The outer atmospheric layers of the early M dwarf Gliese 1

Using infrared and high-resolution optical observations of the M dwarf Gliese 1, we investigated the temperatures in the upper atmospheric layers of this star with low atmospheric activity. To fit the H and metal line profiles, the normal radiative equilibrium temperature decrease must be truncated at about log above which a steep chromospheric (T≈ 4900 K) rise must be imposed. Unfortunately, the position of the onset of the chromosphere depends to some extent on the temperature distribution in the inner parts of the photosphere. The chromosphere is just not sufficiently optically thick to be seen in the infrared up to ≈30 μm. The persistent strength of the TiO bands leads us to check for indications of surface inhomogeneities with negative results.