Dust formation in the oxygen-rich AGB star IK Tauri

Abstract

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.

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

“…9). Therefore, the formation of grains in the extended region found in 2016 may indicate the formation of silicate mantle on Al 2 O 3 grains, instead of pure silicate as modeled by Gobrecht et al (2016). However, given that we cannot differentiate between the models with Al 2 O 3 , Mg 2 SiO 4 , and MgSiO 3 as mentioned in Sect.…”

“…Therefore, our modeling at two epochs suggests the predominance of grains as large as 0.5 µm at pre-maximum light and the predominance of 0.1 µm grains at minimum light. Gobrecht et al (2016) present non-equilibrium gas-dust chemistry models for the Mira star IK Tau. Given its larger variability amplitude and its higher mass-loss rate than that of W Hya, IK Tau is considered to be more evolved than W Hya.…”

“…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.…”

“…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). Therefore, studying time variations in the physical properties of the dust formation region is crucial for understanding the grain nucleation and growth process.…”

Study of the aluminium content in AGB winds using ALMA

“…The angular size was taken as the distance out to 2σ rms . The measurements were also used to compute the azimuthally averaged flux densities to compare with 1D models solving the non-LTE radiative transfer equation for the specific molecule and with models solving the chemical network by Gobrecht et al (2016) in Sect. 3.…”