arXiv:2011.02939v1 [astro-ph.SR] 5 Nov 2020 [email protected] yHvlu.Rcntuto fislgtcre( curve 25 light July its on on of independently object Reconstruction and naked-eye Hevelius. Anthelme, a by by as 20 discovered docu June was earliest 1670 It the be . possibly to mented recently until believed was 1982 1985 n eodmxmma prxmtl . a n1671 5 of in maximum although view, final mag naked-eye a 2.6 from faded approximately subsequently It at March. maximum fading second a a by successively and followed discovery, at mag 3 mately nucsflutltedsoeyi 92o ml ac of patch small a of 1982 in H were discovery object the the recover until to unsuccessful attempts 1679, Since March. 1672 α KVleua ln eerdt sNv upcle1670) Vulpeculae Nova as to referred (long Vulpeculae CK yee sn L using Typeset D 5 ATVERSION RAFT euoiya h ieo h 60eet( event 1670 the of site the at nebulosity inst nttt o srpyis colo hsc A & Physics of School Astrophysics, for Institute Minnesota .P .B K. P. D. .Atoog icsini ie by given is discussion thorough A ). approxi- of maximum brightness visual a had it shows ) h iso h ioa oe mgdi H in imaged lobes bipolar the of tips the Keywords: merge to known. not attributed currently are is ILOTs lumin outbursts the be lower Vul bridge in CK which eruptions that objects pre propose (ILOTs), We than Transients merger. luminous Optical stellar more Galactic far any was or event nova 1670 the that indicating idnln fio eotdhr a eeldhg line-of- high revealed has here reported iron c of be scenarios line alternative bidden that required have observations KVl hsipista h bouevsa antd tth at magnitude visual absolute the that implies This Vul. CK ± 0ya aeie edrv eie siae ihestim with estimate, revised a derive cont of we velocities stark baseline, deprojected in the 10-year are on a Based velocities Vul. high CK Such in velocities . bipolar the of 10k s km 2130 KVleua,wiheutdi D17-1 a ogconside long was 1670-71, AD in erupted which Vulpeculae, CK 8 orl akCnr o srpyis colo hsc and Physics of School Astrophysics, for Centre Bank Jodrell A ANERJEE T E N X OVEMBER twocolumn 1 1. tr:idvda C u)—sas euir—sas winds, : — peculiar stars: — Vul) (CK individual stars: 7 srnm srpyisDvso,Pyia eerhLabo Research Physical Division, Astrophysics & Astronomy colo at n pc xlrto,AioaSaeUniver State Arizona Exploration, Space and Earth of School − 6 . 1 3 5 aut fCmuig niern,adSine Universit Science, and Engineering, Computing, of Faculty INTRODUCTION 4 srpyisGop enr oe aoaois el Uni Keele Laboratories, Jones Lennard Group, Astrophysics eateto hsc,FoiaSaeUiest,7 Chieft 77 University, State Florida Physics, of Department , EELN EAKBYPWRU LS RMTEPAST THE FROM BLAST POWERFUL REMARKABLY A REVEALING suigtepeiul eie nlnto nl f65 of angle inclination derived previously the assuming − 1 .R G R. T. a a bevdb eeisin Hevelius by observed was mag 6 ,2020 6, 2 eiiOsraoyNFsNILb 7 .AoouPae Hi Place, A’ohoku N. 670 NOIRLab, Observatory/NSF’s Gemini tl nAASTeX63 in style ERIFAE PCRSOYO KVULPECULAE: CK OF SPECTROSCOPY NEAR-INFRARED EBALLE , 2 hr tal. et Shara .E A. hr Moffat & Shara .S S. VANS