Discovery of new low-excitation planetary nebulae

Abstract

We report a multi-wavelength study of four new planetary nebula (PN) candidates selected from the INT/WFC Photometric Hα Survey of the Northern Galactic Plane (IPHAS) and Deep Sky Hunter (DSH) catalogues. We present mid-resolution optical spectra of these PNs. The PN status of our sample was confirmed by optical narrow-band images and mid-resolution spectra. Based on the locations of these objects in the log(Hα/[N II]) versus log(Hα/[S II]) diagnostic diagram, we conclude that these sources are evolved low-excitation PNs. The optical and infrared appearances of these newly discovered PNs are discussed. Three of the new nebulae studied here are detected in infrared and have low infrared-to-radio flux ratios, probably suggesting that they are evolved. Furthermore, we derive the dynamical ages and distances of these nebulae and study the spectral energy distribution for one of them with extensive infrared archival data.