Detection of fluorescent molecular hydrogen emission in the planetary nebula Hubble 12

Abstract

The first clear detection of fluorescent molecular hydrogen emission in a planetary nebula, the compact young planetary Hubble 12 is reported. Emission in the near-infrared vibrational-rotational transitions of H2 has been seen previously in numerous planetary nebulae, but has usually been attributed to thermal emission from shocked gas. The alternate mechanism, fluorescence or radiative pumping, is easily recognizable because it produces relatively strong lines from high-lying vibrational levels. A 2.0-2.3 micron spectrum taken on the H2-emitting shell, which encircles the central ionized core of Hubble 12, and which shows prominent emission lines arising from the v = 1, 2, and 3 levels of H2, is presented. Offsetting to a spatial position where the strong nebular line and continuum emission was excluded from the beam was essential for isolating this molecular emission spectrum. The observed line intensities show unambiguously that the dominant excitation mechanism for this object is fluorescence rather than thermal excitation. 23 references.