Formation of SiC grains in pulsation-enhanced dust-driven wind around carbon-rich asymptotic giant branch stars

Abstract

We investigate the formation of silicon carbide (SiC) grains in the framework of dust-driven wind around pulsating carbon-rich asymptotic giant branch (C-rich AGB) stars to reveal not only the amount but also the size distribution. Two cases are considered for the nucleation process: one is the local thermal equilibrium (LTE) case where the vibration temperature of SiCclusters T v is equal to the gas temperature as usual, and another is the non-LTEcase in which T v is assumed to be the same as the temperature of small SiCgrains. The results of the hydrodynamical calculations for a model with stellar parameters of mass M * = 1.0 M ⊙, luminosity L * = 104 L ⊙, effective temperature T eff = 2600K, C/O ratio = 1.4, and pulsation period P = 650days show the following: in the LTEcase, SiCgrains condense in accelerated outflowing gas after the formation of carbon grains, and the resulting averaged mass ratio of SiCto carbon grains of 10 -8 is too small to reproduce the value of 0.01-0.3, which is inferred from the radiative transfer models. On the other hand, in the non-LTEcase, the formation region of the SiCgrains is more internal and/or almost identical to that of the carbon grains due to the so-called inverse greenhouse effect. The mass ratio of SiCto carbon grains averaged at the outer boundary ranges from 0.098 to 0.23 for the sticking probability αs = 0.1-1.0. The size distributions with the peak at ∼0.2-0.3 μm in radius cover the range of size derived from the analysis of the presolar SiCgrains. Thus, the difference between the temperatures of the small cluster and gas plays a crucial role in the formation process of SiCgrains around C-rich AGBstars, and this aspect should be explored for the formation process of dust grains in astrophysical environments. © 2012. The American Astronomical Society. All rights reserved.