Recurrent novae as a progenitor system of Type Ia supernovae. I. RS Ophiuchi subclass --- Systems with a red giant companion

Theoretical light curves of four recurrent novae in outburst are modeled to obtain various physical parameters. They are those with a red giant companion, T CrB, RS Oph, V745 Sco, and V3890 Sgr. Our model includes irradiations of the companion star and the accretion disk together with a shadowing effect on the companion by the accretion disk. The early visual light curves are well reproduced with a thermonuclear runaway model on a very massive white dwarf of 1.37 Mo for T CrB, 1.37 Mo for RS Oph with low metallicity (Z=0.004), 1.35 Mo for V745 Sco, and 1.35 Mo for V3890 Sgr. Each envelope mass at the optical maximum is also estimated to be 3 x 10^{-6}, 2 x 10^{-6}, 5 x 10^{-6}, and 3 x 10^{-6} Mo, indicating an average mass accretion rate of 0.4 x 10^{-7}, 1.2 x 10^{-7}, 0.9 x 10^{-7}, and 1.1 x 10^{-7} Mo/yr during the quiescent phase. Although a large part of the envelope mass is blown in the wind, each WD can retain a substantial part of the envelope mass after hydrogen burning ends. Thus, we have obtained net mass-increasing rates of the WDs as 0.1 x 10^{-7}, 0.12 x 10^{-7}, 0.05 x 10^{-7}, and 0.11 x 10^{-7} Mo/yr. These results strongly indicate that the WDs in the recurrent novae have now grown up to near the Chandrasekhar mass limit and will soon explode as a Type Ia supernova if the WDs consist of carbon and oxygen. We also include a radiation-induced warping instability of the accretion disk to reproduce the second peak of T CrB outbursts. Thus, we have clarified the reason why only T CrB shows a secondary maximum but the other three systems do not.Comment: 41 pages including 18 figures, to appear in Ap

Revised analysis of the supersoft X-ray phase, helium enrichment, and turn-off time in the 2000 outburst of the recurrent nova CI Aquilae

The recurrent nova CI Aquilae entered the final decline phase a bit before May of 2001, showing the slowest evolution among the recurrent novae. Based on the optically thick wind mass-loss theory of the thermonuclear runaway model, we have estimated the turn-off time of the CI Aql 2000 outburst in March of 2001, after a supersoft X-ray source (SSS) phase lasts 150 days from December of 2000 until May of 2001. Fitting our theoretical light curves with both the 1917 and 2000 outbursts, we also obtained the WD mass to be M_{WD}= 1.2 \pm 0.05 M_\sun, helium enrichment of ejecta, He/H ~ 0.5 by number, the mass of the hydrogen-rich envelope on the WD of \Delta M ~ 6 x 10^{-6} M_\sun at the optical maximum, which is indicating an average mass accretion rate of \dot M_{acc} ~ 0.8 x 10^{-7} M_\sun yr^{-1} during the quiescent phase between the 1917 and 2000 outbursts.Comment: 2 pages, to appear in "The Physics of Cataclysmic Variables and Related Objects", eds. B. Gaensicke, K. Beuermann & K. Reinsch, ASP Conference Serie

Prediction of supersoft X-ray phase, helium enrichment, and turn-off time in recurrent nova CI Aquilae 2000 outburst

Recurrent nova CI Aquilae is still bright 300 days after the optical maximum, showing the slowest evolution among recurrent novae. We predict the turn-off time of CI Aql 2000 outburst coming in August 2001 after a supersoft X-ray source (SSS) phase lasts 250 days. We also predict helium enrichment of ejecta, He/H = 0.25 by number. Observational confirmations are urgently required. Based on the optically thick wind mass-loss theory of the thermonuclear runaway model, we have also estimated the WD mass to be 1.2 +- 0.05 Msun by fitting our theoretical light curves with the 1917 and 2000 outbursts. The mass of the hydrogen-rich envelope on the WD is also estimated to be 6 x 10^{-6} Msun at the optical maximum, indicating an average mass accretion rate of 0.7 x 10^{-7} Msun/yr during the quiescent phase between the 1917 and 2000 outbursts. Using these obtained values, we have consistently reproduced the light curve in quiescence as well as the two outburst phases. Thus, we predict the turn-off time to be in August 2001 for the 2000 outburst. We strongly recommend soft X-ray observations to detect SSS until August 2001 because the massive wind phase have already ended in December 2000 followed by an SSS phase that very likely lasts until August 2001.Comment: 8 pages including 5 figures, to appear in the Astrophysical Journal, Letter

