The outer atmosphere of the M supergiant Betelgeuse is puzzling .
Published observations of different kinds have shed light on different aspects of the atmosphere , but no unified picture has emerged .
They have shown , for example , evidence of a water envelope ( MOLsphere ) that in some studies is found to be optically thick in the mid-infrared .
In this paper , we present high-resolution , mid-infrared spectra of Betelgeuse recorded with the TEXES spectrograph .
The spectra clearly show absorption features of water vapor and OH .
We show that a spectrum based on a spherical , hydrostatic model photosphere with T _ { \mathrm { eff } } = 3600 K , an effective temperature often assumed for Betelgeuse , fails to model the observed lines .
Furthermore , we show that published MOLspheres scenarios are unable to explain our data .
However , we are able to model the observed spectrum reasonably well by adopting a cooler outer photospheric structure corresponding to T _ { \mathrm { mod } } = 3250 Â K. The success of this model may indicate the observed mid-infrared lines are formed in cool photospheric surface regions .
Given the uncertainties of the temperature structure and the likely presence of inhomogeneities , we can not rule out the possibility that our spectrum could be mostly photospheric , albeit non-classical .
Our data put new , strong constraints on atmospheric models of Betelgeuse and we conclude that continued investigation requires consideration of non-classical model photospheres as well as possible effects of a MOLsphere .
We show that the mid-infrared water-vapor features have great diagnostic value for the environments of K and M ( super- ) giant star atmospheres .