PoS(GOLDEN 2017)064 ). ⊙ 90 days 0.6M ∼ ∼ https://pos.sissa.it/ and CN in the nova predictions based on 2 s relatively small ( hite-dwarf component should be esence of C O > 1. Furthermore, molecule for- ust formation that started rves and optical spectra of the nova, IV bon and nitrogen for this nova, and they has been detected during its early phase , , 2 m and the relatively small ejected mass from µ e Commons al License (CC BY-NC-ND 4.0). ∗ ) according to the observed isotopic ratios and theoretical ⊙ 1.0M ∼ > [email protected] [email protected] thermonuclear runaways. indicates that its atmosphere was enriched in carbon with C/ mation in the early phase is likely to be associated with the d near the visual-brightness maximum in addition to CN. The pr Speaker. Nova V2676 Oph is the first classical nova in which C are consistent with model predictions.the Based inferred on the mass lightcu of the white-dwarf component of V2676 Oph i after the discovery. We have obtained isotopic ratios of car V2676 Oph support this hypothesis. However, thehigher mass ( of the w The absence of strong [Ne II] emission at 12.8 ∗ Copyright owned by the author(s) under the terms of the Creativ c 11-16 September, 2017 Palermo, Italy The Golden Age of Cataclysmic Variables and Related Objects

Hideyo KAWAKITA Koyama Astronomical Observatory, Kyoto Sangyo University Akira ARAI Koyama Astronomical Observatory, Kyoto Sangyo University Motoyama, Kamigamo, Kita-ku, Kyoto 606-8555, Japan E-mail: Motoyama, Kamigamo, Kita-ku, Kyoto 606-8555, Japan E-mail: A Review of the Evolution ofOph: Classical Formation Nova of V2676 Molecules and Dust Grains Attribution-NonCommercial-NoDerivatives 4.0 Internation PoS(GOLDEN 2017)064 20% 2500 Swift e WD ∼ ∼ oscopic sion lines. es (e.g., [8] nova in the , Fe II, and O re essential for β 8000 K) even at tomic molecules on low-resolution ∼ , H st formation began onfirmed as an “Fe Hideyo KAWAKITA maximum (and the ovae have produced α servations confirmed ova. white dwarfs (WDs), [20]. Emissions from es ( ted at its position, and 1 CO molecular-emission ognized in about ocusing in particular on in the range from ]. nated amorphous carbon tional first-overtone band O = 0) by Japanese amateur ervations of the molecules s of H t ic observations of molecular to the carbon monoxide molecule. were performed by the not mation of CO and other molecular es and , CN, and CO molecules — and larger 2 1 in 2012 [50] reported that the near-infrared spectra (from et al. m. Such CO-forming novae generally produce dust grains in their late phas m) taken on UT 2012 March 28 and 30 showed Fe II emission lines together with µ µ -band magnitudes of the nova were measured [38, 39]. Attempts to detect the 5000 K (e.g., [8, 45, 36, 17] and references therein). Spectroscop U ∼ The classical nova V2676 Oph is unique in that C Nova V2676 Oph was discovered on UT 2012 March 25.8 (day Note that “CO” here refers to elemental carbon and oxygen abundanc Classical novae are hot objects. They exhibit high effective temperatur In this review, we summarize the temporal evolution of the photometric and spectr 1 90 days after the nova explosion [36, 51, 34, 46]. Dust formation is rec molecules such as polycyclic aromatic hydrocarbons (PAHs) or hydroge (HAC) — have all been∼ detected in the same nova. In addition, significant du their visual-brightness maxima, when thephotospheric size temperature of is therefore the a nova minimum). photospherehave Nevertheless, been is some detected at simple in dia a several classical novae with excitation temperatures astronomer H. Nishimura [40]. ImmediatelyII after the type” discovery nova, the according nova wasspectroscopic to c observations the [3]. classification The scheme nova of showed narrow Williams emission [67], line based of classical novae [59] and usually occurs in “CO novae” that involve C whereas dust formation rarely occursCO in molecules “ONe have novae” been involving ONe observedaround in WDs 2.3 several novae, usually as the vibra low-excitation emission lines such as C I, N I, O I, and Ca II, in addition to H I emis Many of the H I,that O I, the and nova N was I anbands emission on Fe lines those had II dates P-Cygni (cf., type profiles. [51]). nova.satellite Although These during X-ray ob However, and the there UV declining observations was phaseonly of no the report nova, of no X-ray source was detec radio domain also failed [39]. bands in novae provide information about(as isotopic intermediate abundances, and products obs inunderstanding the the dust evolution formation from in nova atomic outflows. species to dust grains) a K to and references therein). It has thereforespecies been thought is that associated the with for dustdust formation grains without