The photodissociation of circumstellar CO

The spatial distribution of the molecule CO in the internal layer of the circumstellar shell of the cool carbon-rich star TX Psc is presented. The main source of the photodissociation of CO in the internal layer of the shell is chromospheric UV radiation. The abundance of CO is determined by the photodissociation through absorption of UV photons by line processes in the wavelength range of 1099 to 913.5 Å. We have used a weak extended and a strong extended chromosphere. While in a homogeneous shell, the molecule CO is not photodissociated, in a clumpy envelope, it is destroyed by the chromospheric UV radiation field, depending of the degree of clumpiness of the shell.