The UBV Color Evolution of Classical Novae. II. Color-Magnitude Diagram

We have examined the outburst tracks of 40 novae in the color-magnitude diagram (intrinsic B-V color versus absolute V magnitude). After reaching the optical maximum, each nova generally evolves toward blue from the upper-right to the lower-left and then turns back toward the right. The 40 tracks are categorized into one of six templates: very fast nova V1500 Cyg; fast novae V1668 Cyg, V1974 Cyg, and LV Vul; moderately fast nova FH Ser; and very slow nova PU Vul. These templates are located from the left (blue) to the right (red) in this order, depending on the envelope mass and nova speed class. A bluer nova has a less massive envelope and faster nova speed class. In novae with multiple peaks, the track of the first decay is more red than that of the second (or third) decay, because a large part of the envelope mass had already been ejected during the first peak. Thus, our newly obtained tracks in the color-magnitude diagram provide useful information to understand the physics of classical novae. We also found that the absolute magnitude at the beginning of the nebular phase is almost similar among various novae. We are able to determine the absolute magnitude (or distance modulus) by fitting the track of a target nova to the same classification of a nova with a known distance. This method for determining nova distance has been applied to some recurrent novae and their distances have been recalculated.Comment: 58 pages, 76 figures, to appear in the Astrophysical Journal, Supplement Series, typo, figs.47, 61, 64, 73 are correcte

The UBV Color Evolution of Classical Novae. I. Nova-Giant Sequence in the Color-Color Diagram

We identified a general course of classical nova outbursts in the B-V versus U-B color-color diagram. It is reported that novae show spectra similar to those of A--F supergiants near optical light maximum. However, they do not follow the supergiant sequence in the color-color diagram, neither the blackbody nor the main-sequence sequence. Instead, we found that novae evolve along a new sequence in the pre-maximum and near-maximum phases, which we call "the nova-giant sequence." This sequence is parallel to but \Delta (U-B) \approx -0.2 mag bluer than the supergiant sequence. This is because the mass of a nova envelope is much (\sim10^{-4} times) less than that of a normal supergiant. After optical maximum, its color quickly evolves back blueward along the same nova-giant sequence and reaches the point of free-free emission (B-V=-0.03, U-B=-0.97), which coincides with the intersection of the blackbody sequence and the nova-giant sequence, and remains there for a while. Then the color evolves leftward (blueward in B-V but almost constant in U-B), owing mainly to the development of strong emission lines. This is the general course of nova outbursts in the color-color diagram, which was deduced from eight well-observed novae in various speed classes. For a nova with unknown extinction, we can determine a reliable value of the color excess by matching the observed track of the target nova with this general course. This is a new and convenient method for obtaining the color excesses of classical novae. Using this method, we redetermined the color excesses of twenty well-observed novae. The obtained color excesses are in reasonable agreement with the previous results, which in turn supports the idea of our general track of nova outbursts.Comment: 55 pages, including 52 figures, ApJ, 785, 97, 2014 April 2

A Light Curve Analysis of Classical Novae: Free-free Emission vs. Photospheric Emission

