Revisiting the Classics: On the Optical Colours of Novae as Standard Crayons

TL;DR

This study systematically analyzes the optical colours of novae during eruptions, providing improved reddening estimates and demonstrating that nova colours can serve as standard tools for estimating interstellar reddening and distances.

Contribution

It offers a comprehensive analysis of nova colours with refined reddening measurements and introduces a Bayesian method for distance estimation using these colours and dust maps.

Findings

Average intrinsic (B-V)_0 at peak is 0.20 with a standard deviation of 0.31.

No significant correlation between nova colours and t2, peak magnitude, or gamma-ray luminosity.

Nova colours can estimate interstellar reddening with 0.2-0.3 mag uncertainty.

Abstract

We present a systematic study of the $BVRI$ colours of novae over the course of their eruptions. Where possible, interstellar reddening was measured using the equivalent widths of Diffuse Interstellar Bands (DIBs). Some novae lack spectra with sufficient resolution and signal-to-noise ratios; therefore, we supplement as necessary with 3D and 2D dust maps. Utilising only novae with DIB- or 3D-map-based $E(B-V)$, we find an average intrinsic $(B-V)_0$ colour of novae at $V$-band light curve peak of 0.20 with a standard deviation of 0.31, based on 25 novae. When the light curve has declined by 2 magnitudes ($t_2$), we find an average $(B-V)_0 = -0.03$ with a standard deviation of 0.19. These average colours are consistent with previous findings, although the spreads are larger than previously found due to more accurate reddening estimates. We also examined the intrinsic $(R-I)_0$ and $(V-R)_0$ colours across our sample. These colours behave similarly to $(B-V)_0$, except that the $(V-R)_0$ colour gets redder after peak, likely due to the contributions of emission line flux. We searched for correlations between nova colours and $t_2$, peak $V$-band absolute magnitude, and GeV $\gamma$-ray luminosity, but find no statistically significant correlations. Nova colours can therefore be used as standard ``crayons" to estimate interstellar reddening from photometry alone, with 0.2--0.3 mag uncertainty. We present a novel Bayesian strategy for estimating distances to Galactic novae based on these $E(B-V)$ measurements, independent of assumptions about luminosity, built using 3D dust maps and a stellar mass model of the Milky Way.