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

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.

💡 Research Summary

This paper reports the first direct, wavelength‑dependent measurement of the apparent diameter of the Mira variable o Cet (Mira) using a combination of adaptive optics (AO) and differential spectral imaging (DSI) on the Subaru 8.2 m telescope. The authors employed the AO188 system to correct atmospheric turbulence in real time, achieving near‑diffraction‑limited resolution (~0.06 arcsec) in the near‑infrared K‑band. Simultaneously, the InfraRed Camera and Spectrograph (IRCS) was operated in echelle mode at a spectral resolving power of R≈4000, covering the 2.0–2.4 µm window where strong CO (2‑0) bandheads and numerous H₂O absorption lines are present.

The observing sequence consisted of a series of short exposures totalling about 30 minutes, yielding a signal‑to‑noise ratio above 150 per spectral channel. After standard reduction (dark subtraction, flat‑fielding, wavelength calibration), the authors extracted one‑dimensional spatial profiles for each wavelength bin. The key methodological step is the differential spectral imaging: for each molecular line the spatial profile at the line centre is directly compared to the profile of the adjacent continuum. By subtracting the two, systematic variations of the point‑spread function (PSF) caused by AO performance, seeing, or instrumental drift are largely cancelled. Gaussian fits to the residual profiles provide a precise measurement of the full‑width at half‑maximum (FWHM) as a function of wavelength.

The results reveal a striking increase in apparent stellar size at the wavelengths of CO and H₂O absorption. The measured FWHM in the line cores is 1.5–2.0 times larger than the continuum value, indicating that the star’s effective radius at those wavelengths is up to twice the photospheric radius. This enlargement is interpreted as the presence of optically thick molecular layers that extend above the continuum‑forming photosphere. The CO layer appears more compact, while the H₂O layer is more extended, consistent with theoretical dynamic atmosphere models of Mira variables that predict shock‑driven molecular shells at 1–2 R★ above the photosphere.

A crucial demonstration of the technique is its sensitivity: the differential approach can detect diameter differences that are significantly smaller than the intrinsic PSF width, something that conventional interferometry or imaging alone cannot achieve at comparable spectral resolution. The authors discuss the limitations of a single‑epoch observation—Mira variables undergo large radius changes over their pulsation cycle—and the residual systematic uncertainty (~5 %) due to AO correction variability. They propose future multi‑epoch campaigns, higher spectral resolution (R > 10 000), and extension to longer wavelengths (3–5 µm) to map the temperature and density structure of the molecular shells more accurately.

In conclusion, the combination of high‑order adaptive optics and differential spectral imaging provides a powerful new tool for probing the fine structure of stellar atmospheres. The successful detection of molecular‑layer‑induced diameter expansion in o Cet validates the method and opens the door to systematic studies of other pulsating giants, asymptotic‑giant‑branch stars, and possibly exoplanet host stars with extended atmospheres.