Imaging the transition between pre-planetary and planetary nebulae: integral field spectroscopy of hot post-AGB stars with NIFS

We present 2-2.4 mu m integral field spectroscopy of a sample of hot post-asymptotic giant branch stars with early-B spectral types, using the Near-infrared Integral Field Spectrometer instrument on Gemini North. These stars are just beginning to ionize their immediate environments and turn into planetary nebulae (PNe). We use molecular hydrogen emission lines together with hydrogen and helium recombination lines to explore the distribution of molecular and atomic gas and the extent of the developing ionized region. We see a range of evolutionary stages: IRAS 18062+2410 and IRAS 18379-1707 have recently developed compact and unresolved regions of photoionized H within axisymmetric molecular envelopes, with the former object increasing its Br gamma flux by a factor of 5.3 in 14 years; IRAS 22023+5249 and IRAS 20462+3416 have extended Br gamma nebulae and in the latter object only weak H-2 emission remains; IRAS 19336-0400 is at a more advanced stage of PN formation where H-2 is mostly dissociated and we see structure in both the H and He recombination line nebulae. IRAS 19200+3457 is the only object not to show the He I line at 2.058 mu m and is probably the least evolved object in our sample; the H-2 emission forms a ring around the star and we suggest that this object may be a rare example of a 'round' pre-PN in transition to a 'round' PN.