On the Progenitor System of Nova V2491 Cygni
aa r X i v : . [ a s t r o - ph . S R ] A p r Astronomy&Astrophysicsmanuscript no. 16038 c (cid:13)
ESO 2018August 13, 2018
On the Progenitor System of Nova V2491 Cygni(Research Note)
M. J. Darnley ,⋆ , V. A. R. M. Ribeiro , M. F. Bode , and U. Munari , Astrophysics Research Institute, Liverpool John Moores University, Egerton Wharf, Birkenhead, CH41 1LD, UK INAF Astronomical Observatory of Padova, via dell’Osservatorio, 36012 Asiago (VI), Italy ANS Collaboration c / o Astronomical Observatory, 36012 Asiago (VI), ItalyReceived November 2, 2010; accepted April 12, 2011 ABSTRACT
Nova V2491 Cyg is one of just two detected pre-outburst in X-rays. The light curve of this nova exhibited a rare “re-brightening”which has been attributed by some as the system being a polar, whilst others claim that a magnetic WD is unlikely. By virtue of thenature of X-ray and spectroscopic observations the system has been proposed as a recurrent nova, however the adoption of a 0.1 dayorbital period is generally seen as incompatible with such a system. In this research note we address the nature of the progenitorsystem and the source of the 0.1 day periodicity. Through the combination of Liverpool Telescope observations with published dataand archival 2MASS data we show that V2491 Cyg, at a distance of 10 . −
14 kpc, is likely to be a recurrent nova of the U Sco-class; containing a sub-giant secondary and an accretion disk, rather than accretion directly onto the poles. We show that there islittle evidence, at quiescence, supporting a ∼ . ∼ Key words. stars: novae – stars: individual: V2491 Cyg
1. Introduction
The typical classical nova (CN) system consists of a white dwarf(WD) primary and a main-sequence secondary in a close binaryorbit; the secondary star fills its Roche lobe. Material lost by thesecondary accumulates onto the surface of the WD via an ac-cretion disk around the WD which lies in the orbital plane. Ina small number of cases - the polars - accretion is channelledto the magnetic poles of the WD and no appreciable disk ispresent. Nuclear burning can initiate within this accreted, degen-erate, material, leading to a thermonuclear runaway which willeject some or all of this accreted layer. A nova eruption can reach M V ≃ −
10 (Shafter et al. 2009) and may eject 10 − − − M ⊙ of matter (e.g. Bode 2010, and references therein).The closely related recurrent nova (RN) systems exhibit re-currence timescales of 10 −
100 years. This is attributed to a com-bination of a high mass WD and high accretion rate. The elevatedaccretion rate is caused by the presence of an evolved secondary;either a sub-giant (U Sco-class) or a red giant (RS Oph-class).However, the T Pyx-class of recurrents contain lower mass WDsand main sequence secondaries, but other than their short recur-rence timescales, they are more akin to CNe (Anupama 2008).V2491 Cygni (Nova Cygni 2008 m = . α FHWM ∼ ,
500 km s − ) and was latter classified as a He / N nova(Helton et al. 2008; Munari et al. 2011). The rapid decline frommaximum light classifies V2491 Cyg as a very fast nova. ⋆ e-mail: [email protected]
The optical decline of V2491 Cyg exhibited a secondarymaximum at day ∼
15 (Munari et al. 2011). Such significant re-brightenings have been seen in a number of other novae(V2362 Cyg and V1493 Aql; Strope et al. 2010), but are un-usual, and still poorly understood. Hachisu & Kato (2009) haveproposed magnetic activity as an additional energy source anda possible cause of the re-brightening, a requirement being ahighly magnetised WD (a polar).Following the outburst, it was discovered that there wasa pre-existing X-ray source coincident with the position ofV2491 Cyg (Ibarra & Kuulkers 2008; Ibarra et al. 2008, 2009).This is only the second nova for which pre-outburst X-ray emis-sion has been detected, after V2487 Oph (Hernanz & Sala 2002).V2487 Oph was classified as a recurrent nova when a previousoutburst was indentified from 1900 (Pagnotta et al. 2009).Baklanov et al. (2008) reported a variation in the B - and V -bands between 10 and 20 days after outburst, these had a periodof 0.09580(5) days and amplitude of 0 . − .
