The UBV Color Evolution of Classical Novae. I. Nova-Giant Sequence in   the Color-Color Diagram
  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 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 \Delta (U-B) \approx -0.2 mag bluer than the supergiant sequence. This is because the mass of a nova envelope is much (\sim10^{-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 color-color 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 twenty well-observed novae. The obtained color excesses are in reasonable agreement with the previous results, which in turn supports the idea of our general track of nova outbursts. 
Izumi Hachisu (Univ. of Tokyo), Mariko Kato (Keio Univ.)
astro-ph.SR