Near-infrared spectro-interferometry of Mira variables and comparisons to 1D dynamic model atmospheres and 3D convection simulations

Abstract

Aims. We aim at comparing spectro-interferometric observations of Mira variable asymptotic giant branch (AGB) stars with the latest 1D dynamic model atmospheres based on self-excited pulsation models (CODEX models) and with 3D dynamic model atmospheres including pulsation and convection (CO5BOLD models) to better understand the processes that extend the molecular atmosphere to radii where dust can form. Methods. We obtained a total of 20 near-infrared K-band spectro-interferometric snapshot observations of the Mira variables o Cet, R Leo, R Aqr, X Hya, W Vel, and R Cnc with a spectral resolution of about 1500. We compared observed flux and visibility spectra with predictions by CODEX 1D dynamic model atmospheres and with azimuthally averaged intensities based on CO5BOLD 3D dynamic model atmospheres. Results. Our visibility data confirm the presence of spatially extended molecular atmospheres located above the continuum radii with large-scale inhomogeneities or clumps that contribute a few percent of the total flux. The detailed structure of the inhomogeneities or clumps show a variability on time scales of 3 months and above. Both modeling attempts provided satisfactory fits to our data. In particular, they are both consistent with the observed decrease in the visibility function at molecular bands of water vapor and CO, indicating a spatially extended molecular atmosphere. Observational variability phases are mostly consistent with those of the best-fit CODEX models, except for near-maximum phases, where data are better described by near-minimum models. Rosseland angular diameters derived from the model fits are broadly consistent between those based on the 1D and the 3D models and with earlier observations. We derived fundamental parameters including absolute radii, effective temperatures, and luminosities for our sources. Conclusions. Our results provide a first observational support for theoretical results that shocks induced by convection and pulsation in the 3D CO5BOLD models of AGB stars are roughly spherically expanding and of similar nature to those of self-excited pulsations in 1D CODEX models. Unlike for red supergiants, the pulsation-and shock-induced dynamics can levitate the molecular atmospheres of Mira variables to extensions that are consistent with observations.

Mass loss of stars on the asymptotic giant branch

“…12, middle). These large-scale, outwards-propagating shock fronts probably create local conditions which are comparable to those induced by spherical shock waves resulting from purely radial pulsation (for a discussion see, e.g., Wittkowski et al 2016;Freytag et al 2017). This may explain why spherical dust-driven wind models give mass-loss rates, spectra and photometric variations that agree well with observations, as will be discussed in Sect.…”

Dawes Review 6: The Impact of Companions on Stellar Evolution

“…2D, hydrostatic stellar evolution is also starting to be explored as a natural stepping stone to full 3D modelling (Espinosa Lara & Rieutord, 2013). Convection in giant stars was studied using 3D models by Meakin & Arnett (2007) and more recently by Chiavassa et al (2011) with CO 5 BOLD (Freytag, Steffen, & Dorch, 2002) at relatively low resolution, but high enough for a meaningful comparison with VLTI observations of Wittkowski et al (2016). This revealed the size of the modelled convection plumes to be approximately correct.…”

Dust in brown dwarfs and extrasolar planets

“…. Of particular interest is the question if cloud particle in a carbon-rich atmosphere will remain homogeneous as commonly assumed for AGB star wind modeling (Fleischer et al 1992;Mattsson et al 2010;Wittkowski et al 2016), or if they will be composed of some mix of materials similar to the chemically more complex oxygen-rich case.…”