LOCAL THERMONUCLEAR RUNAWAYS AMONG CLASSICAL NOVAE

A classical nova may accrete a non-homogeneous envelope,which can resu lt in the ignition of a local thermonuclear runaway on the surface of the white dwarf. We studied the propagation of thermal flows along the meridian in the hydrogen rich envelope, to find the conditions under which a thermonuclear runaway is not spherically symmetric. We include d mass accretion and tested the effect of temperature inhomogeneities in the secular evolution of the envelope, supposing perturbations with different wavelengths, ranging from very small to comparable with the radius of the white dwarf. The calculations were stopped at the onset of a thermonuclear runaway, when the hydrodynamic expansion starts. W e found the ranges of accretion rates and masses of white dwarfs for w hich the runaway ignites locally. The propagation time of the runaway along the meridian may be as long as days and weeks. ''Local'' thermon uclear runaways can explain the asymmetries and inhomogeneities of the nova shells and account for the slow rise time to maximum (about one week) of many novae.