We analyzed light curves of seven relatively slower novae, PW Vul, V705 Cas, GQ Mus, RR Pic, V5558 Sgr, HR Del, and V723 Cas, based on an optically thick wind theory of nova outbursts. For fast novae, free-free emission dominates the spectrum in optical bands rather than photospheric emission and nova optical light curves follow the universal decline law. Faster novae blow stronger winds with larger mass loss rates. Since the brightness of free-free emission depends directly on the wind mass loss rate, faster novae show brighter optical maxima. In slower novae, however, we must take into account photospheric emission because of their lower wind mass loss rates. We calculated three model light curves of free-free emission, photospheric emission, and the sum of them for various WD masses with various chemical compositions of their envelopes, and fitted reasonably with observational data of optical, near-IR (NIR), and UV bands. From light curve fittings of the seven novae, we estimated their absolute magnitudes, distances, and WD masses. In PW Vul and V705 Cas, free-free emission still dominates the spectrum in the optical and NIR bands. In the very slow novae, RR Pic, V5558 Sgr, HR Del, and V723 Cas, photospheric emission dominates the spectrum rather than free-free emission, which makes a deviation from the universal decline law. We have confirmed that the absolute brightnesses of our model light curves are consistent with the distance moduli of four classical novae with known distances (GK Per, V603 Aql, RR Pic, and DQ Her). We also discussed the reason why the very slow novae are about 1 mag brighter than the proposed maximum magnitude vs. rate of decline relation.Comment: 31 pages including 36 figures, to appear in Ap

A Light Curve Analysis of Recurrent and Very Fast Novae in our Galaxy, Magellanic Clouds, and M31

We analyzed optical, UV, and X-ray light curves of 14 recurrent and very fast novae in our galaxy, Magellanic Clouds, and M31, and obtained their distances and white dwarf (WD) masses. Among the 14 novae, we found that eight novae host very massive ($\gtrsim 1.35 M_\odot$) WDs and are candidates of Type Ia supernova (SN Ia) progenitors. We confirmed that the same timescaling law and time-stretching method as in galactic novae can be applied to extra-galactic fast novae. We classify the four novae, V745 Sco, T CrB, V838 Her, and V1534 Sco, as the V745 Sco type (rapid-decline), the two novae, RS Oph and V407 Cyg, as the RS Oph type (circumstellar matter(CSM)-shock), and the two novae, U Sco and CI Aql, as the U Sco type (normal-decline). The $V$ light curves of these novae almost overlap with each other in the same group, if we properly stretch in the time direction (timescaling law). We apply our classification method to LMC, SMC, and M31 novae. YY Dor, LMC N 2009a, and SMC N 2016 belong to the normal-decline type, LMC N 2013 to the CSM-shock type, and LMC N 2012a and M31N 2008-12a to the rapid-decline type. We obtained the distance of SMC N 2016 to be $d=20\pm2$ kpc, suggesting that SMC N 2016 is a member of our galaxy. The rapid-decline type novae have very massive WDs of $M_{\rm WD}=1.37-1.385 M_\odot$ and are promising candidates of SN Ia progenitors. This type of novae are much fainter than the MMRD relations.Comment: 64 pages including 51 figures, to appear in ApJ

Light Curve Analysis of Neon Novae

We analyzed light curves of five neon novae, QU Vul, V351 Pup, V382 Vel, V693 CrA, and V1974 Cyg, and determined their white dwarf (WD) masses and distance moduli on the basis of theoretical light curves composed of free-free and photospheric emission. For QU Vul, we obtained a distance of d~2.4 kpc, reddening of E(B-V)~0.55, and WD mass of M_WD=0.82-0.96 M_sun. This suggests that an oxygen-neon WD lost a mass of more than ~0.1 M_sun since its birth. For V351 Pup, we obtained d~5.5 kpc, E(B-V)~0.45, and M_WD=0.98-1.1 M_sun. For V382 Vel, we obtained d~1.6 kpc, E(B-V)~0.15, and M_WD=1.13-1.28 M_sun. For V693 CrA, we obtained d~7.1 kpc, E(B-V)~0.05, and M_WD=1.15-1.25 M_sun. For V1974 Cyg, we obtained d~1.8 kpc, E(B-V)~0.30, and M_WD=0.95-1.1 M_sun. For comparison, we added the carbon-oxygen nova V1668 Cyg to our analysis and obtained d~5.4 kpc, E(B-V)~0.30, and M_WD=0.98-1.1 M_sun. In QU Vul, photospheric emission contributes 0.4-0.8 mag at most to the optical light curve compared with free-free emission only. In V351 Pup and V1974 Cyg, photospheric emission contributes very little (0.2-0.4 mag at most) to the optical light curve. In V382 Vel and V693 CrA, free-free emission dominates the continuum spectra, and photospheric emission does not contribute to the optical magnitudes. We also discuss the Maximum Magnitude versus Rate of Decline (MMRD) relation for these novae based on the universal decline law.Comment: 47 pages including 61 figures, to appear in the Astrophysical Journal, Supplement Serie

