The $UBV$ Color Evolution of Classical Novae. IV. Time-Stretched $(U-B)_0$-$(M_B-2.5log f_{rm s})$ and $(V-I)_0$-$(M_I-2.5log f_{rm s})$ Color-Magnitude Diagrams of Novae in Outburst


Abstract in English

Light curves and color evolutions of two classical novae can be largely overlapped if we properly squeeze or stretch the timescale of a target nova against that of a template nova by $t=t/f_{rm s}$. Then the brightness of the target nova is related to the brightness of the template nova by $(M[t])_{rm template} = (M[t/f_{rm s}] - 2.5 log f_{rm s})_{rm target}$, where $M[t]$ is the absolute magnitude and a function of time $t$, and $f_{rm s}$ is the ratio of timescales between the target and template novae. In the previous papers of this series, we show that many novae broadly overlap in the time-stretched $(B-V)_0$-$(M_V-2.5 log f_{rm s})$ color-magnitude diagram. In the present paper, we propose two other $(U-B)_0$-$(M_B-2.5log f_{rm s})$ and $(V-I)_0$-$(M_I-2.5log f_{rm s})$ diagrams, and show that their tracks overlap for 16 novae and for 52 novae, respectively. Here, $(U-B)_0$, $(B-V)_0$, and $(V-I)_0$ are the intrinsic $U-B$, $B-V$, and $V-I$ colors and not changed by the time-stretch, and $M_B$, $M_V$, and $M_I$ are the absolute $B$, $V$, and $I$ magnitudes. Using these properties, we considerably refine the previous estimates of their distance and reddening. The obtained distances are in reasonable agreement with those of {it Gaia} Data Release 2 catalogue.

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