The excitation mechanisms and evolutionary stages of UWISH2 planetary nebula candidates

Abstract

We present medium-resolution K-band long-slit spectroscopy of 29 true, likely, possible and candidate Galactic Plane planetary nebulae (PNe) from the UWISH2 survey - many of which have only been recently discovered. These objects are bright in molecular hydrogen (H2) emission, and many have bipolar morphologies. Through the detection of the Br{\gamma} emission line, which traces ionized hydrogen, we find that the majority of the candidate PNe are indeed likely to be PNe, while 2 of the targets are more likely young stellar objects (YSOs) or pre-planetary nebulae (pPNe). We detect Br{\gamma} in 13 objects which have no detection in IPHAS or SHS H{\alpha} surveys. This implies they are potential members of the little-known optically-obscured PN population, hidden from wide-field optical surveys. We use the spatial extent of the H2 1-0 S(1) and Br{\gamma} lines to estimate the evolutionary stage of our targets, and find that W-BPNe (bipolar PNe with pinched waist morphologies) are likely to be younger objects, while R-BPNe (bipolar PNe with large ring structures) are more evolved. We use line ratios to trace the excitation mechanism of the H2, and find the 1-0 S(1) / 2-1 S(1) and 1-0 S(1) / Br{\gamma} ratios are higher for R-BPNe, implying the H2 is thermally excited. However, in W-BPNe, these ratios are lower, and so UV-fluorescence may be contributing to the excitation of H2.