The &lt;em&gt;UBV&lt;/em&gt; Color Evolution of Classical and Symbiotic Novae

I Hachisu,M Kato

doi:10.14311/app.2015.02.0273

Abstract

We identified a general course of classical nova outbursts in the B − V vs. U − B diagram. It has been reported that novae show spectra similar to 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 Δ(U − B) ≈ −0.2 mag bluer than the supergiant sequence. 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) and stays there for a while, which is coincident with the intersection of the blackbody sequence and the nova-giant sequence. Then the color evolves leftward (blueward in B − V but almost constant in U − B) due mainly to development of strong emission lines. This is the general course of nova outbursts in the color-color diagram, which is deduced from eight well-observed novae including 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 color excesses of classical novae. Using this method, we redetermined the color excesses of nineteen well-observed novae.

Highlights

A classical nova is a thermonuclear runaway event on a mass-accreting white dwarf (WD) in a binary. Despite of their overall similarity, optical light curves of novae have a wide variety of timescales and shapes
Duerbeck & Seitter (1979) discussed that color evolutions of six novae are remarkably similar in the intrinsic (B − V )[0] versus (U − B)[0] color-color diagram independently of their different nova speed classes. van den Bergh & Younger (1987) derived general trends of color evolutions in nova light curves, i.e., (B − V )0 = 0.23 ± 0.06 at optical maximum and (B − V )0 = −0.02 ± 0.04 at t2, where tm (m = 2 or 3) is the days during which a nova decays by mth magnitude from its optical maximum and (B − V )[0] is the intrinsic B − V color of the nova
E(B − V ), of 23 novae assuming that all novae have the same intrinsic (B − V )[0] color at the stabilization stage, i.e., E(B −V ) = (B −V )ss −(B −V )0 = (B −V )ss +0.11, where (B − V )ss is the observed B − V color at the stabilization stage

Summary

Introduction

A classical nova is a thermonuclear runaway event on a mass-accreting white dwarf (WD) in a binary. Van den Bergh & Younger (1987) derived general trends of color evolutions in nova light curves, i.e., (B − V )0 = 0.23 ± 0.06 at optical maximum and (B − V )0 = −0.02 ± 0.04 at t2, where tm (m = 2 or 3) is the days during which a nova decays by mth magnitude from its optical maximum and (B − V )[0] is the intrinsic B − V color of the nova. These two relations, often show large deviations from the values obtained by other methods.

Nova-giant Sequence in the Color-color Diagram

Common Paths of Novae in the Color-color Diagram

Estimates of Extinctions toward Various Novae