A Light Curve Analysis of 32 Recent Galactic Novae --- Distances and White Dwarf Masses

We obtained the absolute magnitudes, distances, and white dwarf (WD) masses of 32 recent galactic novae based on the time-stretching method for nova light curves. A large part of the light/color curves of two classical novae often overlap each other if we properly squeeze/stretch their timescales. Then, a target nova brightness is related to the other template nova brightness by $(M_V[t])_{\rm template} = (M_V[t/f_{\rm s}] - 2.5 \log f_{\rm s})_{\rm target}$, where $t$ is the time, $M_V[t]$ is the absolute $V$ magnitude, and $f_{\rm s}$ is their timescaling ratio. Moreover, when these two time-stretched light curves, $(t/f_{\rm s})$-$(M_V-2.5 \log f_{\rm s})$, overlap each other, $(t/f_{\rm s})$-$(B-V)_0$ do too, where $(B-V)_0$ is the intrinsic $B-V$ color. Thus, the two nova tracks overlap each other in the $(B-V)_0$-$(M_V-2.5 \log f_{\rm s})$ diagram. Inversely using these properties, we obtain/confirm the distance and reddening by comparing each nova light/color curves with the well calibrated template novae. We classify the 32 novae into two types, LV Vul and V1500 Cyg types, in the time-stretched $(B-V)_0$-$(M_V-2.5 \log f_{\rm s})$ color-magnitude diagram. The WD mass is obtained by direct comparison of the model $V$ light curves with the observation. Thus, we obtain a uniform set of 32 galactic classical novae that provides the distances and WD masses from a single method. Many novae broadly follow the universal decline law and the present method can be applied to them, while some novae largely deviate from the universal decline law and so the method cannot be directly applied to them. We discuss such examples.Comment: 175 pages including 126 figures, to appear in ApJ

Recurrent novae as progenitors of Type Ia supernovae

Recurrent novae are binaries harboring a very massive white dwarf (WD), as massive as the Chandrasekhar mass, because of their short recurrence periods of nova outbursts of 10-100 years. Thus, recurrent novae are considered as candidates of progenitors of Type Ia supernovae (SNe Ia). In fact, the SN Ia PTF11kx showed evidence that its progenitor is a symbiotic recurrent nova. The binary parameters of recurrent novae have been well determined, especially for the ones with frequent outbursts, U Sco and RS Oph, which provide useful information on the elementary processes in binary evolution toward SNe Ia. Therefore we use them as testbeds for binary evolution models. For example, the original double degenerate (DD) scenario cannot reproduce RS Oph type recurrent novae, whereas the new single degenerate (SD) scenario proposed by Hachisu et al. (1999) naturally can. We review main differences between the SD and DD scenarios, especially for their basic processes of binary evolution. We also discuss observational support for each physical process. The original DD scenario is based on the physics in 1980s, whereas the SD scenario on more recent physics including the new opacity, mass-growth efficiency of WDs, and optically thick winds developed in nova outbursts.Comment: 25 pages, 3 figures, to appear in Bull. Astr. Soc. India (2012), 40, 393-41