hr tal. et Shara α TARRFIELD , 3 tooy 1 hrhSre E nvriyo inst,Mi Minnesota, of University SE, Street Church 116 stronomy, n21.Tedpoetdvlcte ftetp r approxim are tips the of velocities deprojected The 2010. in ( .S M. 1985 ABSTRACT HAHBANDEH ). - , 7 srnm,Uiest fMnhse,Mnhse 1 P,U 9PL, M13 Manchester Manchester, of University Astronomy, AND niee.Ln ltifae pcrsoyo for- a of spectroscopy infrared slit Long onsidered. opc ai ore a eotdby reported was source, a with radio coincides about which compact A of symmetric center the feature, structure, emission. disk-like small submillimeter line structure bipolar fine dusty metallic and HCN KVul. CK n svsbei ete h pia o h erI.The database near-IR. SIMBAD the the ( nor in Vul optical incorrect CK is the of neither identification in current visible is and molecules. organic small several from emission shows also eldarc pcrmo yrcros oehrwt H with together hydrocarbons, of spectrum of rich point a the vealed at ( spectroscopy located (IR) source Infrared radio symmetry. compact a faint reported A also remnant. nova classical a being inconsis- 70 Vul dust, from CK emission with to tent due excess large a revealed hssrcue hc lgswt aklnsi h H the in lanes dark with aligns which structure, This H in discovered was .Z A. tducranis f3 of uncertainties, ated adke l 2007 al. et Hajduk aoy arnpr,Amdbd 809 India 380009, Ahmedabad, Navrangpura, ratory, h isadteraglrepninmaue over measured expansion angular their and tips the fSuhWls otpid F71L UK 1DL, CF37 Pontypridd, Wales, South of y att rvosrprso uhlwrexpansion lower much of reports previous to rast st a ewe oa n uenve While supernovae. and novae between gap osity ih eoiis( velocities sight iy o 744 ep,A 58-44 USA 85287-1404, AZ Tempe, 871404, Box sity, vns h rgno h ihylmnu ILOT luminous highly the of origin the events, r eko h 60epoinwas explosion 1670 the of peak e h eta tro KVli epyesrue ndust, in enshrouded deeply is Vul CK of central The umliee htmtyo KVl( Vul CK of photometry Submillimeter i a,Tlaase L33645,USA 32306-4350, FL Tallahassee, Way, ain ′′ , est,Kee tfodhr,S55G UK 5BG, ST5 Staffordshire, Keele, versity, 4 iu siae n rgtrta n classical any than brighter and estimates vious ioa eua retdams xcl - (Figure N-S exactly almost oriented nebula, bipolar IJLSTRA .E W E. C. og otecaso nemdaeLuminosity Intermediate of class the to longs e ob oaotus;hwvr recent however, outburst; nova a be to red nti td,w dp h ai coordinates radio the adopt we study, this In . o aa’,970 USA 96720, Hawai’i, lo, 8 ◦ uflw ifae:stars —infrared: outflows OODWARD o h xso yidia symmetry cylindrical of axis the for ± ∼ itdi al o h eta oreof source central the for 1 Table in listed ) α mgn by imaging , . 0 ms km 900 5 2 .D G D. R. − + 0 0 . . 6 9 p o h itneto distance the for kpc − EHRZ 1 naoi,M 55,USA 55455, MN nneapolis, adke al. et Hajduk fteaseat ansae the of ) M V , 5 vn ta.2016 al. et Evans = .P .E S. P. S. ye tal. et Eyres − vn ta.2002 al. et Evans 12 K . 4 ately − + ( 2 1 2007 YRES . . 4 3 , α ( image, ∼ ,who ), 2018 , 6 re- ) 10 the 1 2 ). ), ′′ ) , 2 D. P. K. BANERJEEETAL.

