Systematic trend of water vapour absorption in red giant

Context. The structures of the outer atmospheres of red giants are very complex. Recent interpretations of a range of different observations have led to contradictory views of these regions. It is, however, clear that classical model photospheres are inadequate to describe the nature of the outer atmospheres. The notion of large optically thick molecular spheres around the stars (MOLspheres) has been invoked in order to explain spectro-interferometric observations, as well as low- and high-resolution spectra. On the other hand high-resolution spectra in the mid-IR do not easily fit into this picture. On the contrary, they rule out any large sphere of water vapour in LTE surrounding red giants. Aims. In order to approach a unified scenario for these outer regions of red giants, more empirical evidence from different diagnostics are needed. Our aim here is to investigate high-resolution, mid-infrared spectra for a range of red giants, spanning spectral types from early-K to mid M. We want to study how the pure rotational lines of water vapour change with effective temperature, and whether we can find common properties that can put new constraints on the modelling of these regions, so that we thereby can gain new insights. Methods. We have recorded mid-infrared spectra at 12.2 − 12.4 µm at high spectral resolution of 10 well-studied bright red giants, with TEXES mounted on the IRTF on Mauna Kea. These stars span effective temperatures from 3450 to 4850 K. Results. We find that all red giants in our study cooler than 4300 K, spanning a large range of effective temperatures (down to 345