Ultraviolet imaging of planetary nebulae with mathit{GALEX}

Abstract

Over four hundred Galactic Planetary Nebulae (PNe) have been imaged by mathit{GALEX} in two ultraviolet (UV) bands, far-UV (FUV, 1344–1786 Å, lambda _{eff}= 1528~{mathring{mathrm{A}}}) and near-NUV (NUV, 1771–2831 Å, lambda _{eff} = 2271~{mathring{mathrm{A}}}). We present examples of extended PNe, for which UV spectroscopy is also available, to illustrate the variety in UV morphology and color, which reflects ionization conditions. The depth of the GALEX imaging varies from flux approx 0.4/5times 10 ^{-18}~mbox{ergs},mbox{cm}^{-2},mbox{s}^{-1},{mathring{mathrm{A}}}^{-1},square ^{primeprime,-1} (mathit{FUV}/mathit{NUV}) for exposures of the order of sim 100 seconds, typical of the survey with the largest area coverage, to sim 0.3/8.3times 10^{-19}~mbox{ergs},mbox{cm}^{-2},mbox{s}^{-1},{mathring{mathrm{A}}}^{-1},square ^{primeprime,-1} (mathit{FUV}/mathit{NUV}) for sim 1500~mbox{sec} exposures, typical of the second largest survey (see Bianchi in Astrophys. Space Sci. 320:11, 2009; Bianchi et al. in Adv. Space Res. 53:900, 2014). mathit{GALEX} broad-band FUV and NUV fluxes include nebular emission lines and in some cases nebular continuum emission. The sensitivity of the mathit{GALEX} instrument and the low sky background, especially in FUV, enable detection and mapping of very faint ionization regions and fronts, including outermost wisps and bow shocks. The mathit{FUV}mbox{-}mathit{NUV} color of the central star provides a good indication of its T_{eff}, because the mathit{GALEX} FUV-NUV color is almost reddening-free for Milky Way type dust (Bianchi et al. in Astrophys. J. Suppl. Ser. 230:24, 2017; Bianchi in Astrophys. Space Sci. 335:51, 2011, Bianchi in Astrophys. Space Sci. 354:103, 2014) and it is more sensitive to hot temperatures than optical colors.