in detectable novae CO, [20], e.g., V1280 even Sco though [7] some and n V339 Delbehavior [53 of V2676 Oph andmolecule discuss formation the physical during evolution the ofcomponent maximum of the V2676 phase. nova, Oph, f and we Finally summarize we the lessons also learned discuss from this the n mass2. of Overview th of V2676 Oph I, with P-Cygni profiles.0.8 Rudy to 2.4 A Review of the Evolution of Classical Nova V2676 Oph 1. Introduction PoS(GOLDEN 2017)064 e ⊙ 0.6 M ∼ 5 − 10 s detected 1934 [35], derived the × es. The mass 10) ed spectrum of = 6.9 – 7.9 kpc d, by Kawakita the assumptions [26]. Therefore, et al. d sed in their study 0.1, together with [47]. The masses frared-wavelength WD of mass 100, in addition to fter discovery was ⊙ Hideyo KAWAKITA pic observations. It ⊙ ± MMRD relation, the ∼ t sufficiently accurate. M t the spectral type of the ecline in the brightness ed class [64]. The rates 8 − ust formation around day = 2.9 view) [20]. V2676 Oph is tem. The nature of the WD 0.07) M V 10 A ± -band at the visual-brightness × during the early-decline phase from their observations during V 1 ) are listed in Table 1. ⊙ − K = 10.6 mag). Raj M based on observational studies (al- determined the total mass of hydro- in 2017 [46] to be 2 V ⊙ d m , and 0.11, from their spectroscopic observa- M 4 − et al. 4 H et al. ± , − recently determined the ejected mass from 10 J , 10 × I × , 0.004 mag day R = 2.35 et al. 2 , ± V V A , B 0.15 by comparing the optical spectrum just after the ± based on the nebular phase spectrum of the nova [47]. They ⊙ 8, as estimated from the Maximum Magnitude Rate of Decline . = 2.65 6 M = 60 – 80 days) and a visual extinction 5 − V − 2 [22]). No other parameters have ever been reported for the binary A t 10 ⊙ -band slope of 0.013 5 to . × V 6 4 . − 1 1.1 M = ∼ in 2015 [37] had obtained . The ∼ 1 − max , V et al. M in 2016 [33], found As shown in Figure 1, the light curves of the nova in the optical and near-in Based on the estimated distance to V2676 Oph, Raj The distance to V2676 Oph was determined by Raj We note that V2676 Oph is similar to the dust-forming nova DQ Her that erupted in . The typical mass of a CO WD is smaller than that of an ONe WD (their masses may b ⊙ 0.01 mag day = 90. The early rate of decline of the brightness between 10 and 80 days a though these facts are not fullyconsidered understood as from a the CO theoretical nova point from of the viewpoint of ejected mass. while those of ONe novae are in the range (1 – 4) A Review of the Evolution of Classical Nova V2676 Oph visual-brightness maximum (which is dominateda by supergiant. absorption) with the synthesiz of the ionized ejecta from typical CO novae are estimated to be in the range (1 – indicates that V2676 Oph can beof classified decline to for the “slow” the or other “very photometric slow” bands spe ( distinguished by based on the slow evolution of its brightness and the dust formation at day M tions, using the same method.et al. Another determination based on a different metho V2676 Oph also probably involves a low-mass WD component, probably a CO the molecule formation just afterin the both visual-brightness novae). maximum Based on (the the CNof “nova molecule V2676 wind” Oph theory wa [24], and the DQ similar Herof rates indicate the of that WD d these component novae of involve WDs DQ with Her similar has mass been determined to be (0.60 secondary star, the orbital period, and inclinationcomponent angle of of V2676 the Oph binary is sys discussed further in the following sections. system hosting V2676 Oph (e.g., the masses of the WD and secondary star, the early-decline phase [46]. Furthermore, Raj V2676 Oph to be regions are characterized by (1) a slow evolution in brightness and (2) d also estimated the dust mass formed in V2676 Oph was (2.1 – 2.7) gen in the ejecta from V2676 Oph to be (0.27 – 3.3) ∼ maximum ( (near the Galactic center), based on the absolute magnitude in the visual extinction by using the “Balmeris decrement” noted method and that their there spectrosco are twomay be issues inaccucate to as be pointed re-examined. outvisual First, some extinction the studies determined [42, MMRD by 55, relation 24]. the u In “Balmerabout addition recombination decrement” to of method the hydrogen (which atoms requires andNagashima physical conditions) may be no the apparent magnitude at the visual-brightness maximum ( (MMRD) relation [12] with t PoS(GOLDEN 2017)064 140 I Hideyo KAWAKITA 1200 J R H K B V 120 1000 -bands) of V2676 Oph up to 150 days K 100 overy. The photometric data are taken from , and H 800 TS/NovaAtlas/atlas.html). , J , I , 80 R 3 , V , B 600 60 Days from discovery Days from discovery 400 40 200 20 I J H K R B V Multi-wavelength lightcurves (in the 0 0

