Interstellar Hα Line Profiles toward HD 93521 and the Lockman Window

摘要

We have used the Wisconsin Hα Mapper (WHAM) facility to measure the interstellar Hα emission toward the high Galactic latitude O star HD 93521 (l = 1831, b = +622). Three emission components were detected along this line of sight. These components have radial velocities of -10, -51, and -90 km s-1 with respect to the local standard of rest (LSR) and have Hα intensities of 0.20, 0.15, and 0.023 R, respectively, corresponding to emission measures of 0.55, 0.42, and 0.06 cm-6 pc. We have also measured the Hα emission toward the direction l = 1485, b = +530, which lies in the region of exceptionally low H?I column density known as the Lockman Window. An emission component is detected in this direction at -1 km s-1 (LSR), with an intensity of 0.20 R (0.55 cm-6 pc). In addition, we studied the direction l = 1635, b = +535. No Galactic emission was detected along this line of sight, and upper limits on the possible intensity of Galactic emission toward this direction are 0.11 R at the LSR and 0.06 R at -50 km s-1. As part of the process of separating the interstellar from the terrestrial emission, we also detected and characterized 12 faint (~0.03-0.15 R), unidentified atmospheric lines present in WHAM Hα spectra. Finally, we have used WHAM to obtain [O?I] λ6300 spectra along the line of sight toward HD 93521. We do not conclusively detect interstellar [O?I] emission toward the star, but place an upper limit of 0.060 R on the [O?I] intensity of the intermediate-velocity (-51 km s-1) component. If the temperature of the gas is 10,000 K, this limit implies that, within the Hα-emitting region, the hydrogen ionization fraction n(H+)(H total) 0.6.