New Measurements of the Radio Photosphere of Mira based on Data from the JVLA and ALMA

TL;DR

This study uses JVLA and ALMA data to measure the radio photosphere of Mira across multiple frequencies, revealing size, shape, and brightness variations, and detecting Mira B at radio wavelengths.

Contribution

First detailed multi-frequency radio measurements of Mira's photosphere, showing size decrease with frequency and surface non-uniformities, demonstrating the capabilities of JVLA and ALMA for stellar atmosphere studies.

Findings

Radio photosphere size decreases at higher frequencies.

The photosphere is slightly elongated with 10-20% flattening.

Mira B's radio-emitting surface radius is approximately 2.0×10^{13} cm.

Abstract

We present new measurements of the millimeter wavelength continuum emission from the long period variable Mira ($o$ Ceti) at frequencies of 46 GHz, 96 GHz, and 229 GHz ($\lambda$~7 mm, 3 mm, and 1 mm) based on observations obtained with the Jansky Very Large Array (JVLA) and the Atacama Large Millimeter/submillimeter Array (ALMA). The measured millimeter flux densities are consistent with a radio photosphere model derived from previous observations, where flux density, $S_{\nu}\propto\nu^{1.86}$. The stellar disk is resolved, and the measurements indicate a decrease in the size of the radio photosphere at higher frequencies, as expected if the opacity decreases at shorter wavelengths. The shape of the radio photosphere is found to be slightly elongated, with a flattening of ~10-20%. The data also reveal evidence for brightness non-uniformities on the surface of Mira at radio wavelengths. Mira's hot companion, Mira B was detected at all three observed wavelengths, and we measure a radius for its radio-emitting surface of $\approx2.0\times10^{13}$ cm. The data presented here highlight the power of the JVLA and ALMA for the study of the atmospheres of evolved stars.