Evaluating the Origin of Water Vapour in Giant Stars

Abstract: The advent of infrared astronomy, unlike optical astronomy that was developed over centuries, was delayed until the space age due to our own atmosphere acting as an efficient shield towards this radiation. With a lot of molecules (including water) emitting in the infrared region, it was first in the late 1990s that water no longer was deemed special and was observed in many cooler (K-M) giant stars. Although it was theoretically predicted that stars of these temperatures would form water, it failed to account for water in the warmer K-type stars, thus challenging our knowledge of the atmosphere of these red giants. In time, two scenarios were developed in order to explain the discrepancies between the theoretical and observed spectra. The first being the addition of a warm molecular envelope known as MOLsphere, and the second being a modified temperature profile of the star. The aim of this project was to shed light and put constraints on these scenarios. With the use of synthetic spectra based upon model atmospheres from the MARCS code and high resolution spectra from the instrument TEXES, we had a lot of well resolved spectral lines across many spectral types, presented for the first time in a collected overview fashion (unlike previous work of scattered single observations). Our results show no emission features, but stronger absorption lines for all spectral types than models suggest. Furthermore, we derive the need of a temperature decrease of approx 500 K in the outer regions of the photosphere and couple it to NLTE-cooling, a physical process that can drive the temperature drop in the outskirts of the stellar atmosphere.