Planetary Nebulae with UVIT: a Progress Report. 6 7 8 N
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Manuscript Click here to access/download;Manuscript;uvitpnrev_v4p.pdf J. Astrophys. Astr. (0000) 000: #### DOI 1 2 3 4 5 Planetary Nebulae with UVIT: A Progress Report. 6 7 8 N. Kameswara Rao1,*, Sutaria F. 1, Murthy J.1, Ray A.2,3 & Pandey G. 1 9 10 1Indian Institute of Astrophysics, Bangalore 560034, India. 11 2Tata Institute of Fundamental Research, Colaba, Mumbai 400005,India. 12 3 Homi Bhabha Centre for Science Education (TIFR) Mumbai 400088, India. 13 *Corresponding author. E-mail: [email protected] 14 15 MS received 7 Nov. 2020; accepted – 16 17 18 Abstract. The spectral region between 1250 Å -3000 Å contains important spectral lines to understand the 19 morphological structures and evolution of planetary nebulae. This is the region sampled by UVIT through various 20 filter bands both in the continuum and in emission lines (e.g.. C iv, He i, Mg ii etc.). We have mapped several 21 planetary nebulae with different characteristics, ranging in morphology from bipolar to wide and diffuse, and in 22 various states of ionization, comparing the UV with the x-ray morphologies wherever the x-ray images were also 23 available. The major unanticipated discovery with UVIT has been the detection of previously undetected, cold, 24 fluorescent, H2 gas surrounding some planetary nebulae. This may be a possible solution to the missing mass 25 problem. Here we present a review of our studies so far done (both published and on going) with UVIT. 26 27 Keywords. Star: AGB and post — AGB stars, winds, outflows — planetary nebulae: ISM: Planetary nebulae: 28 general — planetary nebulae: individual: NGC 6302. 29 30 1. Introduction rial starts to glow as the planetary nebula, and “illumi- 31 nates the pages of the book that tells the star’s story” 32 Planetary nebulae (PNs) are splendid remnants of ex- (Bianchi 2012). In the interacting stellar wind model 33 traordinary deaths of ordinary stars in the mass range (Kwok 1978), it the interaction of the the high speed 34 of 1 – 8 M⊙. They disburse the nucleosynthetically stellar wind and the slowly expanding super-wind ma- 35 processed stellar material like Carbon and s-process el- terial that shapes the planetary nebulae. Presence of 36 ements into the interstellar medium, thus enriching the a companion and or magnetic fields may further al- 37 38 matter which forms the next generation of stars. The ter the morphology of the PN. In general, PNe show 39 extensive, slow, stellar wind, moving at speeds of 10 to very many shapes ranging from spherical to bipolar to −1 −7 −1 40 15 km s , with a mass-loss rate of ∼ 10 M⊙ yr , multipolar, with some even having chaotic geometries. 41 that starts on the thermally pulsing asymptotic giant Morphological studies of these objects reveal their past 42 branch (AGB) – double shell (He & H) burning sources history of mass ejections, their time scales, kinemat- 43 – transforms into a heavy super-wind with mass-loss ics, properties of the ionizing source, wind interac- −4 −1 44 rates of ∼ 10 M⊙ yr (Delfosse et al. 1997) as the tions as well as interactions with interstellar medium 45 star evolves to the tip of AGB in the H-R diagram. In etc.. The UV region is important for the study of both 46 a relatively short time most of the mass is lost through the central stars (CSPNs) as well as the nebula, be- 47 − a super-wind till the envelope mass falls below 10 3 - cause the most important lines of the most abundant 48 −4 ii 49 10 M⊙, when a structural change occurs to the star elements and their ionization states like C 1335 Å, iv ii iii ii 50 as a degenerate CO oxygen core (which ultimately be- C 1550 Å, He 1640 Å,N ] 1760 ÅC ] 2326 Åetc., 51 comes a white dwarf) develops. The photospheric ra- fall in this region. These lines are important for model- 52 dius shrinks and the effective temperature Teff starts to ing the ionization structure, shocked regions, chemical 53 increase keeping the luminosity almost constant. Con- composition etc., and for the estimation of the Teff of 54 sequently, the mass-loss rate stellar wind decreases to the hot CSPNs. Moreover, the interstellar extinction −8 −1 55 about 10 M⊙ yr and the wind speed picks up to 200 through 2179 Åbump can be studied only in the UV 56 to 2000 km s−1. This fast stellar wind plows into the band. The Ultraviolet Imaging Telescope(s) (UVIT) 57 material that was earlier lost through super-wind gener- on ASTROSAT (ref.), with broad and narrow band fil- 58 ating a shock