and 2400 seconds on 1 September. The same exposure times were used at a location 46′′ to the east, selected to minimize the number of stars in the “sky” slit. Exposures were ob- tained in a the standard ABBA manner. The star HIP 98699 (A1V) was observed immediately following CK Vul; the air- mass match was within 0.04 airmasses on each night. On both nights the slit was centered on the radio coordi- nates (Table 1) using a blind offset from a K ≈ 10 mag star Dec located 0.6′ distant. The offsetting accuracy of Gemini is 42 41 19:47:40 39 38 37 36 35 considerably less than a GNIRS pixel. Although CK Vul it- self was not detected in the near-IR, we are confident of the angular distances from it to the spectral features reported in RA 18:10 30 40 50 27:19:00 10 20 this paper. Examination of raw sky-subtracted frames revealed within Figure 1. Hα continuum-subtracted image of CK Vul observed on 10′′ of CK Vul several patches of emission along the slit in 2010 June 22 with the Gemini North Telescope (image kindly pro- the forbidden line of Fe II at 1.25702 µm (vacuum). The H I vided by M. Hajduk) . The position of the compact radio source is Paβ line at 1.28216 µm was also weakly present in a portion indicated by the white dot. The slit (cyan line) was centered on this of this region within a few arcseconds of the radio source. source and passes through the tips of the ansae located near the top Prompted by the report by Hajduk et al. (2013)ofHα line and bottom center of the image. emission with high proper motions at the northern and south- ern tips of the bipolar nebula, we searched the spectral im- ages on both nights and found regions of faint and highly No consensus exists on the cause of the 1670 eruption. − Doppler-shifted (∼±900km s 1) emission in the [Fe II] line, Proposed scenarios include a nova eruption, various types of about one arcsecond in extent, close to the locations of the stellar merger, and a very late thermal pulse in a post-AGB Hα tips. The northern emission stands out fairly well. The star (Shara & Moffat 1982; Shara et al. 1985; Hajduk et al. southern emission has superimposed on part of it the spec- 2007; Kaminski´ et al. 2015, 2017, 2018a; Eyres et al. 2018). trum of a faint star. However, careful data reduction clearly A kinematic study of the nebula could be a means of discrim- established the emission’s existence. We have considered inating between some of the above mechansims. For exam- whether these components are lines of some other atomic ple, stellar mergers rarely produce outflows with velocities − species, but think it highly unlikely and unreasonable that an exceeding 500 kms 1. On the other hand, novaeruptions can − atomic species would selectively create only one-sided emis- generate ejecta velocities of up to several thousand kms 1. sion (i.e. emission in only one of the two ansae) and also that Here we present near-IR spectra of CK Vul, obtained emission would appear only in the tips and not in the bright through a slit positioned as shown in Figure 1. The orienta- central region where it would be expected to be strong. Thus, tion was chosen so that the slit intercepted the radio source we are confident in our identification. at the center of the bipolar structure and the northern and Full reduction of the spectra from each night was limited to southern tips of the ansae, approximately 36′′ from the cen- the wavelength interval containing the [Fe II] and Pa β lines, ter, where the expansion velocities are expected to be largest there being no detected lines outside of that interval. It in- in a homologous or Hubble flow (Balick & Frank 2002, and volved the standard steps of flat-fielding, spatial and spectral references therein). rectification of the images, removal of the effects of cosmic ray hits, wavelength calibration using an argon lamp, and bin- ning of the spectra, in this case into intervals of 0.0002 µm 2. OBSERVATIONS AND DATA REDUCTION (48 kms−1). Spectroscopy of CK Vul was obtained at the Frederick C. Spectra of the twelve regions where the [Fe II] line was Gillett Gemini North Telescope on Maunakea on the nights detected were extracted from the rectified CK Vul reduced of 2020 31 August and 1 September (UT), using the Gem- image from each night. Each was flux-calibrated by dividing ini Near-InfraRed Spectrograph (GNIRS; Elias et al. 2006). by the spectrum of the standard star, including correcting for Sky conditions were excellent throughout the observations. slit loss in the spectrum of the star. The spectra from the The spectrograph configuration included its 99′′ slit oriented two nights were then combined to produce a final spectrum 3◦ west of north, a slit width of 0.45′′, and the short fo- for each region. Pertinent information for each spectrum is cal length blue camera, which provides a plate scale of provided in Table 1. 0.15′′/array pixel. The 110.5 ℓ/mm grating in GNIRS was set to a central wavelength of 1.26 µm, which gave a cov- erage of 1.203−1.317 µm and a resolving power, R, of 4800 (corresponding to resolutions of 0.00026 µmand62 kms−1). Individual exposure times were 300 seconds. The total exposure times on source were 1800 seconds on 31 August INFRAREDSPECTROSCOPYOF CK VULPECULAE 3

Table 1. Observed [Fe II] 1.257 µm line.