05 mag. This pe-riod has been taken by some authors to be the orbital period ofthe system. However, Baklanov (private communication) havenot made this claim and also note that this period became negli-gible as the system waned. Ragan et al. (2010) reported similar R -band periodicity between day 28 and 34.The first indication that V2491 Cyg may be recurrent in na-ture came from Tomov et al. (2008) who noted the spectral sim-ilarities to the RNe U Sco and V394 CrA early after outburst.Additionally, Bode et al. (2009) noted that the V2491 Cyg spec-tra were similar to the early spectra of RS Oph and in particularthose of M31N 2007-12b, a RS Oph-class RN candidate in M31.The progenitor system of V2491 Cyg was identifiedas USNO-B1.0 1223-042965 (Henden & Munari 2008) and M. J. Darnley et al.: On the Progenitor System of Nova V2491 Cygni (RN) there is no evidence of previous outbursts at this posi-tion (Jurdana-Sepic & Munari 2008). Rudy et al. (2008) deter-mined a reddening to V2491 Cyg of E B − V = .
43 via OIline ratios. Helton et al. (2008) used the CN MMRD relation(della Valle & Livio 1995) with this reddening to determine d = . E B − V = . ± .
01 and d =
14 kpc. Ribeiro et al. (2011) re-ported an ejecta morphology consisting of polar blobs and anequatorial ring, with expansion velocities of ∼ ,
000 km s − and an inclination of 80 + − degrees, i.e. close to edge on.Page et al. (2010) reported the results of extensive Swift
X-ray and UV observations of the outburst, these data indicated thatthe WD in the system may be close to the Chandrasekhar mass.By assuming the pre-outburst X-ray emission was due to accre-tion Page et al. (2010) deduced that V2491 Cyg would recur onlonger than typical timescales, centuries rather than decades, ifthe WD mass was indeed high. This paper noted in addition thatX-ray flickering beginning at day 57 implied the resumption ofaccretion in the system. Page et al. (2010) also reported that the ∼ . Swift
UVOT or XRT data. Interestingly, Page et al. (2010) showthat the presence of a highly magnetic WD in the V2491 Cygsystem is unlikely, in direct contradiction to the outburst mecha-nism proposed by Hachisu & Kato (2009).In this research note we address the nature of the progenitorsystem of V2491 Cyg and the ∼ .
2. Observations
All additional data reported in this research note were obtainedby the 2.0m robotic Liverpool Telescope (LT; Steele et al. 2004)located at the Observatorio del Roque de Los Muchachos onthe Canary Island of La Palma, Spain. To investigate the sourceof the ∼ . B -band observationswere taken by the LT on 2010 April 24 and 2010 September11, t =
744 days and t =
884 days post-outburst respectively.These series corresponded to 3.59 hours each, ∼ .
3. Results
For a nova system containing a sub-giant or red giant secondary,one would expect the near-IR emission to be dominated by thesecondary. A solar-like sub-giant star ( M < . M ⊙ ; the uppermass limit dictated by that of the WD primary for stable ac-cretion) would be expected to have a luminosity 2 ≤ M J ≤ ∼
15 kpc a red giant dominatedsystem should be easily detectable within the 2MASS data; asub-giant may be at or around the limit of detectability. A search
Fig. 1. J -band image of the region around V2491 Cyg,the position of V2491 Cyg is marked by the circle. There is afaint source at this position in the 2MASS J data, also in the H and K S data.of the 2MASS catalogue at the position of V2491 Cyg reveals noobjects within a reasonable radius. However, upon inspection ofthe 2MASS images of the region there is a clear, but faint, sourceat the position of the nova, see Figure 1. Photometry of this ob-ject, calibrated to the 2MASS catalogue for a large number ofneighbouring stars, yields m J = . ± . m H = . ± . m K S = . ± .
6. The J -band photometry equates to an absolutemagnitude of M J = . ± . M J = . ± . d = . d =
14 kpc and reddenings E B − V = . E B − V = .
23 respectively. For the H - and K S -bands we find M H = . ± . M H = . ± . M K S = . ± . M K S = . ± . M J = . ± . M H = . ± . M K S = . ± .
4; Hanes 1985; Schaefer 2010).
The lightcurves from the LT B -band observing runs taken 744and 884 days after the outburst of V2491 Cyg are shown inFigure 2, the mean magnitude during each run is shown inTable 1. The photometric uncertainties quoted are dominated bythe calibration of the data, and the brightness of the system de-creased significantly between the two runs. Such a decline mayindicate that the object has not quite returned to quiescence, orit may be indicative of the system’s variation at quiescence. Inaddition to this decline, the B -band emission from the systemis clearly variable, exhibiting an amplitude of ∆ B ≃ . sine curve-like variations seen by Baklanov et al. (2008). Thereis also no distinct period seen in these data; the first run has prop-erties broadly consistent with a ∼ . ∼ .
