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The Progenitor System Of Nova V2491 Cygni (SDSS J194301.96+321913.8)


V2491 Cygni (Nova Cygni 2008 #2; SDSS J194301.96+321913.8) was discovered in outburst on 2008 April 10.8 UT (Nakano et al. 2008). Its nature was confirmed spectroscopically by Ayani & Matsumoto (2008), with a Hα FHWM* of ~4500 km s-1 and was latter classified as a Helium/Nitrogen (He/N) nova (Helton et al. 2008; Munari et al. 2011). The rapid decline from maximum light classifies V2491 Cyg as a very fast classical nova. Rather unusually, the optical decline of V2491 Cyg exhibited a secondary maximum 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 unusual and still poorly understood. Hachisu & Kato (2009) have proposed magnetic activity as an additional energy source and a possible cause of the re-brightening, a requirement being a highly magnetised white dwarf known as a “polar”.

N.B.* FWHM is short for full-width at half maximum, a measure astronomers use to describe resolution of star images.

Classical novae (CNe) systems typically consist of a compact white dwarf (WD) primary and a main-sequence secondary star which usually resembling the same properties of our own Sun (Iben & Tutukov 1996). The system is in a tight binary orbit where the secondary star swells and fills its Roche lobe spilling material onto the surface of the WD primary. This situation leads to a thermonuclear runaway on the surface of the WD, forming an accreted layer. This layer is then expelled and heated to very high temperatures: this is seen by us on Earth as a nova eruption (Iben & Livio 1993; Hellier 2001; Bode & Evans 2008).

Following the initial April 2008 outburst, it was discovered that there was a pre-existing X-ray source coincident with the position of V2491 Cyg (Ibarra & Kuulkers 2008; Ibarra et al. 2008; Ibarra et al. 2009). This is only the second nova for which pre-outburst X-ray emission has been detected, after V2487Oph (Hernanz & Sala 2002). V2487 Oph was classified as a recurrent nova (RN) when a previous outburst was indentified from 1900 (Pagnotta et al. 2009).

The progenitor system of V2491 Cyg was identified as USNO-B1.0 1223-042965 (Henden & Munari 2008) and there is no evidence of previous outbursts at this position (Jurdana-Sepic & Munari 2008). Rudy et al. (2008) determined a sharp reddening of V2491 Cyg via O [I] line ratios. Helton et al. (2008) used the CN “MMRD”† relation (Livio & della Valle 1995) with this reddening to determine d  = 10.5 kpc. Munari et al. (2011) independently derived both the extinction and distance to V2491 Cyg using the interstellar Na [I] line and the van den Bergh & Younger (1987) relation, finding a distance of d = 14 kpc. Ribeiro et al. (2011) reported an ejecta morphology consisting of polar blobs and an equatorial ring, with expansion velocities of ~3000 km s-1 and an inclination of 80°: i.e. close to edge on.

The first indication that V2491 Cyg may be recurrent in nature came from Tomov et al. (2008) who noted the spectral similarities to the RNe U Sco and V394 CrA early after outburst. Additionally, Bode et al. (2009) noted that the V2491 Cyg spectra were similar to the early spectra of RS Oph and in particular those ofM31N 2007-12b, a RS Oph-class RN candidate in M31.

Many properties of V2491 Cyg are directly comparable with the RN U Sco; the optical and near-IR luminosities at quiescence are almost identical, as is the outburst amplitude of the system. As the spectral energy distributions of both V2491 Cyg and U Sco are so similar, we must conclude that the composition of the system is similar; an accretion disc dominating the short wavelength emission the secondary being important at longer wavelengths. Based with the quiescent photometric evidence Darnley et al. (2011) concluded that the V2491 Cyg system is likely itself to be a “clone” of the U Sco system. That is, highly inclinded (edge-on), containing a high mass WD primary, a sub-giant secondary and a bright accretion disc.

Such a system is however incompatible with an orbital period as short as 0.1 days as has been widely assumed for this system, as the radius of the sub-giant star would be larger than the orbital separation. We are therefore given two alternatives: either, V2491 Cyg is a U Sco class RN or, the system is much closer at ~2 kpc. However, there is one parameter of the nova outburst that is both distance and extinction independent, the outburst amplitude; for V2491 Cyg  ~10 magnitudes. Such a small amplitude is incompatible with a very fast CN and is only achievable for the slowest and faintest of novae. Using the outburst amplitude versus rate of decline relationship for CNe (e.g. Warner 2008), the amplitude for a nova with a decline time of ~4.8 days (Munari et al. 2011) and inclination ~80° (Ribeiro et al. 2011) is almost 16 magnitudes. As such, the only scenarios compatible with such a small outburst amplitude are that V2491 Cyg is a (nearby) severely under-luminous and very unusual CN or that it is a (distant) U Sco-class RN. In either case V2491 Cyg is an interesting and important system worthy of further study.

Journal References:

  • Darnley, M. J.; et al. (2011) On The Progenitor System Of Nova V2491 CygniAstronomy & Astrophysics, 530: Article I.D.: 70.
  • Ibarra, A.; et al. (2009) Pre-Nova X-Ray Observations Of V2491 Cygni (Nova Cyg 2008b)Astronomy & Astrophysics, 497 (1): pp.L5-L8.

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