µm) grains at pre-maximum light. Conclusions. The variability phase dependence of the characteristic grain size implies that small grains might just have started to form at minimum light in the wake of a shock, while the pre-maximum light phase might have corresponded to the phase of efficient grain growth.

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“…This process creates a region with enhanced gas densities, referred to as warm molecular layer (WML, Tsuji et al 1997). Although models for the dynamical extended atmospheres of AGB stars have advanced significantly in recent years (see, e.g., Bladh et al 2015;Höfner et al 2016, and references therein), our understanding of this complex region is still incomplete, and calculating the mass-loss rate,Ṁ, of AGB stars from first principles is not possible presently. In this context, empirically characterising the properties of the gas from which dust forms is paramount.…”

Section: Introductionmentioning

confidence: 99%

ALMA observations of the vibrationally excited rotational CO transition v = 1, J = 3 − 2 towards five AGB stars

Khouri

Vlemmings

Ramstedt

et al. 2016

Monthly Notices of the Royal Astronomical Society: Letters

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ALMA observations of the vibrationally-excited rotational CO transition v = 1, J = 3 − 2 towards five AGB stars ABSTRACTWe report the serendipitous detection with ALMA of the vibrationally-excited purerotational CO transition v = 1, J = 3 − 2 towards five asymptotic giant branch (AGB) stars, o Cet, R Aqr, R Scl, W Aql, and π 1 Gru. The observed lines are formed in the poorly-understood region located between the stellar surface and the region where the wind starts, the so-called warm molecular layer. We successfully reproduce the observed lines profiles using a simple model. We constrain the extents, densities, and kinematics of the region where the lines are produced. R Aqr and R Scl show inverse P-Cygni line profiles which indicate infall of material onto the stars. The line profiles of o Cet and R Scl show variability. The serendipitous detection towards these five sources shows that vibrationally-excited rotational lines can be observed towards a large number of nearby AGB stars using ALMA. This opens a new possibility for the study of the innermost regions of AGB circumstellar envelopes.

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Dynamic atmospheres and winds of cool luminous giants

Cited by 101 publications

References 64 publications

Study of the aluminium content in AGB winds using ALMA

Study of the aluminium content in AGB winds using ALMA

Clumpy dust clouds and extended atmosphere of the AGB star W Hydrae revealed with VLT/SPHERE-ZIMPOL and VLTI/AMBER

ALMA observations of the vibrationally excited rotational CO transition v = 1, J = 3 − 2 towards five AGB stars

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