[Previous] | [Session 89] | [Next]
L. Likkel, K. Bartig (U.Wisconsin-Eau Claire), H.L. Dinerstein, D.F. Lester (U.Texas-Austin)
M1-13 is a bipolar planetary nebula which was identified as displaying near-infrared emission from vibrationally-excited H2 in the imaging survey of Kastner et al. (1996, ApJ, 462, 777). We obtained long-slit near-infrared spectroscopic observations of M1-13 in November 2000, using CoolSpec (Lester et al. 2000, PASP, 112, 384) on the 2.7m Harlan J. Smith telescope at McDonald Observatory. We present 2.00-2.27 \micron spectra of M1-13 for an East-West slit through the equatorial waist. The spectrum of M1-13 is unusual in that the H2 emission lines are stronger than the nearby hydrogen and helium recombination lines. The strongest line in the spectrum, 2.121~\micron H2 v=1-0 S(1), is almost twice as strong as H~I Br\gamma, and has a greater spatial extent.
The unusually strong H2 emission from M1-13 indicates the presence of a large amount of molecular material, efficient excitation of H2, or both. UV fluorescence and shock-heated thermal excitation are the two most commonly invoked mechanisms for the excitation of H2 emission. Ratios of various near-infrared lines can be be used to distinguish between these mechanisms. The observed ratio of the 2.121~\micron line to other H2 transitions in M1-13 suggests that, in this object, thermal (shock) excitation dominates. This is also consistent with the high surface brightness of the H2 compared to emission from the ionized gas; strong H2 emission is more easily produced by shock activity than by radiatively-excited gas. Since radiative excitation is known to be dominant in other planetary nebulae, these results emphasize the fact that both shocks and radiative excitation must be considered, and that spectroscopic observations are essential for interpreting H2 emission in planetary nebulae. Funding for this research was provided by NSF grant AST~97-31156 to HLD and a grant to LL from the UW-Eau Claire Office of Research and Sponsored Programs.
[Previous] | [Session 89] | [Next]
Bulletin of the American Astronomical Society, 34, #4
© 2002. The American Astronomical Soceity.