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

TL;DR

This study analyzes the color-magnitude evolution of 40 classical novae, categorizing their tracks into templates to understand nova physics and estimate distances using their evolutionary patterns.

Contribution

It introduces a new classification of nova tracks in the color-magnitude diagram and demonstrates a method to determine nova distances by fitting these templates.

Findings

Nova tracks follow six main templates based on speed class.

Bluer novae have less massive envelopes and are faster.

Absolute magnitude at nebular phase onset is similar across novae.

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.