The nature of the circumstellar CO_2 emission from M giants

TL;DR

This paper analyzes CO2 emission features in M giants' circumstellar environments, proposing a two-layer model to explain observed spectral characteristics and temperature estimations from band widths.

Contribution

It introduces a two-layer circumstellar model for CO2 emission in M giants, explaining spectral features and temperature estimates from band widths.

Findings

CO2 features originate from warm, dense inner layers and cold, extended outer layers.

Spectral band widths can be used to estimate temperatures despite non-LTE conditions.

Different bands indicate varying physical conditions in circumstellar regions.

Abstract

The 13-16 um region observed by the Infrared Space Observatory (ISO) of several ABG stars are discussed. We present and analyse spectra of seven M giants which show carbon dioxide features. To explain the features of the bands, we suggest they originate from two different circumstellar layers, one being a warm and high density layer close to the star, possibly making the 15 um band optically thick, and the other being a large, cold and optically thin layer extending far out in the wind. This could explain the difference in temperatures of the different bands found in the analysis of the spectra and the number of molecules needed for the emission. It is demonstrated that in spite of the bands probably not being formed in vibrational LTE, the temperatures can be estimated from the widths of the bands.