Exploring wind-driving dust species in cool luminous giants

Abstract

Context. Stellar winds observed in asymptotic giant branch (AGB) stars are usually attributed to a combination of stellar pulsations and radiation pressure on dust. Shock waves triggered by pulsations propagate through the atmosphere, compressing the gas and lifting it to cooler regions which creates favourable conditions for grain growth. If sufficient radiative acceleration is exerted on the newly formed grains through absorption or scattering of stellar photons, an outflow can be triggered. Strong candidates for wind-driving dust species in M-type AGB stars are magnesium silicates (Mg 2 SiO 4 and MgSiO 3 ). Such grains can form close to the stellar surface, they consist of abundant materials and, if they grow to sizes comparable to the wavelength of the stellar flux maximum, they experience strong acceleration by photon scattering. Aims. The purpose of this study is to investigate if photon scattering on Mg 2 SiO 4 grains can produce realistic outflows for a wide range of stellar parameters in M-type AGB stars. Methods. We use a frequency-dependent radiation-hydrodynamics code with a detailed description for the growth of Mg 2 SiO 4 grains to calculate the first extensive set of time-dependent wind models for M-type AGB stars. This set includes 139 solar-mass models, with three different luminosities (5000 L , 7000 L , and 10 000 L ) and effective temperatures ranging from 2600 K to 3200 K. The resulting wind properties, visual and near-IR photometry and mid-IR spectra are compared with observations. Results. We show that the models can produce outflows for a wide range of stellar parameters. We also demonstrate that they reproduce observed mass-loss rates and wind velocities, as well as visual and near-IR photometry. However, the current models do not show the characteristic silicate features at 10 and 18 μm as a result of the cool temperature of Mg 2 SiO 4 grains in the wind. Including a small amount of Fe in the grains further out in the circumstellar envelope will increase the grain temperature and result in pronounced silicate features, without significantly affecting the photometry in the visual and near-IR wavelength regions. Conclusions. Outflows driven by photon scattering on Mg 2 SiO 4 grains are a viable wind scenario for M-type AGB stars, given the success of the current models in reproducing observed mass-loss rates, wind velocities, and photometry. Both synthetic and observed photometry suggest that the dusty envelopes of M-type AGB stars are quite transparent at visual and near-IR wavelengths, otherwise the variations in visual flux would not be dominated by molecular features.

Connecting the evolution of thermally pulsing asymptotic giant branch stars to the chemistry in their circumstellar envelopes – I. Hydrogen cyanide

“…Empirically, we find that the relation r cond /R = 0.24 exp(1.87/M ) + 0.027M + 2.68 (where M is current stellar mass) leads to density and temperature profiles suitable for HCN chemistry in O-rich models. We get typical values r cond /R 3 − 4 in line with interferometric observations (Wittkowski et al 2007;Karovicova et al 2011;Norris et al 2012), and theoretical studies (Bladh et al 2013(Bladh et al , 2015. We plan to improve the above prescription on a more physical base in follow-up studies.…”

Dynamic atmospheres and winds of cool luminous giants

“…Norris et al 2012;Scicluna et al 2015;Ohnaka et al 2016). Furthermore, detailed atmosphere and wind models based on this mechanism show good agreement with observations regarding mass loss rates and wind velocities, as well as visual and near-IR colors, and their variation with pulsation phase (Bladh et al 2013(Bladh et al , 2015.…”

“…In a series of papers based on detailed atmosphere and wind models, we have therefore investigated an alternative mechanism, i.e., wind-driving by scattering of stellar photons on Fe-free silicate grains (Höfner 2008;Bladh et al 2013Bladh et al , 2015. This scenario requires grain sizes of about 0.1-1 micron, in order to make the dust particles efficient at scattering radiation in the near-IR wavelength region where the stellar flux peaks.…”

Dust formation in the oxygen-rich AGB star IK Tauri

“…The quantity may vary over orders of magnitude depending on the grain material, with value range of ∼10 −4 -1 (e.g., Leitch-Devlin & Williams 1985;Brune et al 1993;Nagahara & Ozawa 1996). Existing studies on dust A6, page 6 of 15 formation in circumstellar environments often assume a growth process through surface deposition only, and an S value equal to unity or in the range 0.1-1 (e.g., Gail & Sedlmayr 1999;Bladh et al 2015). In view of these uncertainties, we prefer to not consider surface deposition in the formalism at this stage of our study, and discuss the potential effects of accretion on the present results in Sect.…”