Identifying and Quantifying Recurrent Novae Masquerading as Classical Novae

Abstract

Recurrent novae (RNe) are cataclysmic variables with two or more nova eruptions within a century. Classical novae (CNe) are similar systems with only one such eruption. Many of the so-called CNe are actually RNe for which only one eruption has been discovered. Since RNe are candidate Type Ia supernova progenitors, it is important to know whether there are enough in our Galaxy to provide the supernova rate, and therefore to know how many RNe are masquerading as CNe. To quantify this, we collected all available information on the light curves and spectra of a Galactic, time-limited sample of 237 CNe and the 10 known RNe, as well as exhaustive discovery efficiency records. We recognize RNe as having (1) outburst amplitude smaller than 14.5 − 4.5 × log(t 3 ), (2) orbital period >0.6 days, (3) infrared colors of J − H > 0.7 mag and H − K > 0.1 mag, (4) FWHM of Hα > 2000 km s −1 , (5) high excitation lines, such as Fe x or He ii near peak, (6) eruption light curves with a plateau, and (7) white dwarf mass greater than 1.2 M . Using these criteria, we identify V1721 Aql, DE Cir, CP Cru, KT Eri, V838 Her, V2672 Oph, V4160 Sgr, V4643 Sgr, V4739 Sgr, and V477 Sct as strong RN candidates. We evaluate the RN fraction among the known CNe using three methods to get 24% ± 4%, 12% ± 3%, and 35% ± 3%. With roughly a quarter of the 394 known Galactic novae actually being RNe, there should be approximately a hundred such systems masquerading as CNe. Key word: novae, cataclysmic variablesOnline-only material: color figures, machine-readable table 1. RECURRENT NOVA CANDIDATES Both classical and recurrent novae (CNe and RNe, respectively) consist of a white dwarf (WD) accreting material from a companion star. The accreted material accumulates until reaching a critical temperature/pressure at the base of the accreted layer, at which point thermonuclear runaway is triggered and the nova eruption occurs. The outburst mechanism is identical for both CNe and RNe, but the recurrence timescale varies by multiple orders of magnitude, with RNe seen to erupt at least once per century. The systems classified as CNe have only one discovered eruption, but more undiscovered eruptions could have occurred within the last century. The truly classical systems do not have any more eruptions on timescales of less than a century. We note that this century-long timescale is empirically based on observations, and is somewhat arbitrary, arising due more to the history of reliably recorded, large-scale observations (dating back to the 1890s, when the first astronomical plates were made) than to any physical distinction. We anticipate needing to either expand the definition of an RN in the future, as we discover systems with recurrence times just slightly greater than 100 yr, or to alter the nomenclature to something such as Fast Recurrence Time Novae, to distinguish systems whose recurrences we have had time to observe from those for which we still wait, a wait time which may be on the order of 10 5 yr. For this paper, we will continue with the curr...

Recurrent Novae: A Masquerade

“…V2487 Oph is also located in the RN region of this plot, which is one of the characteristics that led us to search for-and find-a previous eruption of this system [Pagnotta et al(2009)]. Figure originally published in [Pagnotta & Schaefer(2014)]. …”

“…The RG threshold (J − H > 0.7 and H − K > 0.1) is based on Figure 1 of [Harrison(1992)]. Figure originally published in [Pagnotta & Schaefer(2014)]. …”

“…One of the two new indicators we introduced in [Pagnotta & Schaefer(2014)] is a high expansion velocity during outburst, as measured by the FWHM of the Hα line near peak. The Hα line is ubiquitous in novae, and the FWHM is both relatively straightforward to measure and commonly reported by observers, making it a useful characteristic.…”

“…Novae are often divided into two general categories: classical novae (CNe), which have only one known eruption, and recurrent novae (RNe), for which multiple eruptions have been observed. The eruption mechanisms in CNe and RNe are identical, but two conditions in RNe contribute to the short amount of time needed for enough matter to accumulate to trigger the next nova eruption: the presence of a high mass WD and a high accretion rate,Ṁ [Pagnotta & Schaefer(2014)]. These conditions are in fact both necessary to have an RN; with only one, the system will still be a CN.…”

On the narrow emission line components of the LMC novae 2004 (YY Doradus) and 2009a

“…The careful reader has certainly noticed that of the seven novae listed above, four are recurrent novae and three are somewhat debated (i.e. LMC 2009a, KT Eri and DE Cir, Jurdana-Sepic et al 2012;Pagnotta & Schaefer 2014). Without entering into the discussion on the possible short recurrent nature of these novae and the recently debated definition of recurrent novae and classes of recurrent novae, we just note here that few of these seven novae show additional common phenomenology such as the expansion velocities (all FWHM are in excess of ∼3500 km s −1 ), a [Ne] dominated nebular spectrum (U Sco, Diaz et al 2011;Mason et al 2012;YY Dor, Walter et al 2012;LMC 2009a, this paper, andDE Cir, Walter et al 2012), slightly evolved secondaries (U Sco, V394 CrA, KT Eri andLMC 2009a, Darnley et al 2012; for the case of KT Eri see also Munari & Dallaporta 2014a) and orbital period of the order of 1 day (U Sco, Schaefer 2010; V394 CrA, Schaefer 2010, andLMC 2009a;Bode et al 2009) which are worth to explore since they suggest possible profound common physical characteristics (e.g.…”