Simulated movies of the gas motions in the radio photosphere of a red
supergiant (RSG) star as observed with the ngVLA Main Array at 46 GHz.
The angular resolution is ~1.5 mas, corresponding to roughly 0.02
stellar radii. Each movie consists of 32 frames spanning ~2 years,
assuming observations every ~3 weeks. The adopted distance was 222 pc,
comparable to the nearby RSG star Betelgeuse.

The RSG phase marks a short-lived period near the end of the lives of massive
stars (>10 solar masses) during which the outer atmosphere becomes enormously
extended (up to 1000 solar radii), as well as dynamic, and unstable, and the
star begins shedding mass at a high rate through a cool, dense wind.
This mass loss has a profound effect the evolutionary path of the star
(including whether or not the star will eventually explode as a supernova),
but its driving mechanism is poorly understood. Observations of radio
continuum emission that arises just inside the wind launch region can provide
vital clues into the mass loss process.

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 RSG star atmosphere from Chiavassa et al. (2009). The
Chiavassa 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 using the SMILI
package of Akiyama et al. (2019) convolved with a half size of the uniform-weighted
beam. 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 A. Chiavassa.
Reference: Akiyama & Matthews, 2019, Next Generation Very Large Array Memo Series, No. 66