Offseta Aperture Vel. of peakb Fluxc Notes arcsec arcsec km s−1 hel. 10−19 W m−2 +36.7 0.45 × 0.90 +869 1.39 blue shoulder +7.9 0.45 × 0.75 -111 0.53 +4.2 0.45 × 0.75 -147 1.48 blue shoulder +4.2 0.45 × 0.75 -40 2.15 +2.9 0.45 × 0.75 -118 6.25 +1.5 0.45 × 0.75 -106 5.53 Pa β at −120 km s−1 +0.6 0.45 × 0.45 -300 0.84 +0.6 0.45 × 0.45 -63 2.62 Pa β at −50 km s−1 0.0 0.45 × 0.45 -302 1.16 0.0 0.45 × 0.45 -47 0.56 Pa β at −50 km s−1 -0.6 0.45 × 0.45 -285 0.25 -3.3 0.45 × 0.75 +68 2.25 -4.1 0.45 × 0.75 +20 3.50 -8.9 0.45 × 0.75 +137 2.05 -37.1 0.45 × 1.20 -922 0.84 may have red shoulder a From radio coordinates of CK Vul (J2000 RA =19:47:38.074, Dec = +27:18:45.16) along slit at position angle 3deg west of north. b Uncertainty ∼10 km s−1. c Typical random uncertainty 0.10 W m−2. Uncertainty in absolute flux calibration ∼20 percent.

3. RESULTS central waist of the hourglass nebula (i.e. the equatorial re- Spectra of [Fe II] 1.257 µm at all positions where the line gion) where low expansion velocities are expected. Hour- was detected are presented in Figure 2. Emission was also glass or bipolar nebulae are believed to be generated when detected in the Paβ line, but only close to the radio source. A the outflowing ejecta from the central source are constricted third line, [Fe II] at 1.279 µm, was also weakly detected, but by a material over-densities in the equatorial planes, i.e. at only in a few arcsecond long segment of the slit where the the waist of the hourglasses. The ejecta expand freely and 1.257 µm line is at its brightest. Figure 3 shows the spectrum rapidly in the polar directions, but slowly in the equatorial of that portion of the central region where all three lines are region. High velocities are expected in the polar regions by present. virtue of the larger distance to the tips of the ansae. As can be seen in Figure 2, the [Fe II] 1.257 µm line emis- For the Hα frame of 2010 June 22 (Figure 1) we mea- sion is prominent at various locations within ±9′′ of the cen- sure the angular separation between the centers of the tips tral source. The velocity pattern within this inner region is of the ansae at the positions where the GNIRS slit inter- ′′ ± ′′ complex, with large variations with position and at some lo- sected them to be 71. 29 0. 20. Using an accurate value ′′ ± ′′ −1 cations more than one component present. Discussion and of 0. 1517 0. 0002 pixel for the plate scale of GNIRS in modeling of the velocity field will be reported elsewhere. the configuration in which it was used, we find an angu- ′′ ± ′′ The highlight of these observations is the high line-of- lar separation of 73. 87 0. 23 for the centers of the tips in sight velocities observed in the tips of the north and south the [Fe II] 1.257 µm emission line. Because [Fe II] emis- − ansae in the [Fe II] line, peaking at ∼ +870 kms 1 and ∼ sion arises in a region where hydrogen is partially ionized −920 kms−1, respectively. Hajduk et al. (2013), using the (Mouri et al. 2000), we assume that Hα and [Fe II] are co- ′′ ± ′′ morpho-kinematic modelling tool SHAPE (Steffen & López spatial. An expansion of 2. 58 0. 30 has therefore occurred 2006) found that that the inclination angle of the bipolar over a 10.2 year period (2010 June 22 to 2020 August 31). Hα nebula is 65◦ relative to the line-of-sight. That implies Assuming constant velocity this implies that the expansion ± that the northern and southern tips are moving outward from began 290 40 years ago. Comparison with the the likely the center at 2130 kms−1 (900 kms−1/cos 65◦). This is date of the ejection event, 1670 or 1671, suggests that the deceleration of the tips, if any, has been rather modest. vastly higher than previously reported expansion velocities −1 −1 Assuming a constant transverse velocity of 1930 kms in CK Vul, which are of order a few hundred km s or less ◦ (Shara et al. 1985; Hajduk et al. 2007). (2130 sin 65 ) between 2010 and 2020 (i.e., no signifi- The reason for these previously reported low velocities is cant deceleration of the ejecta), the distance to CK Vul is now clear: all of them are for regions located around the 3.2 kpc, independent of reddening. This distance is a factor 4 D. P. K. BANERJEEETAL.

