Neutral carbon in the circumstellar envelope of alpha Orionis

We report the detection of emission in the 609 micrometers 3P1 to 3P0 fine-structure line of neutral atomic carbon in the circumstellar envelope of alpha Ori, using the 10.4 m telescope of the Caltech Submillimeter Observatory. The C I line is centered at a radial velocity of +3 km/s, with an intensity of 200 mK, and a width of 30 km/s, characteristic of the expanding circumstellar envelope. The observations show that C I is an important component of the circumstellar gas. The number of C I atoms in the envelope exceeds the number of CO molecules by a factor of approximately 5, and the C I column density is large enough that significant absorption of the stellar and chromospheric spectrum can be expected at wavelengths shorter than the C I ionization edge at 1100 A. The mass-loss rate determined from our observations using the photospheric carbon abundance and assuming full recombination of carbon in the transition region is 2 x 10-6 solar mass/yr. This is in good agreement with the value determined from H I 21 cm observations and confirms that C I is a major carrier of carbon between the chromosphere and the outer envelope.