The Sensitivity of Multidimensional Nova Calculations to the Outer Boundary Condition

Multidimensional reactive flow models of accreted hydrogen rich envelopes ontop of degenerate cold white dwarfs are very effective tools for the study ofcritical, non spherically symmetric, behaviors during the early stages of novaoutbursts. Such models can shed light both on the mechanism responsible for theheavy element enrichment observed to characterize nova envelope matter and onthe role of perturbations during the early stages of ignition of the runaway.The complexity of convective reactive flow in multi-dimensions makes thecomputational model itself complex and sensitive to the details of thenumerics. In this study, we demonstrate that the imposed outer boundarycondition can have a dramatic effect on the solution. Several commonly usedchoices for the outer boundary conditions are examined. It is shown that thesolutions obtained from Lagrangian simulations, where the envelope is allowedto expand and mass is being conserved, are consistent with sphericallysymmetric solutions. In Eulerian schemes which utilize an outer boundarycondition of free outflow, the outburst can be artificially quenched.Comment: 12 Pages 3 figures; Accepted for publication in the Astrophysical Journa