025 day period. These data are too sparseto perform any meaningful period analysis. However, it is clearthat there is no strong eclipse signature seen in these data. Theeclipses predicted by Ribeiro et al. (2011) would be expected tobe as deep as those seen in DQ Her and U Sco; ∼ < .
15 days may be ruledout. These lightcurves are reminiscent of optical flickering seen . J. Darnley et al.: On the Progenitor System of Nova V2491 Cygni (RN) Fig. 2.
Liverpool Telescope B -band light curve of V2491 Cygtaken 744 days (top) and 884 days (bottom) post-outburst. Eachphotometry run covers 3.59 hours, each observation lasted 60seconds. A ruler of length 0.09580(5) days (i.e. the period re-ported by Baklanov et al. 2008) is shown on each plot.once accretion had re-established following the 2006 outburst ofRS Oph (Worters et al. 2007). Whilst there is a vast array of optical data covering the out-burst of V2491 Cyg, there is somewhat a lack of data for thesystem at, or close to, quiescence. In Table 1 we have consoli-dated both pre- and post-outburst data from a number of sources.These sources include, data from the SDSS-II survey and theAsiago 1.82m using the AFOSC spectrograph / imager (both aspublished in Munari et al. 2011). Comparison of the pre-outburstSDSS-II data with the later AFOSC and LT data indicates thatV2491 Cyg is back (or very close) to quiescent levels by day ∼
800 after outburst. As such, we derive the outburst amplitudeof ∆ m V ∼ ∆ m R ∼
10 magnitudes, which is similar to the outburstamplitude observed in U Sco (Schaefer et al. 2010). The typicalamplitudes observed in RS Oph-class RNe is ∼ − m V ∼
10 magnitudes forthe very slowest up to m V ∼
17 magnitudes for the very fastest(Warner 2008). These quiescent data can also, in principle, beused as an aid to determining the nature of the progenitor sys-tem, but one needs to be careful when dealing with the largeuncertainties on the distance and extinction to V2491 Cyg.Shown in Figure 3 is a
V vs. B − V colour-magnitude dia-grams populated with Hipparcos stars (Perryman & ESA 1997).The quiescent positions of the RN prototypes RS Oph ( hatched region) and U Sco (green) and also the RN V2487 Oph (blue)are included for comparison. The position of V2491 Cyg isshown by the base of the red arrow, assuming d = . E B − V = .
43; the red arrow head indicates the position ofthe system for d =
14 kpc and E B − V = .
23. It is clear thatthe uncertainties on distance and extinction have little e ff ect onthe system’s position in a colour-magnitude diagram. This plotsupports the results from the near-IR data that the V2491 Cygsystem does not contain a red giant secondary. However, the qui-escent V2491 Cyg system lies remarkably close to the positionof U Sco. Additionally, V2491 Cyg and V2487 Oph, both of Telescope Filter Time (days Magnitudeafter peak)2MASS J -3,633 16 . ± . H -3,633 16 . ± . K S -3,633 16 . ± . B -6,895 18.3SDSS-II R -6,895 17.4SDSS-II I -6,895 16.9AFOSC V +
831 17.88AFOSC R +
831 17.49AFOSC I +
831 17.14LT B +
744 18 . ± . B +
884 18 . ± . Table 1.
A summary of photometric observations of theV2491 Cyg progenitor system at or close to quiescence usedin this work. Data are taken from Munari et al. (2011) and thiswork.
Fig. 3.
Colour-magnitude diagram using
Hipparcos data(Perryman & ESA 1997). The green point indicates the posi-tion of a quiescent U Sco, the ellipse shows the location ofa queiscent RS Oph and the blue point shows the position ofV2487 Oph. The arrows respresent a quiescent V2491 Cyg: tail( d = . E B − V = . d =
14 kpc, E B − V = . d = E B − V = .
23; seeSection 4.2). The grey dashed region indicates where most qui-escent classical nova systems are found (Darnley et al., in prep).which exhibited pre-outburst X-ray emission, lie within a simi-lar region on this plot.
