Low-Energy X-Rays Ruled Out as Interstellar Ionizing Mechanism Toward K3-50

We have detected the H94a and C94a recombination lines (X3.8 cm) originating in an H I region along the line of sight toward the nebula K3-50. The fraction of ionized atomic hydrogen is determined to be (6.8 I 5.8) x 1(h . In conjunction with a study of neutral hydrogen (X21 cm), this value implies an electron density of 1(h2 in the H I cloud. For this cloud, soft X-rays ( 100 eV) are ruled out as the interstellar heating mechanism.