[Previous] | [Session 6] | [Next]
L. Likkel, K. Bartig (U. Wisconsin, Eau Claire), H.L. Dinerstein, D.F. Lester (U. Texas, Austin)
We are investigating the physical conditions in molecular gas associated with planetary nebulae (PNe) by studying their H2 emission. Molecules in the envelopes of planetary nebulae are subjected to both intense UV radiation from the central star and the possibility of heating by shocks. Both UV radiation (``fluorescence'') and shocks can produce near-infrared H2 line emission, which is seen in dozens of PNe. Interpretation of the emission depends on which mechanism is operating. Since these two basic mechanisms produce different line intensity ratios, detailed spectroscopic observations are the key to understanding this emission.
We present new observations of vibrationally-excited H2 emission from IC~5117, obtained at McDonald Observatory in November 1999 and June 2000 using a modular, long-slit near-infrared spectrometer, ``CoolSpec'' (http://marple.as.utexas.edu:80/~dfl/coolspec/). Our K-band spectra cover the region 1.98--2.26 \mum at \lambda/\Delta\lambda=800, and yield 3\sigma detections of lines as weak as 1\times10-14 erg cm-2 s-1. We find that the line intensities are inconsistent with the thermal ratios which would be expected for pure shock excitation. The strongest evidence comes from our detection of H2 3-2 S(3) 2.201 \mum, adjacent to but separable from a commonly-seen (in PNe) unidentified line at 2.199 \mum. However, the weakness of several K-band v=2 lines, and our upper limits on J-band overtone lines from v=3, 4, are also inconsistent with a pure radiative cascade. Since the ionized gas in IC~5117 has n \geq 105 cm-3, it is likely that the adjacent molecular gas is also dense. At high densities, collisions can redistribute an ensemble of radiatively-excited H2 molecules, altering the level populations and therefore the emergent line intensity ratios. We suggest that such collisional effects are important in the dense molecular regions of young PNe, and that objects such as IC~5117 offer excellent laboratories for studying physical processes in dense, UV-illuminated molecular gas.
This work is supported by NSF grant AST 97-31156 to HLD, and by grants to LL from the University of Wisconsin--Eau Claire.