The Effects of Chromospheric Radiation on the Circumstellar Chemistry of Evolved Stars

The physical and chemical properties of the circumstellar envelopes of evolved stars are strongly affected by the interstellar radiation field. Other sources of UV radiation should be similarly effective, and some examples are nearby stars (including companions), chromospheres, and the central stars of planetary nebulae. We consider the particular case of Alpha Ori, which has a chromosphere and an extended CSE with a small dust to gas ratio. Its properties are dominated by the chromospheric and interstellar radiation fields. The most common species are neutral atoms and first ions, and the electron fraction is high throughout the entire CSE, i.e. at least 10−4. The abundances of neutrals peak in the outer CSE close to where the chromospheric and interstellar radiation fields are equal. An important application is KI, whose density has been measured by scattering. The theory predicts that the slope of the KI density should change from about −1.5 to −3.5 in the outer envelope, the exact values being determined by the temperature distribution. The mass loss rate implied by the KI density is of the order of 4×10−6 M⊙ yr−1.