6 8 4 8 4 6

18 20 22 12 14 16 10 18 14 16 10 12 Magnitudes Magnitudes Figure 1: A Review of the Evolution of Classical Nova V2676 Oph (upper panel) and up to 1300 days (lower panel) after its disc SMARTS [63] (http://www.astro.sunysb.edu/fwalter/SMAR PoS(GOLDEN 2017)064 . 2 = 10 uclear t the system at day tected simul- lly thin wind = 2 – 13 [32]. 1 − t to in 2014 [23]. ands lasted about l-brightness max- 10. Near-infrared Hideyo KAWAKITA ealized for WDs of ∼ covery, the emission km s es are lower than the er just after its visual- ecoming weaker near t ] and V445 Pup [27]. did not find any other ection 2. ar its visual-brightness y several observers around g this period [50]. After ] 1 − d bands during the early-decline , during days and CN were detected on two 1 2 − = 12 (when almost no emission lines 0.004 0.004 0.004 0.004 0.004 0.004 0.004 t ± ± ± ± ± ± ± 400 km s ∼ 0.006 0.013 0.010 0.010 0.013 0.009 0.010 emission indicated a decrease in the expansion to molecule has never been detected previously in 1 2 4 α − = 13 and 14, together with a modeled spectrum of C t = 15 – 19. The CN violet-system absorption may also have 750 km s = 14, together with the spectrum of a typical carbon star (TX t ∼ t I J B R K V H Photometric band Decline rate [mag day = 13 and 14 [36, 32]. The C [23, 31]. It is therefore likely that V2676 Oph was powered by a thermon t ⊙ = 10 – 80). Swan-band absorption and the CN “red system” band absorption were de t 2 Rates of decline of V2676 Oph in the optical and near-infrare = 12, the emission lines began to strengthen again (Figure 2). The C Low-resolution optical spectroscopic observations were performed by These changes in the emission lines observed in V2676 Oph around its visua t taneously at days brightness maximum, and absorption due toone both the week CN [69, violet- and 54, red-system b 58, 6, 2, 56]. In the case of V2676 Oph, C other classical novae, whereas the CN molecule was detected clearly in DQ H Psc). Figure 4 shows the spectra at days The excitation temperatures on those dates were 4500 – 5000 K. Those valu Figure 3 shows the spectrum at day been detected (but not well-confirmed) in other novae, e.g., in GK Per [70 nights only, but they mayspectroscopic have observations been for present days for as long as one week. We the visual-brightness maximum of V2676lines Oph were [36, narrow, 32, and 46]. the P-Cygni Just profile after of its H dis runaway (TNR) on a WD of relatively small mass, as already discussed in S imum can be explainedThey by hypothesized a that model for themaximum, slow internal from novae structure a proposed of static by afirst configuration Hachisu begins slow to & to nova a Ka blow changes during “nova ne thethe wind” static visual-brightness configuration. configuration maximum. just Finally, The after anenters the optica optically explosion, a “thick” b “nova wind wind” starts configuration. tomass blow 0.5 Note – after 0.7 that M such a static solution can be r 3. Near Visual Maximum: Molecule Formation in V2676 Oph were seen in the observed spectrum). The visual maximum occurred at da Table 1: A Review of the Evolution of Classical Nova V2676 Oph phase (days [46]. The emissions became progressively weaker until day spectroscopic observations also reported weakenedday emission lines durin The P-Cygni absorption profile of Na I indicated an outflow velocity of 600 velocity of the nova outflow, from A Review of the Evolution of Classical Nova V2676 Oph Hideyo KAWAKITA 6563 4861 β α Fe II (38) 4523 Fe II (37) 4556 Fe II (38) 4584 Fe II (37) 4629 Fe II (43) 4731 C I 4762,4772 H Fe I 4668 Fe II (42) 4924 Fe I 4985 Fe II (42) 5018 C I 5040+5052 Fe II (42) 5169 Fe II (49) 5198 Fe II (49) 5235 Fe II (49) 5276 Fe II (49) 5317 Fe II (48) 5363 Fe I 5404 Fe II 5428 Fe II (55) 5535 [O I] 5577 Fe II 5588 Sc II 5659 N II 5680 Fe I 5706 Fe I 5753 He I 5876 Na I 5890+5896 Fe II (46) 5991 Fe I 6009 Fe II (74) 6149 Fe II (74) 6248 [O I] 6300 Si II 6347 [O I] 6364 Si II 6371 Fe II (74) 6456 N I 6484 Fe II (40) 6516 H He I 6678 N I 6723 O I 7002 He I 7065 C I 7115 [O II] 7325 Fe II (73) 7469 O I 7774 Mg II 7890

May 26.7

May 23.7

May 22.6 PoS(GOLDEN 2017)064May 18.6 May 04.8

Apr. 28.7 Normalized flux

Apr. 27.7

Apr. 26.8

Apr. 24.8

Apr. 18.7

Apr. 16.7

4500 5000 5500 6000 6500 7000 7500 8000 Wavelength [Å] 6563 4861 β α Fe II (38) 4523 Fe II (37) 4556 Fe II (38) 4584 Fe II (37) 4629 Fe II (43) 4731 C I 4762,4772 H Fe I 4668 Fe II (42) 4924 Fe I 4985 Fe II (42) 5018 C I 5040+5052 Fe II (42) 5169 Fe II (49) 5198 Fe II (49) 5235 Fe II (49) 5276 Fe II (49) 5317 Fe II (48) 5363 Fe I 5404 Fe II 5428 Fe II (55) 5535 [O I] 5577 Fe II 5588 Sc II 5659 N II 5680 Fe I 5706 Fe I 5753 He I 5876 Na I 5890+5896 Fe II (46) 5991 Fe I 6009 Fe II (74) 6149 Fe II (74) 6248 Si II 6347 Si II 6371 Fe II (74) 6456 N I 6484 Fe II (40) 6516 H He I 6678 N I 6723 O I 7002 He I 7065 C I 7115 Fe II (73) 7469 O I 7774 Mg II 7890

Apr. 14.8

Apr. 08.7

Apr. 07.8

Apr. 06.7

Apr. 04.7

Apr. 03.8 Normalized flux

Apr. 01.8

Mar. 28.8

Mar. 28.7

Mar. 27.8

Mar. 27.7

4500 5000 5500 6000 6500 7000 7500 8000 Wavelength [Å]

Figure 2: Optical spectra of V2676 Oph during days t = 2 – 62, modified from Figure 5 of [32]. 5 PoS(GOLDEN 2017)064 and 676 2 -band V ? Where e 1)[43]. absorption 2 8000 K[16]. > otosphere had ∼ OI uter envelope? ximum (C Hideyo KAWAKITA ing may have been 750 res by assuming a hy- ature, indicating an in- v=+3 ximum, ∆ n of a hydrostatic atmo- uring the near-maximum are comparable to the CN = 12 and 14, respectively) and bon-rich (C/O ric temperatures were about t 700 12 mag), and the brightness reached Fe II ∼ CN α V 1.5 mag. There was no significant change H v=+4 650 ∼ ∆ V m 5000 K). Furthermore, the strong C ∆ 6 =3–8( ∼ t 10 (Figure 5). As already noted in the previous sec- ∼ 600 t v=+5 NaI ∆ Wavelength [nm] 550 v=-1 ∆ 10 after brightening by ) color index determined from their photometric observations. After cor- TX Psc (a carbon star) I 2 ∼ − C v=0 t 500 ∆ V β H in 2016 [33] investigated the evolution of the photospheric temperature of V2 Nova V2676 Oph on UT 2012 Apr 6 (upper) and Apr 8 (lower) Nova V2676 Oph on UT 2012 Apr v=+1 et al. ∆ Normalized spectra of V2676 Oph on UT 2012 Apr 6 and 8 (days 450 0

