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

Abstract

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. From the observed intensities we infer that the emitting regions have temperatures 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 location. The total mass of gas being cooled through these atomic fine structure lines is of the order of 10(-4) M_sun. 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]. Based on observations with ISO, an ESA project with instruments funded by ESA Member States (especially the PI countries: France, Germany, the Netherlands and the United Kingdom) with the participation of ISAS and NASA. The SWS is a joint project of SRON and MPE.