3D Radiative Transfer Simulations of the Atmospheric Environment of the Carbon-rich AGB Star RU Vir

Cool evolved stars are the main contributors to the enrichment of the interstellar medium (ISM) through elements produced in their extended atmospheres. Less evolved red giant branch stars, such as K- and M-type giants show inhomogeneities studied in the ultraviolet, for example with the Hubble Space Telescope (e.g., Carpenter et al. 2018; Rau et al. 2018), while the most evolved among these stars are enshrouded in dust that peaks in the infrared.

In particular, Asymptotic Giant Branch (AGB) stars produce in their atmospheres molecules and dust essential to the Galactic ecology (Martin 2004). Indeed, as stars evolve on the AGB, they get colder and brighter, burning faster and faster in their nuclear core. Triggered by large amplitude pulsations and large-scale convective motions, atmospheric shock waves propagate outwards through the stellar atmosphere, lifting gas to distances where the conditions of high density and low temperature are enough for the formation and growth of dust grains. Then, the radiation pressure acting upon the dust grains provides enough momentum to the grains to accelerate them and drag along by collision of the gas, driving an outflow, or stellar wind, from the star into the ISM (e.g., Höfner & Olofsson 2018). Detached shells around AGB stars show observational evidence of the stellar mass-loss history in their circumstellar envelope (e.g., Brunner et al. 2019).

When experiencing the third dredge-up, AGB stars' chemistry can turn from oxygen-rich into carbon-rich (Iben & Renzini 1983). Carbon-rich AGB stars' extended atmospheres produce dust such as amorphous carbon (amC) and silicon carbide (SiC), the latter manifesting in the spectral energy distribution (SED) with a broad dust feature around ∼11.4 μm (e.g., Hackwell 1972; Treffers & Cohen 1974; Goebel 1980; see Figure 1). The precursors of those dust grains are believed to be molecules, such as C₂H₂ and HCN, which form further inside in the atmosphere (e.g., Massalkhi et al. 2018). Therefore, understanding cool evolved stars' atmospheres is fundamental to the comprehension of their mass loss, their evolution, and the enrichment of the ISM (e.g., Herwig 2005; Rau et al. 2019).

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With the newest technological advancements of the interferometric instruments on the biggest ground-based telescope, we are now on the brink of a new era of multi-messenger imaging capability, where the atmospheres of these stars will be imaged for the first time in a multi-wavelength fashion. It is therefore timely that 3D radiative transfer simulations of carbon-rich AGB stars are made. We report here a work in progress on modeling the atmosphere of RU Vir with the latest generation radiative transfer code RADMC-3D (Dullemond et al. 2012).

RADMC-3D is a freely available open source ³ software package for astrophysical radiative transfer calculations in arbitrary 1D, 2D or 3D geometries. We build a model made of gas and dust surrounding the star in a 3D cartesian grid. We used a stellar photospheric size of 500.9 R_⊙ based on findings from Rau et al. (2015), and a spherical symmetric dust distribution model that extends from ∼5 R_⋆ to ∼20 R_⋆, with an effective temperature of 2800 K (Rau et al. 2015).

We included the most prevalent species of gas and dust found in carbon-rich AGB stars: C₂H₂ and HCN for the gas; and for the dust amorphous carbon (amC, featureless) and silicon carbide (SiC, 11.4 μm feature—see also Figure 1). We calculated the opacities using the optical constants of silicon carbide from Mutschke et al. (1999), and of amorphous carbon from Zubko et al. (1996). The number of thermal photons used is 10⁶, and the resulting number of scattering photons per wavelength grid point is 10⁵. The gas and dust average temperature is 1500 K. The density distribution is of the type R⁻². We found a total dust mass lost of 5 · 10⁻⁵ M_⊙ yr⁻¹, and that the SED is well reproduced by the inclusion of such molecular and dust species (G. Rau et al. 2021, in preparation).

Figure 1 shows the simulated images of RU Vir's atmosphere with the configuration just described. The left-hand side of the figure highlights where in the atmosphere of RU Vir is mostly gas (L and M band) or dust (N band).

This work shows preliminary simulations and the capability of RADMC-3D for modeling AGB stars. Efforts are ongoing to extend this work to a more comprehensive modeling of RU Vir, and eventually other stars. This future work will encompass modeling of the spectral energy distribution, reproduction of different gas & dust temperatures, simulations of several slopes of the density function, and modeling of different geometries of the dust distribution (G. Rau et al. 2021, in preparation).

Comparison with future reconstructed interferometric image observations for carbon-rich AGB stars will provide further fundamental constraints on the atmospheres of these stars, essential in order to image the distribution of the molecular and dust environment in this carbon-rich AGB star, and constrain the zone and temperatures of molecule and dust formation. The European Southern Observatory (ESO)ʼs Very Large Interferometric Instrument (VLTI) Multi AperTure mid-Infrared SpectroScopic Experiment (MATISSE), working simultaneously in the L, M, and N bands, will be the ideal instrument to carry out such observations.

The material is based upon work supported by NASA under award number 80GSFC17M0002.

Software: RADMC-3D code (Dullemond et al. 2012).