What was going on during the near-maximum phase that led to molecule formation

0.5 1.0 2.5 2.0 1.5 4.0 3.5 3.0 Normalized intensity Normalized tion, the brightness changed slowly up to the time of visual maximum. For example, th magnitudes were almost constant during days the maximum at day in the photospheric temperature around the maximum. These facts imply that the ph expanded radially without a significant increasecrease in in the the photospheric temper luminosity of V2676 Oph. The energy source for this brighten drostatic stellar photosphere appropriate forsphere a is not supergiant. too bad The because assumptio thephase evolution (e.g., of V2676 as Oph is was the very7000 slow case d K for until a the Cepheid visual variable)[33]. maximum The at photosphe day recting for interstellar reddening, they obtained the photospheric temperatu Oph based on the ( did the molecules formKawakita in the nova envelope: near the photosphere or in the o of a carbon star (TX Psc), modified from Figure 3 of [36]. Figure 3: A Review of the Evolution of Classical Nova V2676 Oph typical temperature of nova photosphere around the visual-brightness ma bands that have never beenabsorption bands, observed probably previously indicate in that the novae, nova and envelope gas which was car V2676 Oph was thus cool enough to form diatomic molecules near the visual ma CN molecules can form at temperatures below PoS(GOLDEN 2017)064 - e re the [32], indica- lecules. The 1 , there were a 580 − ormation of the and Fe I (which rly-decline phase is larger than that ts of gas ejected as Hideyo KAWAKITA guration to a “nova O molecules during (with enhanced CNO brightness maximum, = 5000K) = 4500K) ey pointed out that non- ge radial extension of — ex ex 450 km s lly thin wind had already T T ∼ 560 excitation temperatures were ted the hard radiation from ( ( 2 2 2 C C C . The 2 540 in 1994 [25] discussed the formation of and CN began. The cooling timescale ( a of C 2 et al. 7 520 9 days [33], which is comparable to that due to radiative Wavelength [nm] ∼ 500 in V2676 Oph, modified from Figure 7 of [32]. The dotted lines a 2 = 13 and 14, respectively. t 480 4500K on days ∼ = 14, when the molecular formation of C 460 t The Swan bands of C V2676 Oph on UT2015 Apr 7 V2676 Oph on UT2015 V2676 Oph on UT2015 Apr 8 on UT2015 Apr V2676 Oph = 37 and 38) [51]. Unfortunately, no spectroscopic observations of C t or CN (see Table 2), the CO molecules may have formed earlier, before the f 1 2

Based on their model atmospheres, Hauschildt The photospheric temperature of V2676 Oph decreased after the visual 2

and CN molecules. The UV continuum absorption by neutral atoms such as C I Normalized intensity Normalized 0.5 1.5 2.5 5000K and 2 CN in nova atmospheres theoretically, especially for theLTE effects case in of nova DQ atmospheres Her. are Th large andand very the important huge due temperature to the gradient lar in — the nova photosphere. Their models was seen in the optical spectrathe as nova line photosphere. absorption [33]) This probablyCO may preven have molecule promoted was the successfully formation(days detected of in those V2676 diatomic mo Oph, but only during the ea C the near-maximum phase have been reported so far. cooling by CO molecules [14].of Because C the binding energy of the CO molecule folding time) during this period was observed spectra, and the solid lines are the modeled spectr ∼ up to day Figure 4: A Review of the Evolution of Classical Nova V2676 Oph supplied by a transition inwind” the internal configuration, structure as of noted the byfew nova weak Kato from emission a & lines, static and Hachisu the confi in expansion velocity 2009 was [31]. still as slow At as the same time tive of slowly expanding gasceased around by the that nova time, photosphere. the Ifan observed the optically emission optica thin lines wind may during have an arisen earlier from phase. remnan PoS(GOLDEN 2017)064

ratures

[K] T temperature: Effective molecules eff have been ob- 2 sphere at that time. Hideyo KAWAKITA 12000 11000 10000 9000 8000 7000 6000 5000 4000 3000 ways been observed detected in emission an the typical photo- tures of CO molecules es of the CO molecules ctron temperatures may -decline phase. This fact h of molecule formation eyond the photosphere at 14 e CN absorption bands in the during its near-maximum phase, 12 in novae [56, 32], as well as those of 2 5000 K). On the other hand, the CO vi- ∼ (and probably CN) in V2676 Oph are com- < 2 10 8 8 in V2676 Oph were concentrated near the photosphere 2 8000 K). Otherwise, if the CO molecules were mainly excited 6 ∼ Days from discovery > 4 Swan band [K] 2 2 from corrected (V-I) [K] from the KAO spectrum [K] from the SMARTS spectrum [K] from C 4000 K) in the line-forming region of the nova atmosphere, and such low tempe eff eff Effective temperatures of the nova photosphere of V2676 Oph eff ex Apparent V magnitude (AAVSO) T T T T ∼ 0

