The mysterious ring in the open cluster NGC 3572: Planetary nebula or photoevaporating globule?

We discuss optical and infrared emission from the putative planetary nebula in the young open cluster NGC 3572. Velocity images of [N II] λ6583 obtained with the Rutgers/CTIO Fabry-Perot interferometer reveal that most gas in the nebula is expanding at velocities ≲5 km s^-1, with marginal evidence for bipolar expansion. A few outer condensations are seen at faster redshifted velocities, but their origin is uncertain. Optical spectra reveal a spatial excitation gradient, with higher excitation in a diffuse outer halo and low excitation in the bright inner nebula, suggesting that the nebula is externally ionized by hot stars in the open cluster and that the nebula and cluster are therefore equidistant. The nebula coincides with an infrared source detected by the MSX and IRAS satellites and has a spectral energy distribution implying a total mass of 5-10 M_⊙. MSX also reveals diffuse infrared emission associated with the cluster, and its morphology implies a connection with the ring nebula. We discuss two very different interpretations of this object - it is either a strange planetary nebula or (more probably) a young photoevaporating globule left over from the molecular cloud that formed the cluster.