at the interface, while the stellar radiation ters which cover important spectral lines and contin- 59 60 heats and ionizes the ejecta. The circumstellar mate- uum with an angular resolution of about 1”.5, over a 61 62 ⃝c Indian Academy of Sciences 1 63 64 65 #### Page 2 of 1 J. Astrophys. Astr. (0000) 000: #### 1 28’ field of view, are well suited for the study of PNs. 6302 are to be found in (Kameswara Rao et al 2018a,b), 2 Details of UVIT are provided in Kumar et al (2012) while and for NGC 2818 in (Kameswara Rao et al., 3 and its in-orbit performance is described in Tandon et A& A, submitted). Although observations of NGC 650, 4 + 5 al (2017a) and in Tandon et al. (2020). UVIT is one OH231.8 4.2, Mz3, (and IC 4997) have been done the 6 of the five payloads on the multiwavelength Indian as- data is not yet available from ISSDC. 7 tronomical satellite ASTROSAT that was launched on The compact low excitation planetary nebula, NGC 8 2015 September 28. It consists of two 38 cm aperture 40, was the first object we studied with a view to 9 telescopes, one of which is optimized for FUV, while look for correspondence of high excitation UV line re- 10 the other has a dichroic beam splitter that reflects NUV gions with Chandra X-ray images. It has been im- 11 and transmits the optical. Each UV channel can be aged in the far-ultraviolet filters F169M (UVIT/FUV- 12 studied in five broad and narrow band filters, as well F3 with λeff = 1608 Å ) and F172M (UVIT/FUV- 13 as by low resolution transmission gratings. The Visual F5 with λeff of 1717 Å), as well as in the near- 14 channel (VIS channel), which operates only in the in- ultraviolet (UVIT/NUV) filters N245M (UVIT/NUV- 15 tegration mode, is used for tracking. Our project uses B13) and N279N (UVIT/NUV-N2 with λeff of 2792 Å). 16 iv 17 UVIT imaging of x-ray bright and x-ray faint Planetary The filters selected would allow imaging in C 1550Å ff ii 18 Nebulae of di erent morphologies in various UV emis- (F169M) and C ] 2326Å (N245M) emission lines, as 19 sion lines, particularly C iv 1550 Å, C ii] 2326 Å, [O ii] well as in the continuumn (F172M) and (N263). Mor- 20 2470 Å, Si iv 1400 Å, Mg ii] 2800 Å, He ii 1640 Å etc., phological studies in optical and infrared (IR) show 21 using various filters of the UVIT-FUV and -NUV chan- that NGC 40 has ionized high density central core sur- 22 nels. We aim to study the UV morphologies, shocked rounded by faint filamentary halo with circumnebu- 23 regions and correspondence of UV and X-ray emissions lar rings that are seen only in Hα but not in [O iii]. 24 in PNs, and to that end, several PNs of varied morpho- UVIT studies show that C ii] 2326Å emission is con- 25 logical types in both near (NUV) and far (FUV) UV fined mostly to the core and shows similar morphology 26 ranges have been observed. Unfortunately, the NUV as low excitation lines in optical. However, strong C iv 27 28 channel became dysfunctional after 2017. In this paper, 1550Å emission is present in the core and shows simi- 29 we discuss our observations conducted so far of the se- lar morphology and extent as that of X-ray (0.3-8 keV) 30 lected PNs (Table 1), as well as some of the results and emission observed by Chandra, suggesting interaction 31 surprises that emerged. Detailed studies of individual of the high-speed wind from WC8 central star (CS) 32 objects would be presented else where but some salient with the nebula. An unexpected UVIT discovery is the 33 observational features particularly brought out by UV presence of faint large emission halo in FUV F169M 34 studies are dealt in the current presentation. Detailed surrounding the central core (Figure 2-top). This FUV 35 discussion of NGC 40 and NGC 6302 have been pre- halo is absent in the other filters. This emission halo 36 sented in Kameswara Rao (2018a,b). is unlikely to be due to C iv 1550Å emission, or due to 37 Table 1 shows a broad morphological classification dust scattering. Instead, it most likely is due to UV fluo- 38 39 of the nebulae we observed with UVIT so far which rescence emission from Lyman bands of H2 molecules 40 range from compact bipolar nebulae (B) to large ellip- since a few vib-rotational lines have already been de- 41 tical (E) and round (R) nebulae. Some are irregular. tected in the IR from Spitzer spectra. The FUV halo in 42 Figure 1 illustrates typical nebular emission lines that NGC 40 highlights the extensive existence of cold H2 43 are enclosed by UVIT filters that were used for our PN molecules in the regions even beyond the optical and 44 studies.