[Paper Review] Planetary Nebulae with UVIT II: Revelations from FUV vision of Butterfly Nebula NGC 6302

N. Kameswara Rao, Orsola De Marco+5 more|arXiv (Cornell University)|Oct 4, 2018

Stellar, planetary, and galactic studies48 references4 citations

TL;DR

Using UVIT on ASTROSAT, this study reveals extended far-ultraviolet (FUV) lobes and orthogonal jet-like structures in the Butterfly Nebula NGC 6302, which are undetected in optical and near-UV wavelengths. The FUV lobes, oriented at 113° and extending to 5 arcmin, likely originate from fluorescent H₂ emission and may represent an earlier, distinct ejection phase preceding the optical bipolar outflow triggered ~2200 years ago.

ABSTRACT

The high excitation planetary nebula, NGC 6302, has been imaged in two far-ultraviolet (FUV) filters, F169M (Sapphire; λ$_{ m eff}$: 1608 Å) and F172M (Silica; λ$_{ m eff}$: 1717 Å) and two NUV filters, N219M (B15; λ$_{ m eff}$: 2196 Å) and N279N (N2; λ$_{ m eff}$: 2792 Å) with the Ultra Violet Imaging Telescope (UVIT). The FUV F169M image shows faint emission lobes that extend to about 5 arcmin on either side of the central source. Faint orthogonal collimated jet-like structures are present on either side of the FUV lobes through the central source. These structures are not present in the two NUV filters nor in the FUV F172M filter. Optical and IR images of NGC 6302 show bright emission bipolar lobes in the east-west direction with a massive torus of molecular gas and dust seen as a dark lane in the north-south direction. The FUV lobes are much more extended and oriented at a position angle of 113°. They and the jet-like structures might be remnants of an earlier evolutionary phase, prior to the dramatic explosive event that triggered the Hubble type bipolar flows approximately 2200 years ago. The source of the FUV lobe and jet emission is not known, but is likely due to fluorescent emission from H$_2$ molecules. The cause of the difference in orientation of optical and FUV lobes is not clear and, we speculate, could be related to two binary interactions.

Motivation & Objective

• To investigate the far-ultraviolet (FUV) morphology of the planetary nebula NGC 6302 using UVIT on ASTROSAT.
• To identify and characterize previously undetected FUV structures, such as extended lobes and collimated jets, beyond the known optical and near-UV features.
• To determine the origin of the FUV emission, particularly whether it arises from fluorescent H₂ molecules or other mechanisms.
• To explore the evolutionary history of NGC 6302, especially the potential role of binary interactions in shaping its complex, multi-phase nebular structures.
• To assess whether such FUV structures are common in young planetary nebulae and what they reveal about early mass ejection phases.

Proposed method

• Acquired deep imaging of NGC 6302 in four UV filters: F169M (1608 Å), F172M (1717 Å), N219M (2196 Å), and N279N (2792 Å) using the Ultra-Violet Imaging Telescope (UVIT) on board the ASTROSAT satellite.
• Compared FUV images (F169M and F172M) with near-UV (NUV) and optical data to isolate FUV-specific features such as extended lobes and jet-like structures.
• Analyzed the spatial orientation and extent of FUV structures relative to the known optical bipolar lobes and the dark equatorial torus.
• Evaluated the emission mechanism by comparing FUV fluxes with known fluorescent H₂ emission lines shortward of 1608 Å.
• Used kinematic and photometric data from prior studies (e.g., HST, MIRI, radio) to contextualize the FUV structures within the nebula’s dynamical history.
• Proposed a binary-driven evolutionary model involving two distinct ejection events: one ~5000 years ago (forming the torus and FUV lobes), and a second ~2200 years ago (producing the optical Hubble-type lobes).

Experimental results

Research questions

• RQ1 What FUV structures are revealed in NGC 6302 that are undetected in optical and near-UV wavelengths?
• RQ2 What is the physical origin of the FUV emission in the extended lobes and orthogonal jet-like features?
• RQ3 How do the orientations of the FUV lobes and optical lobes differ, and what does this imply about the nebula’s formation mechanism?
• RQ4 Can the FUV structures be explained by fluorescent H₂ emission, and what does this imply about hidden mass in the nebula?
• RQ5 Is the presence of two distinct, perpendicular ejection phases consistent with a binary star interaction scenario?

Key findings

• UVIT detected faint, extended FUV lobes in the F169M filter (1608 Å) extending up to 5 arcminutes from the central source, oriented at a position angle of 113°.
• Orthogonal, jet-like structures were observed through the central source in the F169M filter but were absent in the F172M, N219M, and N279N filters, indicating a narrow spectral origin.
• The FUV lobes are significantly more extended than the optical bipolar lobes, which are oriented along the east-west direction and separated by a dark equatorial torus.
• The FUV emission is most likely due to fluorescent excitation of H₂ molecules, as no known atomic lines shortward of 1608 Å can account for the observed flux.
• The FUV structures are not visible in any optical or near-UV data, suggesting they represent a distinct, earlier ejection phase from the central star, possibly preceding the optical Hubble-type outflow by ~2200 years.
• The presence of two perpendicular, multi-phase ejection events—possibly from a binary system—may explain the complex morphology, with the FUV lobes and jets originating from a pre-2200-year ejection phase.