Finally, we note that — in novae — the molecular bands of both CN and C

16 14 15 12 13 10 11 Magnitude in V-band in Magnitude parable to the effective temperature(Figure of 5), the it is nova likely photosphere that at both the CN epoc and C brational bands were observed in emissionindicates for that V2676 the Oph distribution during of its CO early Even molecules during extended beyond the the near-maximum nova phase, photo in the some vibrational novae band (e.g., of V705 CO Casthat has [14]), time. been and If it the probably COwould already have molecules extended cooled were b down collisionally the excited, outerobserved radiative envelope. in cooling Indeed, novae by the are excitation the tempera in CO spheric the temperatures range of of novae ( 2500 – 4500 K, significantly lower th are consistent with the excitation temperatures of CN and C served in absorption, while thein vibrational emission. bands Because of the the excitation temperatures CO of molecule C have al when it was cool enough for those molecules to form ( the CO molecule (e.g., [8]). modified from Figure 5 of [33]. Figure 5: A Review of the Evolution of Classical Nova V2676 Oph abundances relative to the solaroptical abundances) wavelength reproduced region not but also only the th infraredbe CO low emission ( bands. The ele in novae may be explained by dilution of the radiation field. by radiation from the central nova, the relatively lower excitation temperatur PoS(GOLDEN 2017)064 α 0.7 600 with ∼ − 1000 km t at line (from − rthermore, such eased. The H ly-decline phase. about Hideyo KAWAKITA cities decreased from s mentioned in Section also observed in DQ Her amine this hypothesis in metimes been recognized a envelope. ould be interpreted as a re- the optical spectra [46] with = 34. The absorption component s and the wavelengths necessary for t = 91) [46]. Figure 6 shows the change in the = 32, whereas the line profile was narrower t 33]). t in 2012 [51] detected CO molecules in V2676 -band lightcurve exhibited fluctuations of V (day 9 1 = 26) [46]. After that, an absorption component was − et al. t (day energy [eV] wavelength [nm] potential [eV] wavelength [nm] 8.64.5 143.6 276.9 1 − 1100 km s − 2 2 660 km s Molecule DissociationCO Equivalent C 11.1 111.8 Atom IonizationH IHe I Equivalent C I 13.6N I 24.6O I 11.3Fe I 14.5 91.2 13.6 50.4 7.90 110.1 85.3 91.0 156.9 CNH 7.7 160.0 in the P-Cygni profiles of Fe II on day − 1 − = 20 [32]. The P-Cygni profile of Na I showed an absorption componen t 90), as shown in Figure 1. Such re-brightening events may be associated in 2017 [46] pointed out that the = 11.5 until about 70 days after the outburst, before beginning a slow dec = 22) and ∼ 950 km s t V − = 67) and then to about Wavelength limits for continuum absorption by neutral atom et al. t (day 70 to 1 ∼ − Raj During the early-decline phase, the expansion velocity of V2676 Oph incr As noted in the previous section, Rudy (day 1 − mag around km s emission line was systematically broader after day Table 2: A Review of the Evolution of Classical Nova V2676 Oph until at least day the re-appearance of P-Cygni profiles,the as optical can lightcurves. be seen Suchexpansion by re-appearance comparing of of the the P-Cygni novafuture profiles photosphere studies c based [60, on 61]. a sophisticated physical However, model we of expanding have nov to ex observed at photodissociation of selected molecules (after Table 2 of [ 4. Early-Decline Phase days of the P-Cygni profiles of the Balmer emission lines accelerated to a velocity of expansion velocities of the nova outflowthe in pre-maximum V2676 to the Oph. maximum The phase,Such expansion and changes velo then in increased the again absorption during components the[1, of ear 35], the P-Cygni which profiles is were one2. of V2676 the Oph slow novae. andacceleration DQ of It Her the also absorption are showed lines quite CN duringin similar absorption, the high-resolution to early-decline a spectra phase each of has novae other so (e.g., in [68, those 4]). respects. Fu s PoS(GOLDEN 2017)064 ) was about K 100 − brightness. Both ion was optically J Hideyo KAWAKITA ence of CO emission is temperature range is lts in thermal emission ved in a nova [51]). The proximately seven times 6 Oph. The fundamental xpanding nova envelope, 80 y dust grains at optical wave- h [32, 46]. The expansion velocity = 90. Furthermore, photometry t (corresponding to the maximum expansion m) may explain the relatively shallower 1 µ − ated after the visual-brightness maximum (day r phase; see Section 6). 60 2.3 ∼ bands) of V2676 Oph increased suddenly at day 10 750 km s K − = 37 and 38). They reported the detection of CO -band brightness), the color index ( t and K = 10 – 80 (see Table 1). H t 40 Days from discovery 20 = 93 (at the peak of the t -band lightcurve for days K Expansion velocities of the nova outflow observed in V2676 Op 0 -band (the first-overtone band of CO is at 0 K

The near-infrared brightness (in the -600 -800 -200 -400

1, corresponding to a blackbody temperature of 1500 – 2000 K [46]. Th 90 [46]. At the same time, the visual lightcurves started to decrease rapidly in 10). The horizontal dotted line corresponds to

-1200 -1400 -1000 . Expansion velocity [km s ] s [km velocity Expansion -1 3 ∼ ∼ behaviors in the lightcurves are associatedfrom with dust grains dust at formation, near-infrared which wavelengths resu lengths. and visual On extinction day b − slope of the decelerated during the pre-maximum phase, while it acceler consistent with dust condensation in the nova outflow around day band is much weaker thanthick the in the first-overtone fundamental band, band. The indicating peakstronger that of than the the the CO CO first-overtone underlying band continuum emiss is (this ap strong is CO the emission highest indicates ratio the ever release obser ofwhich considerable promotes energy from the the cooling e of thein gas the and results in dust formation. The pres emission from the fundamental and the first and second overtones in V267 Figure 6: A Review of the Evolution of Classical Nova V2676 Oph Oph during the early-decline phase (days velocity obtained from the emission lines during the nebula 5. Dust Formation t t PoS(GOLDEN 2017)064 < tometric = 452 and t 13 form when C/O ared spectroscopic of grains in V2676 Hideyo KAWAKITA . tion for grain forma- D. ]. This emission may ase, the observed UIR PAH molecules cannot to exist simultaneously in 12 mation, at days UT 2013 June 20.4 UT 2014 May 16.5 = 452 and 782, modified from Figure t in 2017 [34] also reported the absence 11 nated by telluric ozone absorption around 9.6 m which is sometimes observed in dust- m] µ = 164 and 380, which was reported in [62], 1 (all oxygen atoms being exhausted by the ectra at each epoch. The spectroscopic data used µ t et al. > 11 10 Wavelength [ m, which is usually identified in the late phase of ONe novae. µ m are eliminated for the fit. The filled blue circles are the pho µ 9 8 Mid-infrared low-resolution spectra of V2676 Oph at days -19 -18 -17

