The Ubv Color Evolution of Classical Novae. Ii. Color–magnitude Diagram

Abstract

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.

The UBV Color Evolution of Classical Novae. III. Time-stretched Color–Magnitude Diagram of Novae in Outburst

“…The early pulse (oscillation) in the V light curve makes a loop in the color-magnitude diagram. This kind of loops were also seen in multiple-peak novae like V723 Cas and V5558 Sgr as discussed by Hachisu & Kato (2016b). We list our results in Table 1. 7.6.…”

“…Template nova tracks in the time-stretched color-magnitude diagram Hachisu & Kato (2016b) proposed six template tracks of novae with well-determined distance modulus in the V band (m − M ) V and color excess E(B − V ). They dubbed them the V1500 Cyg, V1668 Cyg, V1974 Cyg, LV Vul, FH Ser, and PU Vul tracks as shown in Figure 6 Hachisu & Kato (2016b). They categorized 40 novae into one of these six subgroups, depending on their similarity in the (…”

“…V533 Her was studied in Paper II on the (B − V ) 0 -M V diagram but we here examine it on the timestretched color-magnitude diagram. Hachisu & Kato (2016b) took E(B − V ) = 0.038 ± 0.002 from the NASA/IPAC galactic absorption map. We adopt their value of E(B − V ) = 0.038.…”

“…On the other hand, the (t/f s )-M V light curves do not overlap because the absolute M V magnitudes are different among novae. This is the reason why Hachisu & Kato (2016b) did not find common tracks in the (B − V ) 0 -M V diagram. If we use the (B − V ) 0 -(M V − 2.5 log f s ) diagram, we could find common tracks because the (t/f s )-(M V − 2.5 log f s ) light curves overlap each other after time-stretch.…”

X-RAY FLASHES IN RECURRENT NOVAE: M31N 2008-12a AND THE IMPLICATIONS OF THE SWIFT NONDETECTION

“…The authors note, however, that the possibility of a face-on subgiant companion remains because U Sco is an eclipsing binary and its edge-on disk may not be bright. Hachisu & Kato (2016b) classified 40 classical novae into six classes according to their evolutionary path in the colormagnitude diagram and found that the different paths correspond to differences in the nova speed class and thus the envelope mass. These authors also displayed the color-magnitude evolution during the 2014 outburst of M31N 2008-12a and found that its characteristic properties are similar to those of U Sco and CI Aql, which are both recurrent novae with a subgiant companion, but different from RS Oph which has a red giant companion (see their Figures 72(c), (d), and 76(b)).…”