The importance of soft X-rays for the excitation of H2 emission in planetary nebulae

We note that H-2 emitting planetary nebulae tend to have Zanstra temperatures T-Z(He II) > 90 kK. This is shown to be consistent with a large evolutionary lifetime, and the kinematic ages of the envelopes. Non-local thermodynamic equilibrium stellar atmospheric modelling also shows that levels of soft X-ray emission increase more rapidly than has previously been assumed, and are preferentially large in H-2 emitting sources. It is suggested that this may hold the key to explaining the strengths of the H-2 transitions.