Molecular hydrogen emission from cold condensations in NGC 2440

Abstract

We report observations of the |$\upsilon\, = 1-0\,S(1)$| line of molecular hydrogen in the high-excitation planetary nebula NGC 2440. The emission is particularly strong at the positions of the two bright condensations which lie well within the Hrr region and close to the position of the very hot T ≈ 350 000 K central star. The emission is consistent with an excited molecular hydrogen mass of |$\approx 2-4\,\times\,{10}^{-5}\,{M}_{\odot}$| in the condensations, and we estimate the total mass of excited molecular hydrogen associated with the H II region to be |$\approx 6.1\,\times\,{10}^{-3}\,{M}_{\odot}$|⁠. We show that radiation pressure from the central star is insufficient to excite the S(1) line emission. We also show that a stellar-wind-driven shock would imply a mass loss rate of |$3\,\times\,{10}^{-7}\,{M}_{\odot}\,\text{yr}^{-1}$| if we adopt a wind velocity of 2000 km s–1.