Long Term Evolution of Surface Features on the Red Supergiant AZ Cyg

TL;DR

This study uses five years of interferometric observations to analyze the surface features of the red supergiant AZ Cyg, revealing complex, evolving surface structures consistent with 3D RHD models and emphasizing the need for short-term imaging to test convection theories.

Contribution

First long-term interferometric monitoring of AZ Cyg's surface features, providing insights into their evolution and comparison with 3D RHD models of red supergiants.

Findings

Surface features vary on timescales less than one year.

Large features persist for over a year, with sizes close to model predictions.

Interferometric imaging aligns with 3D RHD model predictions.

Abstract

We present H-band interferometric observations of the red supergiant (RSG) AZ Cyg made with the Michigan Infra-Red Combiner (MIRC) at the six-telescope Center for High Angular Resolution Astronomy (CHARA) Array. The observations span 5 years (2011-2016), offering insight into the short and long-term evolution of surface features on RSGs. Using a spectrum of AZ Cyg obtained with SpeX on the NASA InfraRed Telescope Facility (IRTF) and synthetic spectra calculated from spherical MARCS, spherical PHOENIX, and SAtlas model atmospheres, we derive $T_{\text{eff}}$ is between $3972 K$ and $4000 K$ and $\log~g$ between $-0.50$ and $0.00$, depending on the stellar model used. Using fits to the squared visibility and Gaia parallaxes we measure its average radius $R=911^{+57}_{-50}~R_{\odot}$. Reconstructions of the stellar surface using our model-independent imaging codes SQUEEZE and OITOOLS.jl show a complex surface with small bright features that appear to vary on a timescale of less than one year and larger features that persist for more than one year. 1D power spectra of these images suggest a characteristic size of $0.52-0.69~R_{\star}$ for the larger, long lived features. This is close to the values of $0.51-0.53~R_{\star}$ derived from 3D RHD models of stellar surfaces. We conclude that interferometric imaging of this star is in line with predictions of 3D RHD models but that short-term imaging is needed to more stringently test predictions of convection in RSGs.