Formation of SiC Grains in Pulsation-Enhanced Dust-Driven Wind Around Carbon Rich Asymptotic Giant Branch Stars

TL;DR

This study models SiC grain formation in pulsation-enhanced dust-driven winds around carbon-rich AGB stars, revealing the importance of non-LTE effects on grain size and abundance matching observational data.

Contribution

It introduces a non-LTE approach to SiC grain formation modeling, showing its impact on grain size distribution and mass ratio, aligning better with observations.

Findings

Non-LTE conditions produce SiC grains with sizes of 0.2-0.3 μm.

Mass ratio of SiC to carbon grains ranges from 0.098 to 0.23 in non-LTE case.

LTE assumption results in too small SiC mass ratio to match observations.

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 in order to reveal not only the amount but also the size distribution. Two cases are considered for the nucleation process; one is the LTE case where the vibration temperature of SiC clusters $T_{\rm v}$ is equal to the gas temperature as usual, and another is the non-LTE case in which $T_{\rm v}$ is assumed to be the same as the temperature of small SiC grains. The results of hydrodynamical calculations for a model with stellar parameters of mass $M_{\ast}$=1.0 $M_{\odot}$, luminosity $L_{\ast}$=10$^{4}$ $L_{\odot}$, effective temperature $T_{\rm eff}$=2600 K, C/O ratio=1.4, and pulsation period $P$=650 days show the followings: In the LTE case, SiC grains condense in accelerated outflowing gas after the formation of carbon grains and the resulting averaged mass ratio of SiC to carbon grains of $\sim$ 10$^{-8}$ is too small to reproduce the value of 0.01-0.3 inferred from the radiative transfer models. On the other hand, in the non-LTE case, the formation region of SiC grains is inner than and/or almost identical to that of carbon grains due to the so-called inverse greenhouse effect. The mass ratio of SiC to carbon grains averaged at the outer boundary ranges from 0.098 to 0.23 for the sticking probability $\alpha_{\rm s}$=0.1-1.0. The size distributions with the peak at $\sim$ 0.2-0.3 $\rm{\mu}$m in radius cover the range of size derived from the analysis of presolar SiC grains. Thus the difference between temperatures of small cluster and gas plays a crucial role in the formation process of SiC grains around C-rich AGB stars, and this aspect should be explored for the formation process of dust grains in astrophysical environments.