Planetary nebulae with UVIT: Far ultra-violet halo around the Bow Tie nebula (NGC 40)

Abstract

Context. NGC 40 is a planetary nebula with diffuse X-ray emission, suggesting an interaction of the high-speed wind from WC8 cen- tral star (CS) with the nebula. It shows strong C IV 1550 Å emission that cannot be explained by thermal processes alone. We present here the first map of this nebula in C IV emission using broad band filters on the Ultra-Violet Imaging Telescope (UVIT). Aims. We aim 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 between the nebula and the ISM. We also aim to illustrate the potential of UVIT for nebular studies. Methods. We carry out a morphological study of images of the nebula obtained at an angular resolution of about 1.3 00 in four UVIT filter bands that include C IV 1550 Å and [C II ] 2326 Å lines as well as UV continuum. We also make comparisons with X-ray, optical, and IR images from the literature. Results. The [C II ] 2326 Å images show the core of the nebula with two lobes on either side of CS similar to [N II ]. The C IV emission in the core shows similar morphology and extent to that of diffuse X-ray emission concentrated in nebular condensations. A surprising UVIT discovery is the presence of a large faint far UV (FUV) halo in an FUV filter with λ e ff of 1608 Å. The UV halo is not present in any other UV filter. The FUV halo is most likely due to UV fluorescence emission from the Lyman bands of H 2 molecules. Unlike the optical and IR halo, the FUV halo trails predominantly towards the south-east side of the nebular core, opposite to the CS’s proper motion direction. Conclusions. Morphological similarity of C IV 1550 Å and X-ray emission in the core suggests that it results mostly from the interac- tion of strong CS wind with the nebula. The FUV halo in NGC 40 highlights the extensive existence of H 2 molecules in the regions even beyond the optical and IR halos. Thus UV studies are important to estimate the amount of H 2 , which is probably the most dominant molecule and significant for mass-loss studies.