ISO-SWS Observations of CO_2 and H_2O in R Cassiopeiae

Abstract

We present ISO-SWS spectra of the O-rich Mira variable R Cas, showing CO_2 in absorption and emission, and H_2O in absorption. The CO_2 absorption feature is the 01¹0-00⁰0 ro-vibrational band at 14.97 μm. The emission features are the 10⁰0-01¹0 and 11¹0-02²0 ro-vibrational transitions at 13.87 and 13.48 μm respectively. The water absorption spectrum shows the nu_1 and nu_3 ro-vibrational bands in the 2.75-3 μm region. Using LTE models, we derive physical parameters for the features. We find the CO_2 emission temperature to be ~ 1100 K. We discuss the nature of the CO_2 feature at 15 μm and show that it can be modeled as an emission/absorption band by deviating from thermal equilibrium for the population of the 01¹0 vibrational level. The H_2O absorption spectrum is shown to arise from gas at different temperatures, but can be fit reasonably well with two components at T=950 K and T=250 K. The CO_2 emission and hot H_2O absorption temperatures are similar, suggesting that these features probe the same region of the inner envelope. We discuss the inner envelope chemistry using molecular equilibrium calculations and recent modeling work by Duari et al. (1999), and find our observations consistent with the results. We also report the detection of the CO_2 01¹0-00⁰0 ro-vibrational band in absorption towards another oxygen-rich Mira, IRC+10011. Based on observations with ISO, an ESA project with instruments funded by ESA Member States (especially the PI countries: France, Germany, the Netherlands and the United Kingdom) with the participation of ISAS and NASA. The SWS is a joint project of SRON and MPE.