of 4.5-6 times larger than other estimates (Shara et al. 1985; Hajduk et al. 2013). The uncertainty in the derived inclination angle was not given by Hajduk et al. (2013) and is not incorporated in the above distance uncertainty. At high inclination angles, ex- pansion velocity, distance, and absolute magnitude all have non-linear dependenceson this angle. An uncertainty of 5◦ in ◦ +0.9 the assumed 65 gives the distance to CK Vul as 3.2−0.6 kpc; further allowing for the uncertainty in reddening gives the − +1.3 absolute visual magnitude at outburst as MV = 12.4−2.4. +0.9 The above distance of 3.2−0.6 kpc is in apparent conflict with distance estimates by Hajduk et al. (2013) of ∼500 pc and ∼2 kpc to two variable field stars, which they argued lie behind some of the ejecta of CK Vul, which has caused their recent brightness fluctuations and apparent Li overabun- dances. However, the former of these has a Gaia parallax (Bailer-Jones et al. 2018) of 4.9 kpc and thus no longer con- flicts. The latter star is too faint for Gaia and a more precise distance would need to be determined by some other means.

4. DISCUSSION 4.1. Line emission in the central few arcseconds Here we confine our discussion to the central region and the spectrum in Figure 3, in which all three lines ([Fe II] 1.257 and 1.279 µm, and Paβ) are present. This spectrum includes contributions from several disparate areas. Thus the properties derived here are mean values and do not nece- saarly represent any individual region. Based on the strength Figure 2. Spectra of the [Fe II] 1.257 µm line in the CK Vul nebula of [Fe II] 1.257 µm relative to Paβ, we assume that the gas in at the twelve locations along the slit where the line was detected. this region is largely collisionally excited. However, the pres- See Table 1 for details. ence of a compact radio source (Hajduk et al. 2007) indicates the possibility of some photoionization. As discussed by Mouri et al. (2000), the [Fe II] emission arises in a region in which hydrogen is partially ionized. The critical density for collisional de-excitation of the upper level of the 1.257 µm line is

0.66 4 Te −3 ncrit =5.6 × 10 cm , 10,000 K

where Te is the electron temperature. Use- ful diagnostics are the [Fe II] 1.257 µm/Paβ and [Fe II] 1.279 µm/[Fe II] 1.257 µm flux ratios, which are 4.9 ± 0.6and 0.07 ± 0.02 respectively. The [Fe II] 1.257 µm / Paβ flux ratio suggests shock velocities of ∼ 70 ± 5 kms−1 marginally below the threshold (75 kms−1) for an ionizing shock (Mouri et al. 2000). Note that this value is for solar abundances, which may not be applicable to these ejecta. The [Fe II] 1.279 µm/[Fe II] 1.257 µm flux ratio indicates Te ≃ 6000 K, although the deduced Te is virtually independent of ′′ the flux ratio. This temperature gives a critical density of Figure 3. Spectrum extracted over a 3. 6 region near the center, 4 −3 ncrit ≃ 4 × 10 cm for the upper level of the 1.257 µm line. showing three lines ([Fe II] 1.257 µm and 1.279 µm, and H I Paβ 1.282 µm). Dotted line: same spectrum scaled down by a factor of 4.2. The outer tips four. The morphologies of the [Fe II] 1.257 µm tips in the CK Vul nebula closely resemble the “fingertips" observed in INFRAREDSPECTROSCOPYOF CK VULPECULAE 5 the Orion Molecular Cloud by Bally et al. (2015, see e.g., not exceeding 400 kms−1. In the few published spectra their Fig. 9) in the shock-excited forbidden [Fe II] 1.644 µm of extragalactic LRNe (e.g M31 RV and PTF10fqs) also line. There over one-hundred high density clumps of gas are only narrow emission lines are present, indicating veloci- being ejected from the cloud at typical speeds of 300 kms−1, ties similar to those of their Galactic counterparts (Rich et al. which Bally et al. concluded is the