4. Discussion
Many properties of V2491 Cyg are directly comparable with theRN U Sco; the optical and near-IR luminosities at quiescence arealmost identical, as is the outburst amplitude of the system. Asthe spectral energy distributions of both V2491 Cyg and U Scoare so similar, we must conclude that the composition of the sys-tem is similar; an accretion disk dominating the short wavelengthemission the secondary being important at longer wavelengths.Based with the quiescent photometric evidence we conclude thatthe V2491 Cyg system is likely itself to be a “clone” of the U Sco
M. J. Darnley et al.: On the Progenitor System of Nova V2491 Cygni (RN) system. That is, highly inclinded (edge-on), containing a highmass WD primary, a sub-giant secondary and a bright accretiondisk, and as such on this evidence alone we would expect thesystem to be recurrent on relatively short timescales.Such a system is however incompatible with an orbital pe-riod as short as 0 . ∼
40 indicating that be-fore this the central system was still likely to be obscured. Thenature of the optical spectra at this time (P Cygni profiles visi-ble and an absence of high excitation lines; Munari et al. 2011)support this. The U Sco system is well known for its eclips-ing nature. However, during the 2010 outburst of U Sco, theeclipses were masked until the ejecta had cleared the centralsystem (around day 15; Schaefer et al. in prep). Indeed theseeclipses re-appeared three days after the SSS of U Sco was re-vealed (day 12; Schlegel et al. 2010). As the ejecta velocitiesseen in U Sco ( ∼ − ; see e.g. Anupama 2010) aregreater than those of V2491 Cyg, a simple approach would leadto the central system of the latter taking longer to be unveiled.The early post-outburst light curve of U Sco (days 0 −
9) itself ex-hibited variation on a timescale much shorter than that system’sorbital period. As such, the early variability seen in both systemsis most likely connected to the outburst itself. The quasi -periodicvariability seen in the LT light curve of V2491 Cyg at quiescenceis most likely due to “flickering” of an accretion disk.Given the similarities of this V2491 Cyg to U Sco, we wouldexpect the orbital period of V2491 Cyg to be similar to that ofthe former, i.e. days rather than hours. Additionally, the largerdistance derived by Munari et al. (2011) has potential implica-tions to the nature of the system. Page et al. (2010) predicted arecurrence timescale for V2491 Cyg, based on a WD mass of ∼ . M ⊙ and a distance of 10.5 kpc, of >
100 years. However,with V2491 Cyg as distant as 14 kpc, such a recurrence timescaledecreases significantly to <
100 years, on a par with the knownrecurrents, i.e. the inferred accretion rate is higher.
A rather important caveat to the above argument is the dis-tance to V2491 Cyg. Both the Helton et al. (2008, 10.5 kpc) andMunari et al. (2011, 14 kpc) distances are essentially derived viathe same methodology; use of the MMRD relationship. It is onlyin their estimation of the reddening (by Rudy et al. 2008, for theformer) that they di ff er. It should be noted that both these dis-tances are broadly consistent if one applies the ∼ . very fast novae like V2491 Cyg,the atypical light curve may also have a ff ected the reliability.Schaefer (2010) has shown that the MMRD may perform poorlyfor recurrent novae significantly overestimating the distance insome cases (e.g. U Sco). V2491 Cyg is indeed not a typical nova,the re-brightening complicates the determination of the speedclass, and may invalidate the MMRD for this nova. In deriv-ing the distance to V2491 Cyg of 14 kpc, Munari et al. (2011)made a number of observations. Given the Galactic coordinatesof the system ( l = . b = + . = / N nova. However, both V477 Sct (Munari et al. 2006) andV2672 Oph (Munari et al. 2010), both He / N novae, also occupysuch lofty positions above the plane.We are given two alternatives. Either, V2491 Cyg is a U Sco-class RN and the MMRD works well for this particular recur-rent; or, the system is much closer, ∼ ∆ m V ∼
10 magnitudes. Such a small amplitude is incompati-ble with a very fast
CN and is only achievable for the slowest(faintest) of novae. Using the outburst amplitude versus rate ofdecline relationship for CNe (see Warner 2008), the amplitudefor a nova with a decline time t = . ∼
80 degrees (Ribeiro et al. 2011) is almost 16magnitudes. As such, the only scenarios compatible with sucha small outburst amplitude are that V2491 Cyg is a (nearby)severely under-luminous and very unusual CN or that it is a (dis-tant) U Sco-class RN. In either case V2491 Cyg is an interestingand important system worthy of further study.
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