10 10 10

Direct evidence for the existence of dust grains was obtained by mid-infr The unidentified-IR (UIR) emission line at 11.4

µ Flux density [W/cm / [W/cm density Flux m] 2 m and by UIR emission at 11.4 µ 782 [34]. Both amorphous carbonV2676 grains Oph and silicate (Figure grains 7). weretion, found According only to carbon-rich the grains traditional form paradigm in of gas with CO C/O satura formation of CO molecules), and only silicate grains (containing oxygen atoms) and photometric observations of V2676 Oph, carried out after dust for 1 (all carbon atoms being fixedOph in indicates CO non-thermal-equilibrium molecules). conditions for The the existence dust of formation both [20] types of strong [Ne II] emission at 12.8 be due to free-flying PAHsurvive molecules in or the HAC strong grains. radiation fieldfeature Because from may free-flying the be central due nova during to its HAC later grains ph [13]. Kawakita This fact strongly supports the conclusion that V2676 Oph involves a CO W measurements. forming novae (e.g., V705 Cas [15]), was also detected in V2676 Oph [34 also confirms the presence of dust grains. for the fit are shown in green; the wavelength regions contami 1 of [34]. The black solid lines represent the synthesized sp Figure 7: A Review of the Evolution of Classical Nova V2676 Oph in the near-infrared wavelength region between days PoS(GOLDEN 2017)064 t 1.6 750 sian ± ± , with 1 − ). The line 1 e observed), − re 9. Although coefficients for ents (which have A 185 km s ystem is 86.5 . Alternatively, the Hideyo KAWAKITA thus originate from nent of the same [S lines is about energy level). Since − le of [S III] at 953.1 [S III] are consistent jecta, viewed nearly try of the nova ejecta gnized in other weak 1 wind that collides with − vel, their emission-line equatorial plane of the 200 km s N II] at 658.3 nm. The shown in Figures 8 and ]) is consistent with the cement of the brightness of V2676 Oph. Figure 9 − , and and cool environments are p in the optical lightcurves , their profiles are basically ach profile is normalized at 1 − quatorially concentrated ring . Figure 8 shows the emission- 1 200 km s − . Although the emission lines of [S ±∼ 1 21 km s − − , 1 − , respectively. emission line at 656.3 nm, we extracted the 1 − 200 km s α 12 coefficients for the respective transitions). We also ±∼ A 90 degrees for the binary system hosting the explosion and ∼ 1 − , and 367 km s 1 − 0km s ∼ , 149 km s in 2017 [11] recently proposed that dust formation in novae occurs in the 1 in 2017 [46] reported a medium-resolution spectrum of V2676 Oph at day − 3). We fitted the profile with three Gaussian components. As shown in Figure ∼ et al. et al. , and each emission profile could be fitted with a combination of two or three Gaus 1 − Derdzinski In the nebular phase of a nova (during which the optically thin emission lines ar The double-peaked profiles seen in the [N II] emission lines are also reco of V2676 Oph). The two velocity components that peak at degrees, close to the line-of-sight9 [26]. are also The consistent lineedge-on profiles with (i.e., of an at V2676 expanding an Oph ring inclination of angle of equatorially concentrated e dense, cool environments created by radiative shocks. A dense and e of ejecta from the nova explosionthe is slower followed ejecta, by a producing faster shocks and insuitable isotropic the nova for equatorial plane. dust Such formation. dense binary Therefore, system associated if with our the line-of-sight nova, weproduced is by expect close to the strong observe to a extinction, the and deep simultaneouslyin dro we the expect infrared-wavelength an enhan region.dust-forming This nova picture DQ Her, (see in also which the Figure inclination 1 angle of of the [11 related binary s profile of [S III] at 906.9nm nm may be is affected similar by to strong thatblue telluric of component absorption [N lines of II], in the but this [S the wavelengthIII] III] region emission line at profi due, 953.1 e.g., nm to may a be difference in weaker opacities. than the red compo those transitions ( intensity ratio of those two [N II] lines is consistent with the ratio of Einstein’s blue-side profile of the [N II] at 654.8 nm and the red-side profile of the [ FWHMs of 302 km s 8, the resultant components are centered at 213 km s A Review of the Evolution of Classical Nova V2676 Oph 6. Nebular Phase III] at 906.9 nm and 953.1profiles nm are are associated slightly with different the for same the upper energy blue le components (peaked at about line profiles of [N II] (atparts 654.8 of and 658.3 those nm, emission associated lines with the overlap same with upper the H = 1138, for which the instrumental resolution was about 100 km s components, at velocities of km s the shapes of some emission lines provide(e.g., useful [21]). information about Raj the geome emission lines (such as [Ocompares I] the and emission [S lines of III]) [N inits II], peak the [OI], (before nebular-phase and normalization, spectrum [S the III]. intensitywith In ratios the this of figure, corresponding the e ratios doublet of of [O Einstein’s I] and shifted the wavelengths of the [O I] and [S III] lines to align their red compon positive velocities) with the redthe peak S/N of ratios the for [N the [O II]consistent I] line with and in [S one the III] another. velocity lines The axis are not Full of as Width Figu good at as Zero for Intensity [N (FWZI) II] of the PoS(GOLDEN 2017)064 nova explosions bundances of the egion of the nova Hideyo KAWAKITA 1000 tar (as the secondary mponent, which may t of carbon relative to cause matter accreted The measured FWZIs e 6). The red (far-side) may correspond to gas the surface materials of 1) [36], based on theoret- 1 er than carbon-rich ejecta, − > e. At least two velocity compo- 0 km s 500 ∼ [NII]@658.3nm x 1.00 [NII]@658.3nm 3.16 [NII]@654.8nm x btraction of a baseline, fitting with Gaussian , comparable to the expansion velocities -1 1 sian functions (see text). − . The original data was provided by Raj (2017, private 1 13 0 − Velocity [km s ] 750 km s ± and CN (along with CO) in V2676 Oph is interesting, because it 2 -500 -1000 The [N II] emission lines of V2676 Oph during the nebular phas