result of a violent event 1989; Kasliwal 2012). In the case of PTF10fqs, in addi- ∼500 yr ago during the formation of a dense cluster of mas- tion to the narrow Balmer and calcium emission features, sive stars. thereis a 10,000 kms−1 wide unidentified emission feature at We propose that excitation of the upper level of the [Fe II] ∼8600 Å, possibly suggestive of a terminal explosive event 1.257 µm line in the CK Vul tips is the result of the same (Kasliwal et al. 2011a; Kasliwal 2012). In general, though, phenomenon as that proposed by Bally et al. (2015) for the the highest velocities in CK Vul are considerably higher than [Fe II] line in Orion. In each of the tips a low velocity re- those in LRNe. verse shock is being driven into a high density gas clump The peak absolute luminosity of CK Vul is also much ejected from CK Vul as each clump encounters much lower higher than the Galactic LRNe, the brightest of which was density interstellar gas. The shock ionizes and collisionally V838 Mon with MV = −9.8mag(Bond et al. 2003). Kasliwal excites the Fe and, at least to some extent, the hydrogen. It is (2012) has shown that there exists a group of objects with possible that lines of molecular hydrogen are also associated that are intermediate between the brightest no- with these clunps, as has been observed in detail in Orion vae and faintest supernovae (i.e. with peak MV between – by Tedds et al. (1999). Observations of them could provide 9 and –14, Kasliwal 2012, Fig. 1). Soker & Kashi (2011) constraints on the physical properties of the clumps. have termed these Intermediate-Luminosity Optical Tran- sients (ILOTs). In the literature ILOTs are interchangeably 4.3. The nature of CK Vul called Intermediate-Luminosity Red Transients or LRNe. The greatly increased distance to CK Vul derived here V838 Mon, M31 RV, and M85OT are all proposed members has dramatic implications for the dust mass and luminosity of this group. It is suggested (Kasliwal 2012) that the less in the bipolar nebula and for the luminosities of the explo- luminous of these outbursts result from mergers of main se- sive events in 1670-71. First, the dust mass in the compact quence stars: “mergeburst” events (Soker & Tylenda 2007). bipolar dusty nebula reported by Eyres et al. (2018) now be- The strongest evidence for a mergeburst is the case of −3 −3 comes 4.3 × 10 M⊙, of which 3.3 × 10 M⊙ lies in the V1309 Sco, where a decaying orbital period prior to erup- −3 diffuse emission, and 1.0 × 10 M⊙ lies in the disk. The tion was clearly detected (Tylenda et al. 2011), indicating present luminosity of the central object, as determined from the components of the binary were spiraling into each other. the spectral energy distribution of the dust is 20 L⊙ (see However, V1309 Sco is estimated to have a peak MV of only Kaminski´ et al. 2015). ∼ −6 mag (although the distance to the object is uncertain). Second, assuming the maximum visual brightness mV in To explain the outbursts of the more luminous ILOTs, al- 1671 was 2.6 ± 0.3 (Shara et al. 1985), and that the visual ternative routes are needed. One suggested channel is elec- extinction AV in the direction of CK Vul at a distance d = tron capture in an O-Ne-Mg core in extreme asymptotic gi- 3.2 kpc is 2.47 ± 0.45 (Marshall et al. 2006), the absolute ant branch (eAGB) stars (Kasliwal 2012). The eAGB stars + − 1.3 are massive AGB stars (5-10 M⊙, Soker & Kashi 2011). Al- magnitude of CK Vul at maximum was 12.4−2.4. It is thus clear that CK Vul underwent a much more luminous sequence ternatively, Soker & Kashi discuss the possibility that such of explosions in 1670-71 than hitherto recognized. At peak outbursts could be related to major instabilities of the en- its absolute magnitude was at least 