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N.R. Hausen, R.J. Reynolds, S.L. Tufte, L.M. Haffner (University of Wisconsin--Madison)
Observations of absorption and emission lines yield insight into the properties of interstellar gas. Spitzer and Fitzpatrick (1993) conducted a detailed investigation of absorption lines along the line of sight toward the O9.5 star HD93521. The development of the Wisconsin H\alpha Mapper (WHAM) facility has now made it possible to obtain data for faint optical emission lines as well. In this paper, we present WHAM measurements of H\alpha emission line profiles toward HD93521 (l = 183.1\circ, b = +62.2\circ) and the direction l = 148.5\circ, b = +53.0\circ. The latter direction is in the region of exceptionally low HI column density and weak interstellar H\alpha intensity known as the Lockman Window.
Our spectra from the direction of HD93521 reveal three H\alpha emission components. There is a ``slow'' component at --3 ± 4 km s-1 with respect to the local standard of rest. This component has a width of 28 ± 8 km s-1 and an intensity of 0.22 ± 0.06 R (1 R = 106/4\pi photons cm-2 s-1 sr-1). A ``fast'' component occurs at --49 ± 3 km s-1, has a width of 43 ± 5 km s-1 and an intensity of 0.16 ± 0.03 R. A fainter, higher-velocity emission component is also clearly present in our data. This component is at --90 ± 5 km s-1, has a width of 25 ± 5 km s-1 and an intensity of 0.023 ± 0.007 R.
We have also detected H\alpha emission from the Lockman Window. This emission is present at +4 ± 5 km s-1 and has a width of 20 ± 5 km s-1 and an intensity of 0.20 ± 0.06 R. It has been our careful analysis of the Lockman Window direction that has enabled us to measure reliably the intensity of extremely faint emission lines such as those toward HD93521. By using the Lockman direction as a ``standard,'' we are able to effectively remove contamination from weak atmospheric lines in our data. This technique will in addition play an important role in the data reduction for the recently-completed WHAM H\alpha sky survey.
This work has been supported by the National Science Foundation through grant AST 96-19424, by the University of Wisconsin Graduate School, and by an Excellence in Education Foundation Barry M. Goldwater Scholarship.
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