Titanium and vanadium chemistry in low-mass dwarf stars

The equilibrium gas and condensation chemistry of titanium and vanadium in M, L, and T dwarf atmospheres is computed. The calcium titanates (Ca3Ti2O7 and Ca4Ti3O10) are identified for the first time as important Ti-bearing condensates in addition to perovskite (CaTiO3) and Ti oxides in dwarf atmospheres. The chemistry of Ti is intimately coupled to that of refractory condensates containing Ca and Al, whose chemistry is much more intricate than commonly assumed. The TiO gas abundances in equilibrium with Ca3Ti2O7 or Ca4Ti3O10 are lower than the TiO gas abundances in equilibrium with perovskite. Consequently, this implies that TiO opacities are lower than previously assumed at temperatures and pressures where the new Ca titanates are stable. Vanadium condenses into solid solution with Ti-bearing condensates (as observed in meteorites) and not as pure vanadium oxide, as commonly assumed. The calculated TiO (gas) and VO (gas) abundances are used to constrain temperatures at the M/L dwarf transition. Adopted temperatures are 2010 K (with an upper limit of 2245 K) for M8 and 1960 K for M9. Temperatures at the beginning of the L sequence are less than 1950 K and are tentatively placed at similar to1910 K for L0.