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L. Likkel (U. Wisconsin -- Eau Claire), H.L. Dinerstein (U. Texas at Austin), D. Lester (U. Texas at Austin), A. Kindt (U. Wisconsin -- Eau Claire)
We present long-slit spectroscopic observations of near-infrared emission from two planetary nebulae (PNe), Vy 2-2 and J 900. The data were obtained in 1998 with the new long-slit spectrometer, CoolSpec, on the 2.7 m Harlan Smith telescope at the University of Texas' McDonald Observatory (see poster by Lester {et al.} for a description of CoolSpec). Our observations had a spectral resolving power R/\DeltaR = 640 in H-band and 567 in K-band.
The two major excitation mechanisms that produce near-infrared molecular hydrogen emission lines in PNe are UV pumping (``fluorescence") and thermal excitation (shocks). Identification of the dominant excitation mechanism in individual PNe must be confirmed spectroscopically, and we examine the intensity ratios among the K-band H2 lines to address this issue. If the density is high enough, fluorescent excitation can masquerade as thermal heating, but H-band observations can provide definitive proof of UV pumping, by the detection of lines from higher vibrational levels that thermal excitation will not populate. Since fluorescent excitation is a relatively inefficient mechanism, it can only produce strong near-infrared line emission if there is a substantial amount of material in the neutral/molecular envelope (``photodissociation region").
The prototype UV-pumped planetary nebula, found by Dinerstein {et al.} (1988, ApJ, 327, L27), is Hubble 12. J 900, in contrast, is likely to be strongly dominated by shock excitation. Vy 2-2, like BD+30 3639 (Shupe { et al.} 1998, ApJ, 498, 267), may be an intermediate case. One revelation from our relatively high spectral resolution data is the extent to which telluric absorption features and blending of lines from other species affect the measurements of the H2 line ratios, and therefore the conclusions about excitation mechanisms.
The author(s) of this abstract have provided an email address for comments about the abstract: likkel@uwec.edu