Cool luminous stars: the hybrid nature of their infrared spectra -- Carbon, oxygen, and their isotopic abundances in 23 K - M giant stars

TL;DR

This study reveals that the infrared spectra of cool luminous stars are hybrid, comprising photospheric and warm molecular envelope components, which complicates abundance analysis and requires considering non-photospheric contamination.

Contribution

The paper empirically identifies a line strength limit for reliable abundance analysis and demonstrates the influence of warm molecular envelopes on spectral lines in K-M giants.

Findings

Weak lines (logW/nu < -4.75) are suitable for abundance analysis.

Strong lines are contaminated by warm molecular layers.

Infrared spectra of K-M giants are hybrid, involving photospheric and molecular envelope contributions.

Abstract

We identified a possible origin of the difficulty in abundance analysis of cool luminous stars. We found purely empirically that there is a limit of logW/nu = -4.75 (W: equivalent width, nu: wavenumber) above which the observed lines do not follow the line formation theory based on the classical micro-turbulent model and that the abundance analysis can be done only with the lines of logW/nu < -4.75. The C, O, and their isotopic abundances determined from such weak lines of CO and OH in 23 K - M giants are roughly consistent with the predictions of evolutionary models. However, the stronger lines of logW/nu > -4.75 cannot be analyzed at all by the classical line formation theory. From the behavior of these lines and considering other observations such as the detections of H2O lines, not only in the late M giants but also in the early M and K giants, we found that these lines are badly disturbed by the contamination from extra molecular layers. We already know that the very strong lines of logW/nu > -4.4 are contaminated by the contribution from the extra warm molecular layers, but we now show that such contamination should be prevailing not only in the strong low excitation lines but also in the intermediate-strength lines (-4.75 < logW/nu > -4.4) as well. The reason why these lines cannot be used for determining photospheric abundances is simply because they include the contamination of the non-photospheric origin. Instead they can be new proves of the warm molecular envelope for which little is known yet. An important conclusion is that the infrared spectra of K - M giants are a hybrid of at least two components originating in the photosphere and the warm molecular envelope. In the interpretation and analysis of the infrared spectra of cool luminous stars, it is essential to keep their hybrid naure in mind.