Planetary Nebulae with Ultra-Violet Imaging Telescope (UVIT): Far Ultra-violet halo around the Bow Tie nebula (NGC 40)

摘要

Context. NGC 40 is a planetary nebula with diffuse X-ray emission, suggestingan interaction of the high speed wind from WC8 central star (CS) with thenebula. It shows strong Civ 1550 {\AA} emission that cannot be explained bythermal processes alone. We present here the first map of this nebula in C IVemission, using broad band filters on the UVIT. Aims. To map the hot C IV emitting gas and its correspondence with soft X-ray(0.3-8 keV) emitting regions, in order to study the shock interaction with thenebula and the ISM. This also illustrates the potential of UVIT for nebularstudies. Methods. Morphological study of images of the nebula obtained at an angularresolution of about 1.3" in four UVIT filter bands that include C IV 1550 {\AA}and C II] 2326 {\AA} lines and UV continuum. Comparisons with X-ray, optical,and IR images from literature. Results. The C II] 2326 {\AA} images show the core of the nebula with twolobes on either side of CS similar to [N II]. The C IV emission in the coreshows similar morphology and extant as that of diffuse X-ray emissionconcentrated in nebular condensations. A surprising UVIT discovery is thepresence of a large faint FUV halo in FUV Filter with {\lambda}eff of 1608{\AA}. The UV halo is not present in any other UV filter. FUV halo is mostlikely due to UV fluorescence emission from the Lyman bands of H2 molecules.Unlike the optical and IR halo, FUV halo trails predominantly towardssouth-east side of the nebular core, opposite to the CS's proper motiondirection. Conclusions. Morphological similarity of C IV 1550 {\AA} and X-ray emissionin the core suggests that it results mostly from interaction of strong CS windwith the nebula. The FUV halo in NGC 40 highlights the existence of H2molecules extensively in the regions even beyond the optical and IR halos.