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THE EFFECT OF TELLURIC ABSORPTION ON THE INTENSITY OF HE ILAMBDA 10830

The observed intensity of He I 10830 in planetary nebulae is roughly half of that expected by standard-level-population calculations. Several recent studies suggest that the physical processes populating excited states of helium are not now understood. In this paper,absorption of 108320 by water vapor in the earth's atmosphere is examined as a means of explaining this discrepancy. This effect can reduce the intensity of He I 10830 by as much as 20-25 percent in the well-studied planetary nebula NGC 7027, depending on the time of year. This effect accounts for about 50 percent of the discrepancy discussed by previous workers. The ramifications of these results are discussed, and radial-velocity 'danger zones', in which the line is most affected, are given to aid in the assessment of this effect in other objects. Tables are provided to determine the observing seasons when the line is least affected by telluric absorption.