Astronomy and Astrophysics, volume 609, L1-1 (2018/1-1)
Planetary nebulae with UVIT: Far ultra-violet halo around the Bow Tie nebula (NGC 40).
RAO N.K., SUTARIA F., MURTHY J., KRISHNA S., MOHAN R. and RAY A.
Abstract (from CDS):
Context. NGC 40 is a planetary nebula with diffuse X-ray emission, suggesting an interaction of the high-speed wind from WC8 central star (CS) with the nebula. It shows strong CIV 1550Å emission that cannot be explained by thermal processes alone. We present here the first map of this nebula in CIV emission using broad band filters on the Ultra-Violet Imaging Telescope (UVIT).Aim. We aim to map the hot CIV-emitting gas and its correspondence with soft X-ray (0.3-8keV) 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'' in four UVIT filter bands that include CIV 1550Å and [CII] 2326Å lines as well as UV continuum. We also make comparisons with X-ray, optical, and IR images from the literature.Results. The [CII] 2326Å images show the core of the nebula with two lobes on either side of CS similar to [NII]. The CIV 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 λeff of 1608Å. 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 H2 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 CIV 1550Å and X-ray emission in the core suggests that it results mostly from the interaction of strong CS wind with the nebula. The FUV halo in NGC 40 highlights the extensive existence of H2 molecules in the regions even beyond the optical and IR halos. Thus UV studies are important to estimate the amount of H2, which is probably the most dominant molecule and significant for mass-loss studies.