New insights into the dust formation of oxygen-rich AGB stars

Abstract

Context. Asymptotic giant branch (AGB) stars are one of the major sources of dust in the universe. The formation of molecules and dust grains and their subsequent expulsion into the interstellar medium via strong stellar winds is under intense investigation. This is in particular true for oxygen-rich stars, for which the path of dust formation has remained unclear. Aims. We conducted spatially and spectrally resolved mid-infrared multi-epoch interferometric observations to investigate the dust formation process in the extended atmospheres of oxygen-rich AGB stars. Methods. We observed the Mira variable AGB stars S Ori, GX Mon, and R Cnc between February 2006 and March 2009 with the MIDI instrument at the VLT interferometer. We compared the data to radiative transfer models of the dust shells, where the central stellar intensity profiles were described by dust-free dynamic model atmospheres. We used Al 2 O 3 and warm silicate grains, following earlier studies in the literature. Results. Our S Ori and R Cnc data could be well described by an Al 2 O 3 dust shell alone, and our GX Mon data by a mix of an Al 2 O 3 and a silicate shell. The best-fit parameters for S Ori and R Cnc included photospheric angular diameters Θ Phot of 9.7 ± 1.0 mas and 12.3 ± 1.0 mas, optical depths τ V (Al 2 O 3 ) of 1.5 ± 0.5 and 1.35 ± 0.2, and inner radii R in of 1.9 ± 0.3 R Phot and 2.2 ± 0.3 R Phot , respectively. Best-fit parameters for GX Mon were Θ Phot = 8.7 ± 1.3 mas, τ V (Al 2 O 3 ) = 1.9 ± 0.6, R in (Al 2 O 3 ) = 2.1 ± 0.3 R Phot , τ V (silicate) = 3.2 ± 0.5, and R in (silicate) = 4.6 ± 0.2 R Phot . Our data did not show evidence of intra-cycle and cycle-to-cycle variability or of asymmetries within the error-bars and within the limits of our baseline and phase coverage. Conclusions. Our model fits constrain the chemical composition and the inner boundary radii of the dust shells, as well as the photospheric angular diameters. Our interferometric results are consistent with Al 2 O 3 grains condensing close to the stellar surface at about 2 stellar radii, co-located with the extended atmosphere and SiO maser emission, and warm silicate grains at larger distances of about 4-5 stellar radii. We verified that the number densities of aluminum can match that of the best-fit Al 2 O 3 dust shell near the inner dust radius in sufficiently extended atmospheres, confirming that Al 2 O 3 grains can be seed particles for the further dust condensation. Together with literature data of the mass-loss rates, our sample is consistent with a hypothesis that stars with low mass-loss rates form primarily dust that preserves the spectral properties of Al 2 O 3 , and stars with higher mass-loss rate form dust with properties of warm silicates.

Dust formation in the oxygen-rich AGB star IK Tauri

“…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). This latter interferometric study points to the formation of alumina dust at ∼2 R and of silicates at ∼5 R from the stellar surface for the few sources showing the typical alumina and silicate features in the mid-infrared.…”

What causes the large extensions of red supergiant atmospheres?

“…In the case of RSGs, the proposed radiative pressure on molecular lines may only levitate the molecular atmosphere up to radii where dust can form (as suggested by Josselin & Plez 2007), analogous to shock fronts for Miras, albeit not lifting the material outside the gravitational potential. However, the detailed dust formation, condensation sequence, and massloss mechanisms are also not yet fully understood in the case of oxygen-rich AGB stars (cf., e.g., Karovicova et al 2013, for a recent discussion). Recent indications based on a polarimetric aperture masking technique (Norris et al 2012) and on mid-infrared interferometry (Wittkowski et al 2007;Karovicova et al 2011Karovicova et al , 2013 point toward transparent dust grains forming already at relatively small radii of about 1.5 stellar radii.…”

“…However, the detailed dust formation, condensation sequence, and massloss mechanisms are also not yet fully understood in the case of oxygen-rich AGB stars (cf., e.g., Karovicova et al 2013, for a recent discussion). Recent indications based on a polarimetric aperture masking technique (Norris et al 2012) and on mid-infrared interferometry (Wittkowski et al 2007;Karovicova et al 2011Karovicova et al , 2013 point toward transparent dust grains forming already at relatively small radii of about 1.5 stellar radii. These may be grains of amorphous Al 2 O 3 and/or magnesiumrich iron-free ("forsterite") silicates, while iron-rich silicates form at larger radii.…”

Study of the aluminium content in AGB winds using ALMA

“…Observations indicate that low mass-loss rate stars form primarily dust that preserves the spectral properties of Al 2 O 3 , and stars with higher mass-loss rates form dust with properties of warm silicate oxides (Karovicova et al 2013) 1 . It is absolutely unknown if the silicates form via a heteromolecular homogeneous nucleation process consuming Mg, SiO, and H 2 O molecules (Goumans & Bromley 2012) or if the silicates form on top of the alumina grains (heterogeneous growth), hence gradually accelerating the wind.…”