The Evolving Radio Photospheres of Long-Period Variable Stars

TL;DR

This study uses high-frequency radio observations to measure and analyze the changing shapes, sizes, and surface features of four long-period variable stars, revealing irregularities likely caused by convective flows.

Contribution

It provides new measurements of radio photospheres over time and introduces a sparse modeling imaging technique to detect surface asymmetries in long-period variable stars.

Findings

Stars show shape and size variability over years.

Brightness asymmetries suggest large-scale convective flows.

No evidence of binary companions affecting proper motion.

Abstract

Observations with the Karl G. Jansky Very Large Array at 46 GHz (7 mm) have been used to measure the size and shape of the radio photospheres of four long-period variable stars: R Leonis (R Leo), IRC+10216 (CW Leo), chi Cygni (chi Cyg), and W Hydrae (W Hya). The shapes of the stars range from nearly round to ellipticities of ~0.15. Comparisons with observations taken several years earlier show that the photospheric parameters (mean diameter, shape, and/or flux density) of each of the stars have changed over time. Evidence for brightness asymmetries and non-uniformities across the radio surfaces are also seen in the visibility domain and in images obtained using a sparse modeling image reconstruction technique. These trends may be explained as manifestations of large-scale irregular convective flows on the stellar surface, although effects from non-radial pulsations cannot be excluded. Our data also allow a new evaluation of the proper motion of IRC+10216. Our measurement is in agreement with previous values obtained from radio wavelength measurements, and we find no evidence of statistically significant astrometric perturbations from a binary companion.