Direct Observation of the Extended Molecular Atmosphere of o Cet by Differential Spectral Imaging with an Adaptive Optics System

TL;DR

This study uses adaptive optics and differential spectral imaging to directly measure the extended molecular atmosphere of o Cet, revealing that molecular layers cause significant size variations at specific wavelengths.

Contribution

First direct differential spectroscopic imaging of stellar extension resolving molecular layers with high spectral resolution.

Findings

Molecular layers cause the star's apparent size to double at certain wavelengths.

The technique detects size differences smaller than the point spread function.

Molecular atmospheres significantly influence stellar diameter measurements.

Abstract

We present new measurements of the diameter of o Cet (Mira) as a function of wavelength in the 2.2 micron atmospheric window using the adaptive optics system and the infrared camera and spectrograph mounted on the Subaru Telescope. We found that the angular size of the star at the wavelengths of CO and H2O absorption lines were up to twice as large as the continuum photosphere. This size difference is attributable to the optically thick CO and H2O molecular layers surrounding the photosphere. This measurement is the first direct differential spectroscopic imaging of stellar extension that resolves individual molecular lines with high spectral-resolution observations. This observation technique is extremely sensitive to differences in spatial profiles at different wavelengths; we show that a difference in diameter much smaller than the point spread function can be measured.