Pure fluorescent H2 emission from Hubble 12

Abstract

An improved K-band (2.0–2.4 μm) spectrum of the planetary nebula Hubble 12 has been obtained at a position near where Dinerstein et al. found the H2 line emission to be fluorescent. The new spectrum is consistent with the conclusion of Dinerstein et al. and, in addition, provides further evidence that the emission is essentially purely fluorescent, through the presence of several weak H2 lines of higher excitation than have been detected previously and a measured ortho/para ratio of 1.7. Analysis of the spectrum demonstrates that the gas density is |$\sim 10^{4}-10^{5} \text {cm} ^{-3}$|⁠.