An extensive grid of DARWIN models for M-type AGB stars

Abstract

Context. The stellar winds of asymptotic giant branch (AGB) stars are commonly attributed to radiation pressure on dust grains, formed in the wake of shock waves that arise in the stellar atmospheres. The mass loss due to these outflows is substantial, and modelling the dynamical properties of the winds is essential both for studies of individual stars and for understanding the evolution of stellar populations with low to intermediate mass. Aims. The purpose of this work is to present an extensive grid of dynamical atmosphere and wind models for M-type AGB stars, covering a wide range of relevant stellar parameters. Methods. We used the DARWIN code, which includes frequency-dependent radiation-hydrodynamics and a time-dependent description of dust condensation and evaporation, to simulate the dynamical atmosphere. The wind-driving mechanism is photon scattering on submicron-sized Mg2SiO4 grains. The grid consists of ~4000 models, with luminosities from L⋆ = 890 L⊙ to L⋆ = 40 000 L⊙ and effective temperatures from 2200 to 3400 K. For the first time different current stellar masses are explored with M-type DARWIN models, ranging from 0.75 M⊙ to 3 M⊙. The modelling results are radial atmospheric structures, dynamical properties such as mass-loss rates and wind velocities, and dust properties (e.g. grain sizes, dust-to-gas ratios, and degree of condensed Si). Results. We find that the mass-loss rates of the models correlate strongly with luminosity. They also correlate with the ratio L*∕M*: increasing L*∕M* by an order of magnitude increases the mass-loss rates by about three orders of magnitude, which may naturally create a superwind regime in evolution models. There is, however, no discernible trend of mass-loss rate with effective temperature, in contrast to what is found for C-type AGB stars. We also find that the mass-loss rates level off at luminosities higher than ~14 000 L⊙, and consequently at pulsation periods longer than ~800 days. The final grain radii range from 0.25 to 0.6 μm. The amount of condensed Si is typically between 10 and 40%, with gas-to-dust mass ratios between 500 and 4000.

Gas and dust from metal-rich AGB stars

“…The results in terms of dust production presented here must be taken with some caution, particularly for what attains the production of carbonaceous species. Indeed the present findings are based on the rate of mass loss calculated by means of the Wachter et al (2002Wachter et al ( , 2008 works, which, as discussed in section 2, neglects any role of the carbon excess: this assumption may lead to an overestimation of theṀ's found here (Bladh et al 2019b), considering that in the few models that reach the C-star stage the carbon excess is small, owing to the large oxygen content of the surface regions of the star. It goes without saying that the growth of dust particles, particularly of the carbon grains, might be overestimated as well.…”

Chemical equilibrium in AGB atmospheres: successes, failures, and prospects for small molecules, clusters, and condensates

“…Effective temperatures of AGB stars are usually in the range 2000-3000 K (Bergeat et al 2001). Here we adopt an effective temperature of 2500 K. The temperature gradient throughout the extended atmosphere can be usually well accounted for using a power law, that is, T (r) ∝ r −α , with values of α in the range 0.5-Article number, page 3 of 63 Figure 1 of Bladh et al 2019). The green and blue dashed curves show the profiles resulting from a 3D model of an AGB atmosphere with and without radiation pressure on dust (models st28gm06n06 and st28gm06n26 from Freytag et al 2017, where profiles are averaged over spherical shells and time).…”

“…For example, we compare in the lower panel of Fig. 1 the various dashed curves, which correspond to a 1D model by Bladh et al (2019) at two different phases and to two 3D models from Freytag et al (2017). High angular resolution observations from radio to infrared wavelengths can provide constraints on the densities in the extended atmosphere of AGB stars (see the thin solid curves in the lower panel of Fig.…”

DEATHSTAR: Nearby AGB stars with the Atacama Compact Array

“…Mass-loss rates,Ṁ, on the AGB are found to range from ∼10 −8 -10 −4 M yr −1 (e.g., Höfner & Olofsson 2018, and references therein). It is challenging to find reliable observational methods to measure mass-loss rates covering this wide range (Ramstedt et al 2008), but it is crucial since the measurements will provide key constraints for theoretical models (e.g., Eriksson et al 2014;Marigo et al 2016;Bladh et al 2019). Wind formation is studied using dynamical wind models (e.g., Höfner 2008;Eriksson et al 2014;Bladh et al 2015;Höfner et al 2016) with the ultimate goal of developing a predictive theory of AGB mass loss.…”

“…However, with the derivedṀ uncertainties reaching as high as a factor of three (within the adopted spherically symmetric model, Ramstedt et al 2008), the dynamical wind models cannot be sufficiently well constrained (e.g., Fig. 7 in the recent paper by Bladh et al 2019 shows how the wind parameters vary with model input parameters).…”