0 2 1 3 The detection of both C Relative intensity [arbit. units] [arbit. intensity 0.5 Relative 1.5 2.5 3.5 communication) and processed by theprofiles, authors, etc. e.g., The solid by and the dashed su lines are the best-fit Gaus the near-side and far-side ejecta (the velocity component at moving in directions perpendicular to the equatorialof plane, as the in bipolar forbidden jets). emission lines are about 750 km s nents are prominent. The FWZI is about ical calculations of chemical evolution inoxygen nova outflows in [43]. V2676 The Oph enrichmen maystar) have to originated the in WD in mass the transfernova binary from envelope system must a associated also with carbon have V2676 been s Oph. affectedfrom by The the the elemental composition a secondary of star the (even WDthe be if WD it by diffusion has before solar-like the TNR abundances)based ignited mixes on on with the TNR WD. theory Traditional indicate simulations of that oxygen-rich ejecta are expected, rath indicates that the atmosphere of the nova envelope is carbon-rich (C/O measured during the pre-maximum phase and thecomponent early-decline of phase [N (Figur II] emissionbe is caused slightly weaker by than extinction the producedenvelope. blue by (near-side) dust co grains that remain in the emission r 7. Lessons Learned from V2676 Oph: Is the WD mass small or large? Figure 8: A Review of the Evolution of Classical Nova V2676 Oph PoS(GOLDEN 2017)064 n t time the 676 Oph is also s [5, 22, 10, 30]. inted out by Star- the secondary star d. Hideyo KAWAKITA these ratios in nova the WD component 1000 s that differ strongly ova explosions (and of iscrepancies may be due t simultaneous determination [S III] (normalized at their peaks) [OI]@630.0nm [OI]@636.4nm [NII]@654.8nm [NII]@658.3nm [SIII]@906.9nm [SIII]@953.1nm r [N II] (the same as in Figure 8, but 500 ) and the top profile is for [S III] (with an offset -1 f the three Gaussian components fitted to [N II]. 14 1 for the outer layers of CO WDs [30]. Therefore, 0 ) ONe WDs (e.g., [28, 10]). However, prominent [Ne > C ratio has been determined in several novae from ob- ⊙ 13 Velocity [km s ] C/ 12 1.35 M ∼ (consistent with a CO WD), from the viewpoint of the photometric 2) in V2676 Oph, based on the profiles of the molecular-absorption -500 ⊙ ∼ N 15 in 2015 [32] have determined the isotopic ratios of carbon and nitrogen m was not observed in the mid-infrared spectra of V2676 Oph (see Sectio 0.6 M µ N/ ∼ 14 et al. and CN observed in the nova. These observations determined for the firs 4 and -1000 2 Comparison of the emission-line profiles of [N II], [O I], and ∼ C N in a classical nova, while the

