Simulated movies of the gas motions in the radio photosphere of an asymptotic
giant branch (AGB) star as observed with the ngVLA Main Array at 46 GHz. The
angular resolution is ~1.5 mas, corresponding to roughly 0.04 stellar radii.
Each movie covers a single 1.3-year pulsation period for a 1 solar mass model
AGB star, assuming observations every 2-3 weeks. The adopted distance was 150 pc.

AGB star atmospheres are highly dynamic owing to a combination of radial
pulsations, shocks, and large-scale convective processes. These stars also shed
large amounts of matter through cool, dense winds. Radio waves are emitted from
a portion of their atmospheres that lie at a radius of roughly twice the
visible photosphere, and just interior to the wind launch region.

The adopted brightness distribution for the radio photosphere model (the
"ground truth model" shown in the left hand frame) was based on a 3D
hydrodynamic model of an AGB star atmosphere from Freytag et al. (2017). The
Freytag et al. model is not specific to millimeter wavelengths, but was adapted
here for illustrative purposes. The second frame shows the ground truth model
blurred with a Gaussian kernel.

The two frames on the right show image reconstructions of the model based
on simulated ngVLA Main Array observations (baselines up to ~1000 km).
The integration time was 2 hours per frame and the assumed bandwidth was 10 GHz
in dual polarizations. The second frame from the right shows a multi-scale CLEAN
reconstruction at uniform weighting, while the frame on the right shows
the result of a regularized maximum likelihood reconstruction convolved
with a half size of the uniform-weighted beam using the SMILI package of
Akiyama et al. (2019). While both methods do well at reproducing many of
the key features of the model, the SMILI version achieves superior angular
resolution.

These results underscore that the exquisite sensitivity and resolution of the
ngVLA will enable studies of the physical properties and kinematics of the
atmospheres of evolved stars in unprecedented detail.

Credit: K. Akiyama and L. D. Matthews, based on models adapted from B. Freytag.
Reference: Akiyama & Matthews, 2019, Next Generation Very Large Array Memo Series, No. 66