On the Formation of Graphite Particles in the Atmospheres of Mira Variables

Abstract

The observed molecular abundance ratios H 2 O/H 2 , CO/H 2 , CN/H 2 in α -Orionis are explained on the basis of an equilibrium composition of the elements with atomic ratios O/H ≅ 10 −4 , O/C ≅ 1.03, N/C ≅ 0.1 and a total hydrogen density ∼6.10 15 cm −3 . Provided that O/C < 1.07, N/C < 1.0 and the total hydrogen density exceeds ∼10 13 cm −3 , it is shown that the free carbon in an M star atmosphere becomes saturated with respect to bulk graphite at T ≅ 1800°K. About 0.1−1 per cent of the total atmospheric carbon may condense into graphite flakes of radii ∼10 −6 cm.