THE IDENTIFICATION OF GALACTIC PLANE RADIO-SOURCES - DISTANT PLANETARY-NEBULAE AT VERTICAL-BAR-B-VERTICAL-BAR-LESS-THAN-2 DEGREES

Expanding on our pilot study employing narrowband, near-IR filters to identify highly obscured radio sources in the Galactic plane, we repor t here the identification of 36 new planetary nebulae at /b/<2 degrees . We measure telluric extinction coefficients for narrowband filters c entered at [S III](lambda 9069), [S III](lambda 9532), and Pa9 (lambda 9225), showing that they differ substantially from an extrapolation f rom continuum measurements at shorter wavelengths as a consequence of narrow atmospheric absorption lines; these same Lines prevent the deri vation of quantitative near-IR line ratios from sources whose Doppler shifts are unknown a priori. Nonetheless, we demonstrate that it is po ssible to distinguish PNe from compact H II regions (the other sources of extended radio and far-IR emission in the plane): objects with F{[ S III]}/F{Pa9}greater than or equal to 20 are PNe, while objects with lower values are H II regions. We also report on an investigation of s erendipitous objects with enhanced [S III] fluxes which appear in our images, concluding that most are highly reddened stars. We conclude wi th a brief discussion of the utility of this technique for identifying the radio and IR source populations of the Galactic plane which has h ereby increased threefold the number of identified PNe at low Galactic latitudes. (C) 1995 American Astronomical Society.