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

TL;DR

This paper introduces time-stretched color-magnitude diagrams for classical novae, demonstrating their effectiveness in overlapping nova tracks and refining distance and reddening estimates, consistent with Gaia data.

Contribution

It extends previous work by proposing and validating new $(U-B)_0$-$(M_B-2.5 ext{log}f_s)$ and $(V-I)_0$-$(M_I-2.5 ext{log}f_s)$ diagrams for nova analysis.

Findings

Time-stretched color-magnitude tracks overlap for many novae.

Refined distance and reddening estimates agree with Gaia DR2.

New diagrams improve understanding of nova evolution.

Abstract

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 $(U-B)_0$-$(M_B-2.5\log f_{\rm s})$ and $(V-I)_0$-$(M_I-2.5\log f_{\rm s})$ 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.