Ionization conditions in the expanding envelopes of O and B stars

The presence of O VI, N V, and Si IV ions in the expanding envelopes of early-type stars is correlated with their location in the Hertzsprung--Russell diagram. The O VI and N V lines are observed in the region to the left of the dividing line from (log T/sub eff/, M/sub bol/) = (4.50, -6.0) to (4.20, -9.0). The Si IV lines are observed in nearly all stars which show mass-loss effects, i.e., M/sub bol/ less than or equal to -6.0. The estimated ionization fractions of these ions in the enveloes were compared with calculations for the assumption that the temperature of the envelope is T/sub env/ = 0.80T/sub eff/ and that the radiative ionizations are due to the diluted photospheric flux. The observed degree of ionization is much higher than can be accounted for by radiative ionizations; thus, the envelopes are hot. The stars which have O VI and N V lines must have a /T/sub env/ approx. = (4 +- 2) x 105 K, and the stars which have Si IV lines but no O VI or N V lines must have T/sub env/ approx. = (7 +- 3) x 104 K. Some alternative possibilities to explain the high degree of ionization are discussed and rejected. The consequences of the hot envelopes for the four stellar wind models are discussed. 3 figures, 3 tables