Charge-controlled grain destruction during the 'isothermal' phase of novae

The role played by grain charge in the dust-shell phase of moderate speed novae is examined. Under suitable conditions, photoemission dominates the charging equilibrium during the initial stages when the shell is optically thin, and the grains acquire a positive charge. This inhibits proton-grain collisions and prevents rapid grain erosion, which is discussed in terms of grain-surface reactions (chemisputtering). However, when the dust shell becomes optically thick, the grain charge in the outer part of the shell is reduced and chemisputtering proceeds in this region. The destruction phase is concluded when the shell once again becomes optically thin. Application of this mechanism to the nova NQ Vul 1976 indicates that the derived sputtering is obtained if the plasma density is approx. 108 cm-3. (author)