Classical and Recurrent Novae
National Institute of Astrophysics (INAF), Astronomical Observatory of Padova, 36012 Asiago (VI), Italy
Invited review paper, received June 12, 2012
The physical nature and principal observational properties of novae are reviewed. Suggested improvments to optical photometry and discovery strategies are discussed. Nova eruptions occur in close binary systems, in which a white dwarf (WD) steadily accretes material on its surface from a lower mass cool companion. The accreted envelope is in electron degenerate conditions and grows steadily in mass with time, until a critical amount is accreted (which is inversely related to the WD mass). At that point, a fast evolving thermo-nuclear runaway starts burning hydrogen, in a short flash lasting about a hundred seconds, which is terminated by the violent ejection into the surrounding space (at a speed in excess of the escape velocity) of the whole accreted envelope (or a sizeable fraction of it). The nova is discovered only when, several hours or a few days later, the expansion and cooling of the fireball ejecta make them emit profusely at optical wavelengths; the later decline in brightness is regulated by interplay between dilution of the ejecta into surrounding space, gas and dust opacities, and temperature/luminosity of the central WD when the ejecta eventually become optically thin. The time interval between consecutive outbursts from the same nova is usually (far) longer than recorded history, but for a small number of objects (named recurrent novae) it is short enough that more than one outburst has been observed for them.