3 mag brighter than the velopes of eAGB stars leading to ejections of their envelopes. brightest Galactic classical nova, which generally have MV at The focus on eAGB stars arises because the mid-IR progen- maximum in the range –6.5 to –9.5 (see e.g. Bode & Evans itors of two of the nearest extragalactic ILOTs, NGC300- 2012). A few luminous extragalactic novae have been known OT and SN 2008S, are at the extremely luminous and red to approach MV = −10 (e.g. Kasliwal et al. 2011b). ends of the AGB branch (Prieto 2008; Thompson et al. 2009; CK Vul might be considered as an extreme example of a Kasliwal 2012). Other suggested mechanisms are lumi- “” (LRN; Kulkarni et al. 2007). Thus, nous blue variable eruptions, asteroids falling onto white it is of interest to compare its peak nebular expansion ve- dwarfs, accretion-induced collapses, peculiar core-collapse locity of ∼ 2000 kms−1 with those of others of that class, supernovae, and peculiar classical novae (Kasliwal 2012, and such as V1309 Sco, V4332 Sgr and V838 Mon. V4332 Sgr references therein). produced an Hα line with full width at zero intensity ∼ We make no attempt to classify the nature of the CK Vul 550 kms−1 (Martini et al. 1999), a far lower expansion ve- outburst (e.g., non-terminal Luminous Blue Variable, termi- locity than CK Vul. Similarly, the P Cygni profiles in an nal events such as SN 2008S and NGC 300 OT2008-1). Dis- early spectrum of V838 Mon (Lynch et al. 2004) have sepa- criminating between these is challenging even for present rations between their absorption and emission peaks of 540± day events, despite having diagnostic tools such as panchro- 140 kms−1. Recent millimeter/submillimeter-wave observa- matic outburst spectra, archival images to identify the nature tions of V4332 Sgr, V838 Mon, and V1309 Sco taken 22, 14, of the progenitor (e.g., Smith et al. 2010; Adams et al. 2016; and 8 yr, respectively, after their eruptions (Kaminski´ et al. Jencson et al. 2019; Andrews et al. 2020), and in some cases 2018b), reveal molecular emission lines having full widths the presence of a stellar remnant. 6 D. P. K. BANERJEEETAL.

Any proposed mechanism/model to account for the prop- program of NSF’s NOIRLab, which is managed by the Asso- erties of the CK Vul explosion must be able to account ciation of Universities for Research in Astronomy (AURA) for its key features. These include the high peak out- under a cooperative agreement with the National Science burst magnitude; the multiple peaks of the outburst; the Foundation on behalf of the Gemini Observatory partner- sustained brightness of the outburst for a remarkably long ship: the National Science Foundation (United States), Na- period (∼ 700 days; see the reconstructed light curve in tional Research Council (Canada), Agencia Nacional de In- Shara et al. 1985); the high peak expansion velocities ex- vestigación y Desarrollo (Chile), Ministerio de Ciencia, Tec- ceeding 2000 kms−1; the formation and maintenance of jets, nología e Innovación (Argentina), Ministério da Ciência, bubbles, and dust cocoons; and the production of the short- Tecnologia, Inovações e Comunicações (Brazil), and Korea lived radio-nuclide 26Al during its outburst (Kaminski´ et al. Astronomy and Space Science Institute (Republic of Korea). 2018a). DPKB is supported by a CSIR Emeritus Scientist grant-in- aid, which is being hosted by the Physical Research Labo- ratory, Ahmedabad. We thank M. Hajduk for helpful com- ACKNOWLEDGMENTS ments on the manuscript. Facilities: Gemini:Gillett This Letter is based on observations obtained for program Software: Figaro (Shortridge et al. 1992), Gemini IRAF GN-2020B-Q-207 at the international Gemini Observatory, a Package

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