13 0 2 1 3 Kawakita 15 Normalized intensity Normalized 0.5 1.5 2.5 3.5 -0.5 C/ N/ 12 II] emission at 12.8 bands of C and spectroscopic evolution of theto nova incorrect (see assumptions Sections employed 2 in and therfield simulations 3). in of 2008 These nova [57], explosions. d thefrom As outer unity, po regions in of contrast to the the cores usualRecent of assumptions stellar-evolution CO employed models in WDs predict traditional have C/O simulation C/Othe ratio carbon-rich ejecta from V2676has Oph a may solar-like originate from composition. athe Accordingly, CO prior further WD, stellar theoretical even evolution leading if studies to of the n formation of WDs) are to be encourage 5), indicating that V2676 Oph involves aconsistent CO with WD. a The CO estimated nova ejecta (see massmay Section of 2). be V2 Furthermore, as the small estimated as mass of Figure 9: A Review of the Evolution of Classical Nova V2676 Oph ( during the nebular phasenormalized), of the middle V2676 one Oph. is for [O The I] (with bottom an profile offset of is 1.0 fo of 2.0). The vertical dashed lines indicate the velocities o except for a few cases with heavy ( of the isotopic ratios of both carbon and nitrogen have allowed us to compare servations of the CO first-overtone vibrational band (Table 3). The firs 14 PoS(GOLDEN 2017)064 . , probably ⊙ e investigated ic ratio of car- ains). According V2676 Oph with e relatively small 0.6 M Hideyo KAWAKITA (for either CO WD ∼ nt of V2676 Oph is less photometric and ther hand, strong CO ⊙ t phase of V2676 Oph e WD mass for V2676 theory. Thus, the mass evolution of the optical rved emission spectrum of carbon isotopic ratios ary in order to explain the en proposed theoretically rt of the hypothesis that our N ratio in V2676 Oph can be 15 N/ , CN [32] 2 14 N ratios may have originated in the 15 N/ N Remarks N ratios observed in novae. 14 15 15 2 CN [56] 2 C N/ N/ N ratios. Thus, the spectroscopic observations ∼ ≥ 14 N is produced in TNRs with higher temperatures 14 15 15 15 N/ C N ratios found in V2676 Oph are similar to those of 0.3 CO [48] 0.4 CO [65, 66] 14 C and 13 C and low 15 ± ± 13 21.5 CO [9] CO [44] 5 CO [14] 1.5 3 CO [18] 13 1.5 CO [8] 4 1.3 CO [49, 52] C/ C ratios seem too small in comparison with the observed N/ C/ ∼ ∼ > ≥ ∼ ≥ 12 ≥ > C/ 13 14 12 12 C/ 12 C and low C and could be responsible for the observed isotopic ratios in V2676 Oph. 13 13 ⊙ C/ C/ 12 Summary of 12 V5668 Sgr V2676 Oph V2615 Oph V496 Sct V705 Cas V2274 Cyg 1.2 NQ Vul V842 Cen 2.9 Nova DQ Her and CO are critically important for checking the accuracies of those values 2 Table 3: N ratio, because more abundant 15 N/ 14 C ratio in V2676 Oph (and also compared to the values found in pre-solar gr We can also compare the observed isotopic ratios of carbon and nitrogen in 13 C/ pre-solar grains with low reproduced by WDs having relatively heavy masses such as 1.0 – 1.25 M of V2676 Oph have provided the first observational evidence in suppo solar system consists in part ofemission materials bands formed were in successfully classical detected novae.of in On CO V2676 the Oph (Figure o [51]. 10)bon, seems The for obse good example, as enough in to thedetermined enable from case C of the V2274 determination Cyg of [48]. the Future comparisons isotop ejecta with those of “pre-solar” grainsthat found some in meteorites pre-solar [41]. grains It with has low be theoretical predictions [29, 10]. Based on [29], the observed Basically, higher masses for the WD component of V2676 Oph are necess observed TNR of novae [29]. The or ONe WD), while the predicted 12 to [10], a CO WD with 1.15M a CO WD. Theejected absence mass of of [Ne V2676 II] Oph supportOph emission this presents in hypothesis. an the This interesting mid-infrared discrepancy challengeof spectra in for th the and nucleosynthesis WD th based component onfrom of TNR the V2676 viewpoint Oph of is nucleosynthesisconsidered still in as unknown classical CO (Table novae. WD 4) based The andspectroscopic on WD remains monitoring the compone observations to observations, of b and the its binarycan be mass system performed during is (e.g., the unclear DQ quie un Her [26]). A Review of the Evolution of Classical Nova V2676 Oph and longer durations [29].lightcurves As of discussed V2676 in Oph Section indicates 2, a low-mass however, WD the component slow in this nova, PoS(GOLDEN 2017)064 s r gratitude to ), equatorially 1 blication of his sics) for provid- − ova originated is ust formation. In everal kinds of di- Hideyo KAWAKITA s observed during the or discovery and pho- d also thank Dr. Richard 750 km s h on UT 2012 May 1 (Rudy ∼ from the optical lightcurves C ratios were determined si- ⊙ 13 and CN. Those ratios are similar C/ 2 12 0.6 M ∼ N and 15 N/ 14 16 from the observed isotopic ratios of carbon and nitrogen). The ⊙ 1.2 M ∼ , CN, CO) were detected during the near-maximum and early-decline phase 2 Emission spectrum of the first-overtone band of CO in V2676 Op The authors would like to thank Dr. Ashish Raj (Indian Institute of Astrophy The dust-forming nova V2676 Oph is a unique classical nova, because s ing us the optical spectra ofRudy V2676 for Oph providing published us in hisdata. the recent CO We paper, are an emission also spectrum gratefultometric/spectroscopic of to monitoring V2676 many of Oph amateur prior astronomers novae. observing to novae Finally the f pu we would like to express ou Acknowledgments concentrated ring of ejecta and a bipolar outflow. geometry of the nova ejecta has been inferred from the emission-line profile nebular phase of V2676 Oph; it is consistent with a slowly expanding ( and spectra, in contrast to multaneously from the molecular-absorption-band profiles of C to the ratios found inconsidered some as type CO of WD, although pre-solar its grains. mass The is still WD unknown from ( which the n of the nova, andparticular, HAC for grains the (or first PAH time molecules) for were novae, both also the detected after d atomic molecules (C Figure 10: A Review of the Evolution of Classical Nova V2676 Oph 2017, private communication). 8. Summary PoS(GOLDEN 2017)064 SvA , and CN 2 (2002) 685 star from the (2012) 2 h. ⊙ 330 Hideyo KAWAKITA Herculis 1934 3072 orts to find C MNRAS CBET , , ] ⊙ Radial Velocities from 0.6 [23, 26, 22] ∼ [M Evolution of a 3-M diffusion (1935) 205 Wilson, nuto, 47 PASP , Origins of Absorption Systems of Classical Nova 17 (2016) 30 830 , 2 1 ∼ ApJ − , N m in 15 1 µ ⊙ N/ > Nova Ophiuchi 2012 = PNV J17260708-2551454 M 14 5 Mass and other characteristics of the WD component of V2676 Op 4, − ∼ 10 -band, indicating An Analysis of the CN Absorption Bands in the Spectrum of Nova 0.04 mag day C V × ± 4 13 . Table 4: 1 C/ in the a slow nova the early-decline phase: 0.013 (3) Total ejected mass:∼ (4) No significant [Ne II]emission Ok at 12.8 Ok(5) Carbon-rich ejecta of Xthe nova: C/O X Ok(6) Low isotopic ratios ofboth ? carbon and nitrogen: 12 Ok Ok ? 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