THEUBVCOLOR EVOLUTION OF CLASSICAL NOVAE. I. NOVA-GIANT SEQUENCE IN THE COLOR-COLOR DIAGRAM

Abstract

We identified a general course of classical nova outbursts in the B − V versus U − B color-color diagram. It is reported that novae show spectra similar to those of 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 mainsequence sequence. Instead, we found that novae evolve along a new sequence in the pre-maximum and nearmaximum phases, which we call "the nova-giant sequence." This sequence is parallel to but ∆(U − B) ≈ −0.2 mag bluer than the supergiant sequence. This is because the mass of a nova envelope is much (∼ 10 −4 times) less than that of a normal supergiant. 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), which coincides with the intersection of the blackbody sequence and the nova-giant sequence, and remains there for a while. Then the color evolves leftward (blueward in B − V but almost constant in U − B), owing mainly to the development of strong emission lines. This is the general course of nova outbursts in the colorcolor diagram, which was deduced from eight well-observed novae in 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 the color excesses of classical novae. Using this method, we redetermined the color excesses of 20 well-observed novae. The obtained color excesses are in reasonable agreement with the previous results, which in turn support the idea of our general track of nova outbursts. Additionally, we estimated the absolute V magnitudes of about 30 novae using a method for time-stretching nova light curves to analyze the distance-reddening relations of the novae.

Multiwavelength observations of V407 Lupi (ASASSN-16kt) – a very fast nova erupting in an intermediate polar

“…The fast expanding envelope moves together with the optical photosphere (Hachisu & Kato 2006, in what is known as the "fireball stage". During this the brightness of the star increases by 8 up to 15 mag (Payne-Gaposchkin 1964) and reaches its maximum visual brightness when the optical photosphere attains its maximum radius (Warner 1995;Hachisu & Kato 2014). Following the TNR, the remaining hydrogen-rich, accreted envelope continuously burns on the WD surface and may become visible when the expanding ejecta become optically thin.…”

“…The blue horizontal, dashed line indicates the estimated maximum absolute magnitude. The blue vertical line indicates the intrinsic colours for an optically thick wind free-free emission (see e.g Hachisu & Kato 2014;. Darnley et al 2016 and references therein).…”

Discovery of a long-lived, high-amplitude dusty infrared transient

“…2). In dust producing novae, the broad-band colours generally become bluer with time as the optical depth of dust drops over time (Hachisu & Kato 2014). We conclude from the constant (or slight reddening) colour that the local dust cloud around CX330 is not expanding, but may be cooling.…”

Classical nova V339 Del (Nova Del 2013) - a short review

“…2. V339 Del after maximum moved along the nova-giant sequence introduced by Hachisu and Kato [37]. This sequence is parallel to, but bluer in (U − B) ≈ -0.2 mag than the supergiant sequence, because the mass of a nova envelope is 10 000 times lower than that of a normal supergiant.…”