The coronal line regions of planetary nebulae NGC 6302 and 6537: 3-13μm grating and echelle spectroscopy

We report on advances in the study of the cores of NGC 6302 and 6537 using infrared grating and echelle spectroscopy. In NGC 6302, emission lines from species spanning a large range of ionization potential, and in particular [Siix] 3.934μm, are interpreted using photoionization models (including cloudy), which allow us to re-estimate the temperature of the central star to be about 250 000K. All of the detected lines are consistent with this value, except for [Alv] and [Alvi]. Aluminium is found to be depleted to one hundredth of the solar abundance, which provides further evidence for some dust being mixed with the highly ionized gas (with photons harder than 154eV). A similar depletion pattern is observed in NGC 6537. Echelle spectroscopy of IR coronal ions in NGC 6302 reveals a stratified structure in ionization potential, which confirms photoionization to be the dominant ionization mechanism. The lines are narrow (<22kms^-1 FWHM), with no evidence of the broad wings found in optical lines from species with similar ionization potentials, such as [Nev]3426Å. We note the absence of a hot bubble, or a wind-blown bipolar cavity filled with a hot plasma, at least on 1arcsec and 10kms^-1 scales. The systemic heliocentric velocities for NGC 6302 and 6537, measured from the echelle spectra of IR recombination lines, are found to be -34.8+/-1kms^-1 and -17.8+/-3kms^-1. We also provide accurate new wavelengths for several of the infrared coronal lines observed with the echelle.