The sensitivity of multidimensional nova calculations to the outer boundary condition

Multidimensional reactive flow models of accreted hydrogen-rich envelopes on top of degenerate cold white dwarfs are very effective tools for the study of critical, nonspherically symmetric behaviors during the early stages of nova outbursts. Such models can shed light on both the mechanism responsible for the heavy-element enrichment observed to characterize nova envelope matter and the role of perturbations during the early stages of ignition of the runaway. The complexity of convective reactive flow in multidimensions makes the computational model itself complex and sensitive to the details of the numerics. In this study, we demonstrate that the imposed outer boundary condition can have a dramatic effect on the solution. Several commonly used choices for the outer boundary conditions are examined. It is shown that the solutions obtained from Lagrangian simulations, where the envelope is allowed to expand and mass is being conserved, are consistent with spherically symmetric solutions. In Eulerian schemes, which utilize an outer boundary condition of free outflow, the outburst can be artificially quenched.