Atomic fine-structure lines in the ISO-SWS spectra of the supergiants alpha Orionis and alpha Scorpii

We report the detection of infrared fine-structure lines of [Fe II] and [Si II] in the ISO-SWS spectra of two supergiants, alpha Ori and alpha Sco. Fr om the observed intensities we infer that the emitting regions have tempera tures in the range 1200-1800 K and are located within about 20 R* from the star. This is interior to the region where dust is supposed to condense in the expanding envelope so that Fe and Si are not yet incorporated into the dust. The gas density required to thermalize these lines is of order 10(6) cm(-3) consistent with the expected density in the outflow at the adopted l ocation. The total mass of gas being cooled through these atomic fine struc ture lines is of the order of 10(-4) Mo. Our calculation shows that cooling through the [O I] 63 mu m line extends over a larger radius than for both [Fe II] and [Si II].